Backport this series to 6.1&6.6 because LoongArch gets build errors with latest binutils which has commit 599df6e2db17d1c4 ("ld, LoongArch: print error about linking without -fPIC or -fPIE flag in more detail").
CC .vmlinux.export.o UPD include/generated/utsversion.h CC init/version-timestamp.o LD .tmp_vmlinux.kallsyms1 loongarch64-unknown-linux-gnu-ld: kernel/kallsyms.o:(.text+0): relocation R_LARCH_PCALA_HI20 against `kallsyms_markers` can not be used when making a PIE object; recompile with -fPIE loongarch64-unknown-linux-gnu-ld: kernel/crash_core.o:(.init.text+0x984): relocation R_LARCH_PCALA_HI20 against `kallsyms_names` can not be used when making a PIE object; recompile with -fPIE loongarch64-unknown-linux-gnu-ld: kernel/bpf/btf.o:(.text+0xcc7c): relocation R_LARCH_PCALA_HI20 against `__start_BTF` can not be used when making a PIE object; recompile with -fPIE loongarch64-unknown-linux-gnu-ld: BFD (GNU Binutils) 2.43.50.20241126 assertion fail ../../bfd/elfnn-loongarch.c:2673
In theory 5.10&5.15 also need this, but since LoongArch get upstream at 5.19, so I just ignore them because there is no error report about other archs now.
Weak external linkage is intended for cases where a symbol reference can remain unsatisfied in the final link. Taking the address of such a symbol should yield NULL if the reference was not satisfied.
Given that ordinary RIP or PC relative references cannot produce NULL, some kind of indirection is always needed in such cases, and in position independent code, this results in a GOT entry. In ordinary code, it is arch specific but amounts to the same thing.
While unavoidable in some cases, weak references are currently also used to declare symbols that are always defined in the final link, but not in the first linker pass. This means we end up with worse codegen for no good reason. So let's clean this up, by providing preliminary definitions that are only used as a fallback.
Ard Biesheuvel (3): kallsyms: Avoid weak references for kallsyms symbols vmlinux: Avoid weak reference to notes section btf: Avoid weak external references
Signed-off-by: Ard Biesheuvel ardb@kernel.org Signed-off-by: Huacai Chen chenhuacai@loongson.cn --- include/asm-generic/vmlinux.lds.h | 28 ++++++++++++++++++ kernel/bpf/btf.c | 7 +++-- kernel/bpf/sysfs_btf.c | 6 ++-- kernel/kallsyms.c | 6 ---- kernel/kallsyms_internal.h | 30 ++++++++------------ kernel/ksysfs.c | 4 +-- lib/buildid.c | 4 +-- 7 files changed, 52 insertions(+), 33 deletions(-) --- 2.27.0
From: Ard Biesheuvel ardb@kernel.org
[ Upstream commit 951bcae6c5a0bfaa55b27c5f16178204988f0379 ]
kallsyms is a directory of all the symbols in the vmlinux binary, and so creating it is somewhat of a chicken-and-egg problem, as its non-zero size affects the layout of the binary, and therefore the values of the symbols.
For this reason, the kernel is linked more than once, and the first pass does not include any kallsyms data at all. For the linker to accept this, the symbol declarations describing the kallsyms metadata are emitted as having weak linkage, so they can remain unsatisfied. During the subsequent passes, the weak references are satisfied by the kallsyms metadata that was constructed based on information gathered from the preceding passes.
Weak references lead to somewhat worse codegen, because taking their address may need to produce NULL (if the reference was unsatisfied), and this is not usually supported by RIP or PC relative symbol references.
Given that these references are ultimately always satisfied in the final link, let's drop the weak annotation, and instead, provide fallback definitions in the linker script that are only emitted if an unsatisfied reference exists.
While at it, drop the FRV specific annotation that these symbols reside in .rodata - FRV is long gone.
Tested-by: Nick Desaulniers ndesaulniers@google.com # Boot Reviewed-by: Nick Desaulniers ndesaulniers@google.com Reviewed-by: Kees Cook keescook@chromium.org Acked-by: Arnd Bergmann arnd@arndb.de Link: https://lkml.kernel.org/r/20230504174320.3930345-1-ardb%40kernel.org Signed-off-by: Ard Biesheuvel ardb@kernel.org Signed-off-by: Masahiro Yamada masahiroy@kernel.org Signed-off-by: Huacai Chen chenhuacai@loongson.cn --- include/asm-generic/vmlinux.lds.h | 19 +++++++++++++++++++ kernel/kallsyms.c | 6 ------ kernel/kallsyms_internal.h | 30 ++++++++++++------------------ 3 files changed, 31 insertions(+), 24 deletions(-)
diff --git a/include/asm-generic/vmlinux.lds.h b/include/asm-generic/vmlinux.lds.h index 63029bc7c9dd..64a80059d7fa 100644 --- a/include/asm-generic/vmlinux.lds.h +++ b/include/asm-generic/vmlinux.lds.h @@ -447,11 +447,30 @@ #endif #endif
+/* + * Some symbol definitions will not exist yet during the first pass of the + * link, but are guaranteed to exist in the final link. Provide preliminary + * definitions that will be superseded in the final link to avoid having to + * rely on weak external linkage, which requires a GOT when used in position + * independent code. + */ +#define PRELIMINARY_SYMBOL_DEFINITIONS \ + PROVIDE(kallsyms_addresses = .); \ + PROVIDE(kallsyms_offsets = .); \ + PROVIDE(kallsyms_names = .); \ + PROVIDE(kallsyms_num_syms = .); \ + PROVIDE(kallsyms_relative_base = .); \ + PROVIDE(kallsyms_token_table = .); \ + PROVIDE(kallsyms_token_index = .); \ + PROVIDE(kallsyms_markers = .); \ + PROVIDE(kallsyms_seqs_of_names = .); + /* * Read only Data */ #define RO_DATA(align) \ . = ALIGN((align)); \ + PRELIMINARY_SYMBOL_DEFINITIONS \ .rodata : AT(ADDR(.rodata) - LOAD_OFFSET) { \ __start_rodata = .; \ *(.rodata) *(.rodata.*) \ diff --git a/kernel/kallsyms.c b/kernel/kallsyms.c index 18edd57b5fe8..22ea19a36e6e 100644 --- a/kernel/kallsyms.c +++ b/kernel/kallsyms.c @@ -325,12 +325,6 @@ static unsigned long get_symbol_pos(unsigned long addr, unsigned long symbol_start = 0, symbol_end = 0; unsigned long i, low, high, mid;
- /* This kernel should never had been booted. */ - if (!IS_ENABLED(CONFIG_KALLSYMS_BASE_RELATIVE)) - BUG_ON(!kallsyms_addresses); - else - BUG_ON(!kallsyms_offsets); - /* Do a binary search on the sorted kallsyms_addresses array. */ low = 0; high = kallsyms_num_syms; diff --git a/kernel/kallsyms_internal.h b/kernel/kallsyms_internal.h index 27fabdcc40f5..85480274fc8f 100644 --- a/kernel/kallsyms_internal.h +++ b/kernel/kallsyms_internal.h @@ -5,27 +5,21 @@ #include <linux/types.h>
/* - * These will be re-linked against their real values - * during the second link stage. + * These will be re-linked against their real values during the second link + * stage. Preliminary values must be provided in the linker script using the + * PROVIDE() directive so that the first link stage can complete successfully. */ -extern const unsigned long kallsyms_addresses[] __weak; -extern const int kallsyms_offsets[] __weak; -extern const u8 kallsyms_names[] __weak; +extern const unsigned long kallsyms_addresses[]; +extern const int kallsyms_offsets[]; +extern const u8 kallsyms_names[];
-/* - * Tell the compiler that the count isn't in the small data section if the arch - * has one (eg: FRV). - */ -extern const unsigned int kallsyms_num_syms -__section(".rodata") __attribute__((weak)); - -extern const unsigned long kallsyms_relative_base -__section(".rodata") __attribute__((weak)); +extern const unsigned int kallsyms_num_syms; +extern const unsigned long kallsyms_relative_base;
-extern const char kallsyms_token_table[] __weak; -extern const u16 kallsyms_token_index[] __weak; +extern const char kallsyms_token_table[]; +extern const u16 kallsyms_token_index[];
-extern const unsigned int kallsyms_markers[] __weak; -extern const u8 kallsyms_seqs_of_names[] __weak; +extern const unsigned int kallsyms_markers[]; +extern const u8 kallsyms_seqs_of_names[];
#endif // LINUX_KALLSYMS_INTERNAL_H_
[ Sasha's backport helper bot ]
Hi,
The upstream commit SHA1 provided is correct: 951bcae6c5a0bfaa55b27c5f16178204988f0379
WARNING: Author mismatch between patch and upstream commit: Backport author: Huacai Chen chenhuacai@loongson.cn Commit author: Ard Biesheuvel ardb@kernel.org
Status in newer kernel trees: 6.12.y | Present (exact SHA1) 6.6.y | Not found
Note: The patch differs from the upstream commit: --- 1: 951bcae6c5a0b ! 1: a89be9ce6914a kallsyms: Avoid weak references for kallsyms symbols @@ Metadata ## Commit message ## kallsyms: Avoid weak references for kallsyms symbols
+ [ Upstream commit 951bcae6c5a0bfaa55b27c5f16178204988f0379 ] + kallsyms is a directory of all the symbols in the vmlinux binary, and so creating it is somewhat of a chicken-and-egg problem, as its non-zero size affects the layout of the binary, and therefore the values of the @@ Commit message Link: https://lkml.kernel.org/r/20230504174320.3930345-1-ardb%40kernel.org Signed-off-by: Ard Biesheuvel ardb@kernel.org Signed-off-by: Masahiro Yamada masahiroy@kernel.org + Signed-off-by: Huacai Chen chenhuacai@loongson.cn
## include/asm-generic/vmlinux.lds.h ## @@ ---
Results of testing on various branches:
| Branch | Patch Apply | Build Test | |---------------------------|-------------|------------| | stable/linux-6.6.y | Success | Success | | stable/linux-6.1.y | Success | Success |
[ Sasha's backport helper bot ]
Hi,
The upstream commit SHA1 provided is correct: 951bcae6c5a0bfaa55b27c5f16178204988f0379
WARNING: Author mismatch between patch and upstream commit: Backport author: Huacai Chenchenhuacai@loongson.cn Commit author: Ard Biesheuvelardb@kernel.org
Status in newer kernel trees: 6.13.y | Present (exact SHA1) 6.12.y | Present (exact SHA1) 6.6.y | Not found
Note: The patch differs from the upstream commit: --- 1: 951bcae6c5a0b ! 1: d6ccfcaa675df kallsyms: Avoid weak references for kallsyms symbols @@ Metadata ## Commit message ## kallsyms: Avoid weak references for kallsyms symbols
+ [ Upstream commit 951bcae6c5a0bfaa55b27c5f16178204988f0379 ] + kallsyms is a directory of all the symbols in the vmlinux binary, and so creating it is somewhat of a chicken-and-egg problem, as its non-zero size affects the layout of the binary, and therefore the values of the @@ Commit message Link: https://lkml.kernel.org/r/20230504174320.3930345-1-ardb%40kernel.org Signed-off-by: Ard Biesheuvel ardb@kernel.org Signed-off-by: Masahiro Yamada masahiroy@kernel.org + Signed-off-by: Huacai Chen chenhuacai@loongson.cn
## include/asm-generic/vmlinux.lds.h ## @@ ---
Results of testing on various branches:
| Branch | Patch Apply | Build Test | |---------------------------|-------------|------------| | stable/linux-6.6.y | Success | Failed | | stable/linux-6.1.y | Success | Failed |
Build Errors: Build error for stable/linux-6.6.y: ssh: connect to host 192.168.1.58 port 22: No route to host
Build error for stable/linux-6.1.y: ssh: connect to host 192.168.1.58 port 22: No route to host
From: Ard Biesheuvel ardb@kernel.org
[ Upstream commit 377d9095117c084b835e38c020faf5a78e386f01 ]
Weak references are references that are permitted to remain unsatisfied in the final link. This means they cannot be implemented using place relative relocations, resulting in GOT entries when using position independent code generation.
The notes section should always exist, so the weak annotations can be omitted.
Acked-by: Arnd Bergmann arnd@arndb.de Signed-off-by: Ard Biesheuvel ardb@kernel.org Signed-off-by: Masahiro Yamada masahiroy@kernel.org Signed-off-by: Huacai Chen chenhuacai@loongson.cn --- kernel/ksysfs.c | 4 ++-- lib/buildid.c | 4 ++-- 2 files changed, 4 insertions(+), 4 deletions(-)
diff --git a/kernel/ksysfs.c b/kernel/ksysfs.c index 1d4bc493b2f4..347beb763c59 100644 --- a/kernel/ksysfs.c +++ b/kernel/ksysfs.c @@ -226,8 +226,8 @@ KERNEL_ATTR_RW(rcu_normal); /* * Make /sys/kernel/notes give the raw contents of our kernel .notes section. */ -extern const void __start_notes __weak; -extern const void __stop_notes __weak; +extern const void __start_notes; +extern const void __stop_notes; #define notes_size (&__stop_notes - &__start_notes)
static ssize_t notes_read(struct file *filp, struct kobject *kobj, diff --git a/lib/buildid.c b/lib/buildid.c index 9fc46366597e..ebfc8a28e46a 100644 --- a/lib/buildid.c +++ b/lib/buildid.c @@ -208,8 +208,8 @@ unsigned char vmlinux_build_id[BUILD_ID_SIZE_MAX] __ro_after_init; */ void __init init_vmlinux_build_id(void) { - extern const void __start_notes __weak; - extern const void __stop_notes __weak; + extern const void __start_notes; + extern const void __stop_notes; unsigned int size = &__stop_notes - &__start_notes;
build_id_parse_buf(&__start_notes, vmlinux_build_id, size);
[ Sasha's backport helper bot ]
Hi,
The upstream commit SHA1 provided is correct: 377d9095117c084b835e38c020faf5a78e386f01
WARNING: Author mismatch between patch and upstream commit: Backport author: Huacai Chen chenhuacai@loongson.cn Commit author: Ard Biesheuvel ardb@kernel.org
Status in newer kernel trees: 6.12.y | Present (exact SHA1) 6.6.y | Not found
Note: The patch differs from the upstream commit: --- 1: 377d9095117c0 ! 1: c735ac7f41f90 vmlinux: Avoid weak reference to notes section @@ Metadata ## Commit message ## vmlinux: Avoid weak reference to notes section
+ [ Upstream commit 377d9095117c084b835e38c020faf5a78e386f01 ] + Weak references are references that are permitted to remain unsatisfied in the final link. This means they cannot be implemented using place relative relocations, resulting in GOT entries when using position @@ Commit message Acked-by: Arnd Bergmann arnd@arndb.de Signed-off-by: Ard Biesheuvel ardb@kernel.org Signed-off-by: Masahiro Yamada masahiroy@kernel.org + Signed-off-by: Huacai Chen chenhuacai@loongson.cn
## kernel/ksysfs.c ## @@ kernel/ksysfs.c: KERNEL_ATTR_RW(rcu_normal); ---
Results of testing on various branches:
| Branch | Patch Apply | Build Test | |---------------------------|-------------|------------| | stable/linux-6.6.y | Success | Success | | stable/linux-6.1.y | Success | Success |
From: Ard Biesheuvel ardb@kernel.org
[ Upstream commit fc5eb4a84e4c063e75a6a6e92308e9533c0f19b5 ]
If the BTF code is enabled in the build configuration, the start/stop BTF markers are guaranteed to exist. Only when CONFIG_DEBUG_INFO_BTF=n, the references in btf_parse_vmlinux() will remain unsatisfied, relying on the weak linkage of the external references to avoid breaking the build.
Avoid GOT based relocations to these markers in the final executable by dropping the weak attribute and instead, make btf_parse_vmlinux() return ERR_PTR(-ENOENT) directly if CONFIG_DEBUG_INFO_BTF is not enabled to begin with. The compiler will drop any subsequent references to __start_BTF and __stop_BTF in that case, allowing the link to succeed.
Note that Clang will notice that taking the address of __start_BTF can no longer yield NULL, so testing for that condition becomes unnecessary.
Signed-off-by: Ard Biesheuvel ardb@kernel.org Signed-off-by: Daniel Borkmann daniel@iogearbox.net Acked-by: Andrii Nakryiko andrii@kernel.org Acked-by: Arnd Bergmann arnd@arndb.de Acked-by: Jiri Olsa jolsa@kernel.org Link: https://lore.kernel.org/bpf/20240415162041.2491523-8-ardb+git@google.com Signed-off-by: Huacai Chen chenhuacai@loongson.cn --- kernel/bpf/btf.c | 7 +++++-- kernel/bpf/sysfs_btf.c | 6 +++--- 2 files changed, 8 insertions(+), 5 deletions(-)
diff --git a/kernel/bpf/btf.c b/kernel/bpf/btf.c index c8828016a66f..f231e2a273a1 100644 --- a/kernel/bpf/btf.c +++ b/kernel/bpf/btf.c @@ -5574,8 +5574,8 @@ static struct btf *btf_parse(const union bpf_attr *attr, bpfptr_t uattr, u32 uat return ERR_PTR(err); }
-extern char __weak __start_BTF[]; -extern char __weak __stop_BTF[]; +extern char __start_BTF[]; +extern char __stop_BTF[]; extern struct btf *btf_vmlinux;
#define BPF_MAP_TYPE(_id, _ops) @@ -5724,6 +5724,9 @@ struct btf *btf_parse_vmlinux(void) struct btf *btf = NULL; int err;
+ if (!IS_ENABLED(CONFIG_DEBUG_INFO_BTF)) + return ERR_PTR(-ENOENT); + env = kzalloc(sizeof(*env), GFP_KERNEL | __GFP_NOWARN); if (!env) return ERR_PTR(-ENOMEM); diff --git a/kernel/bpf/sysfs_btf.c b/kernel/bpf/sysfs_btf.c index ef6911aee3bb..fedb54c94cdb 100644 --- a/kernel/bpf/sysfs_btf.c +++ b/kernel/bpf/sysfs_btf.c @@ -9,8 +9,8 @@ #include <linux/sysfs.h>
/* See scripts/link-vmlinux.sh, gen_btf() func for details */ -extern char __weak __start_BTF[]; -extern char __weak __stop_BTF[]; +extern char __start_BTF[]; +extern char __stop_BTF[];
static ssize_t btf_vmlinux_read(struct file *file, struct kobject *kobj, @@ -32,7 +32,7 @@ static int __init btf_vmlinux_init(void) { bin_attr_btf_vmlinux.size = __stop_BTF - __start_BTF;
- if (!__start_BTF || bin_attr_btf_vmlinux.size == 0) + if (bin_attr_btf_vmlinux.size == 0) return 0;
btf_kobj = kobject_create_and_add("btf", kernel_kobj);
[ Sasha's backport helper bot ]
Hi,
The upstream commit SHA1 provided is correct: fc5eb4a84e4c063e75a6a6e92308e9533c0f19b5
WARNING: Author mismatch between patch and upstream commit: Backport author: Huacai Chen chenhuacai@loongson.cn Commit author: Ard Biesheuvel ardb@kernel.org
Status in newer kernel trees: 6.12.y | Present (exact SHA1) 6.6.y | Not found
Note: The patch differs from the upstream commit: --- 1: fc5eb4a84e4c0 ! 1: 0637e4a3f4e0a btf: Avoid weak external references @@ Metadata ## Commit message ## btf: Avoid weak external references
+ [ Upstream commit fc5eb4a84e4c063e75a6a6e92308e9533c0f19b5 ] + If the BTF code is enabled in the build configuration, the start/stop BTF markers are guaranteed to exist. Only when CONFIG_DEBUG_INFO_BTF=n, the references in btf_parse_vmlinux() will remain unsatisfied, relying @@ Commit message Acked-by: Arnd Bergmann arnd@arndb.de Acked-by: Jiri Olsa jolsa@kernel.org Link: https://lore.kernel.org/bpf/20240415162041.2491523-8-ardb+git@google.com + Signed-off-by: Huacai Chen chenhuacai@loongson.cn
## kernel/bpf/btf.c ## @@ kernel/bpf/btf.c: static struct btf *btf_parse(const union bpf_attr *attr, bpfptr_t uattr, u32 uat ---
Results of testing on various branches:
| Branch | Patch Apply | Build Test | |---------------------------|-------------|------------| | stable/linux-6.6.y | Success | Success | | stable/linux-6.1.y | Success | Success |
On Fri, Dec 06, 2024 at 04:58:07PM +0800, Huacai Chen wrote:
Backport this series to 6.1&6.6 because LoongArch gets build errors with latest binutils which has commit 599df6e2db17d1c4 ("ld, LoongArch: print error about linking without -fPIC or -fPIE flag in more detail").
CC .vmlinux.export.o UPD include/generated/utsversion.h CC init/version-timestamp.o LD .tmp_vmlinux.kallsyms1 loongarch64-unknown-linux-gnu-ld: kernel/kallsyms.o:(.text+0): relocation R_LARCH_PCALA_HI20 against `kallsyms_markers` can not be used when making a PIE object; recompile with -fPIE loongarch64-unknown-linux-gnu-ld: kernel/crash_core.o:(.init.text+0x984): relocation R_LARCH_PCALA_HI20 against `kallsyms_names` can not be used when making a PIE object; recompile with -fPIE loongarch64-unknown-linux-gnu-ld: kernel/bpf/btf.o:(.text+0xcc7c): relocation R_LARCH_PCALA_HI20 against `__start_BTF` can not be used when making a PIE object; recompile with -fPIE loongarch64-unknown-linux-gnu-ld: BFD (GNU Binutils) 2.43.50.20241126 assertion fail ../../bfd/elfnn-loongarch.c:2673
In theory 5.10&5.15 also need this, but since LoongArch get upstream at 5.19, so I just ignore them because there is no error report about other archs now.
Odd, why doesn't this affect other arches as well using new binutils? I hate to have to backport all of this just for one arch, as that feels odd.
thanks,
greg k-h
Hi, Greg,
On Fri, Dec 6, 2024 at 9:04 PM Greg Kroah-Hartman gregkh@linuxfoundation.org wrote:
On Fri, Dec 06, 2024 at 04:58:07PM +0800, Huacai Chen wrote:
Backport this series to 6.1&6.6 because LoongArch gets build errors with latest binutils which has commit 599df6e2db17d1c4 ("ld, LoongArch: print error about linking without -fPIC or -fPIE flag in more detail").
CC .vmlinux.export.o UPD include/generated/utsversion.h CC init/version-timestamp.o LD .tmp_vmlinux.kallsyms1 loongarch64-unknown-linux-gnu-ld: kernel/kallsyms.o:(.text+0): relocation R_LARCH_PCALA_HI20 against `kallsyms_markers` can not be used when making a PIE object; recompile with -fPIE loongarch64-unknown-linux-gnu-ld: kernel/crash_core.o:(.init.text+0x984): relocation R_LARCH_PCALA_HI20 against `kallsyms_names` can not be used when making a PIE object; recompile with -fPIE loongarch64-unknown-linux-gnu-ld: kernel/bpf/btf.o:(.text+0xcc7c): relocation R_LARCH_PCALA_HI20 against `__start_BTF` can not be used when making a PIE object; recompile with -fPIE loongarch64-unknown-linux-gnu-ld: BFD (GNU Binutils) 2.43.50.20241126 assertion fail ../../bfd/elfnn-loongarch.c:2673
In theory 5.10&5.15 also need this, but since LoongArch get upstream at 5.19, so I just ignore them because there is no error report about other archs now.
Odd, why doesn't this affect other arches as well using new binutils? I hate to have to backport all of this just for one arch, as that feels odd.
The related binutils commit is only for LoongArch, so build errors only occured on LoongArch. I don't know why other archs have no problem exactly, but may be related to their CFLAGS (for example, if we disable CONFIG_RELOCATABLE, LoongArch also has no build errors because CFLAGS changes).
On the other hand, Ard's original patches are not for LoongArch only, so I think backport to stable branches is also not for LoongArch only.
Huacai
thanks,
greg k-h
On Sat, Dec 07, 2024 at 05:21:00PM +0800, Huacai Chen wrote:
Hi, Greg,
On Fri, Dec 6, 2024 at 9:04 PM Greg Kroah-Hartman gregkh@linuxfoundation.org wrote:
On Fri, Dec 06, 2024 at 04:58:07PM +0800, Huacai Chen wrote:
Backport this series to 6.1&6.6 because LoongArch gets build errors with latest binutils which has commit 599df6e2db17d1c4 ("ld, LoongArch: print error about linking without -fPIC or -fPIE flag in more detail").
CC .vmlinux.export.o UPD include/generated/utsversion.h CC init/version-timestamp.o LD .tmp_vmlinux.kallsyms1 loongarch64-unknown-linux-gnu-ld: kernel/kallsyms.o:(.text+0): relocation R_LARCH_PCALA_HI20 against `kallsyms_markers` can not be used when making a PIE object; recompile with -fPIE loongarch64-unknown-linux-gnu-ld: kernel/crash_core.o:(.init.text+0x984): relocation R_LARCH_PCALA_HI20 against `kallsyms_names` can not be used when making a PIE object; recompile with -fPIE loongarch64-unknown-linux-gnu-ld: kernel/bpf/btf.o:(.text+0xcc7c): relocation R_LARCH_PCALA_HI20 against `__start_BTF` can not be used when making a PIE object; recompile with -fPIE loongarch64-unknown-linux-gnu-ld: BFD (GNU Binutils) 2.43.50.20241126 assertion fail ../../bfd/elfnn-loongarch.c:2673
In theory 5.10&5.15 also need this, but since LoongArch get upstream at 5.19, so I just ignore them because there is no error report about other archs now.
Odd, why doesn't this affect other arches as well using new binutils? I hate to have to backport all of this just for one arch, as that feels odd.
The related binutils commit is only for LoongArch, so build errors only occured on LoongArch. I don't know why other archs have no problem exactly, but may be related to their CFLAGS (for example, if we disable CONFIG_RELOCATABLE, LoongArch also has no build errors because CFLAGS changes).
does LoongArch depend on that option? What happens if it is enabled for other arches? Why doesn't it break them?
On the other hand, Ard's original patches are not for LoongArch only, so I think backport to stable branches is also not for LoongArch only.
Maybe Ard can answer that.
thanks,
greg k-h
On Sat, 2024-12-07 at 10:32 +0100, Greg Kroah-Hartman wrote:
On Sat, Dec 07, 2024 at 05:21:00PM +0800, Huacai Chen wrote:
Hi, Greg,
On Fri, Dec 6, 2024 at 9:04 PM Greg Kroah-Hartman gregkh@linuxfoundation.org wrote:
On Fri, Dec 06, 2024 at 04:58:07PM +0800, Huacai Chen wrote:
Backport this series to 6.1&6.6 because LoongArch gets build errors with latest binutils which has commit 599df6e2db17d1c4 ("ld, LoongArch: print error about linking without -fPIC or -fPIE flag in more detail").
CC .vmlinux.export.o UPD include/generated/utsversion.h CC init/version-timestamp.o LD .tmp_vmlinux.kallsyms1 loongarch64-unknown-linux-gnu-ld: kernel/kallsyms.o:(.text+0): relocation R_LARCH_PCALA_HI20 against `kallsyms_markers` can not be used when making a PIE object; recompile with -fPIE loongarch64-unknown-linux-gnu-ld: kernel/crash_core.o:(.init.text+0x984): relocation R_LARCH_PCALA_HI20 against `kallsyms_names` can not be used when making a PIE object; recompile with -fPIE loongarch64-unknown-linux-gnu-ld: kernel/bpf/btf.o:(.text+0xcc7c): relocation R_LARCH_PCALA_HI20 against `__start_BTF` can not be used when making a PIE object; recompile with -fPIE loongarch64-unknown-linux-gnu-ld: BFD (GNU Binutils) 2.43.50.20241126 assertion fail ../../bfd/elfnn-loongarch.c:2673
In theory 5.10&5.15 also need this, but since LoongArch get upstream at 5.19, so I just ignore them because there is no error report about other archs now.
Odd, why doesn't this affect other arches as well using new binutils? I hate to have to backport all of this just for one arch, as that feels odd.
The related binutils commit is only for LoongArch, so build errors only occured on LoongArch. I don't know why other archs have no problem exactly, but may be related to their CFLAGS (for example, if we disable CONFIG_RELOCATABLE, LoongArch also has no build errors because CFLAGS changes).
does LoongArch depend on that option?
"That option" is -mdirect-extern-access. Without it we'll use GOT in the kernel image to address anything out of the current TU, bloating the kernel size and making it slower.
The problem is the linker failed to handle a direct access to undefined weak symbol on LoongArch. With GCC 14.2 and Binutils 2.43:
$ cat t.c extern int x __attribute__ ((weak));
int main() { __builtin_printf("%p\n", &x); } $ cc t.c -mdirect-extern-access -static-pie -fPIE $ ./a.out 0x7ffff27ac000
The output should be (nil) instead, as an undefined weak symbol should be resolved to address 0. I'm not sure why the kernel was not blown up by this issue.
With Binutils trunk, an error is emitted instead of silently producing buggy executable. Still I don't think emitting an error is correct when linking a static PIE (our vmlinux is a static PIE). Instead the linker should just rewrite
pcalau12i rd, %pc_hi20(undef_weak)
to
move rd, $zero
Also the "recompile with -fPIE" suggestion in the error message is completely misleading. We are *already* compiling relocatable kernel with -fPIE.
I'm making some Binutils patches to implement the rewrite and reword the error message (for instances where emitting an error is the correct thing, e.g. someone attempts to build a dynamically linked program with -mdirect-extern-access).
What happens if it is enabled for other arches? Why doesn't it break them?
The other arches have copy relocation, so their -mdirect-extern-access is intended to work with dynamically linked executable, thus it's the default and not as strong as ours. On them -mdirect-extern-access still uses GOT to address weak symbols.
We don't have copy relocation, thus our default is -mno-direct-extern- access, and -mdirect-extern-access is only intended for static executables (including OS kernel, embedded firmware, etc). So it's designed to be stronger, unfortunately the toolchain failed to implement it correctly.
On Sat, 7 Dec 2024 at 11:46, Xi Ruoyao xry111@xry111.site wrote:
On Sat, 2024-12-07 at 10:32 +0100, Greg Kroah-Hartman wrote:
On Sat, Dec 07, 2024 at 05:21:00PM +0800, Huacai Chen wrote:
Hi, Greg,
On Fri, Dec 6, 2024 at 9:04 PM Greg Kroah-Hartman gregkh@linuxfoundation.org wrote:
On Fri, Dec 06, 2024 at 04:58:07PM +0800, Huacai Chen wrote:
Backport this series to 6.1&6.6 because LoongArch gets build errors with latest binutils which has commit 599df6e2db17d1c4 ("ld, LoongArch: print error about linking without -fPIC or -fPIE flag in more detail").
CC .vmlinux.export.o UPD include/generated/utsversion.h CC init/version-timestamp.o LD .tmp_vmlinux.kallsyms1 loongarch64-unknown-linux-gnu-ld: kernel/kallsyms.o:(.text+0): relocation R_LARCH_PCALA_HI20 against `kallsyms_markers` can not be used when making a PIE object; recompile with -fPIE loongarch64-unknown-linux-gnu-ld: kernel/crash_core.o:(.init.text+0x984): relocation R_LARCH_PCALA_HI20 against `kallsyms_names` can not be used when making a PIE object; recompile with -fPIE loongarch64-unknown-linux-gnu-ld: kernel/bpf/btf.o:(.text+0xcc7c): relocation R_LARCH_PCALA_HI20 against `__start_BTF` can not be used when making a PIE object; recompile with -fPIE loongarch64-unknown-linux-gnu-ld: BFD (GNU Binutils) 2.43.50.20241126 assertion fail ../../bfd/elfnn-loongarch.c:2673
In theory 5.10&5.15 also need this, but since LoongArch get upstream at 5.19, so I just ignore them because there is no error report about other archs now.
Odd, why doesn't this affect other arches as well using new binutils? I hate to have to backport all of this just for one arch, as that feels odd.
The related binutils commit is only for LoongArch, so build errors only occured on LoongArch. I don't know why other archs have no problem exactly, but may be related to their CFLAGS (for example, if we disable CONFIG_RELOCATABLE, LoongArch also has no build errors because CFLAGS changes).
does LoongArch depend on that option?
"That option" is -mdirect-extern-access. Without it we'll use GOT in the kernel image to address anything out of the current TU, bloating the kernel size and making it slower.
An alternative to this might be to add
-include $(srctree)/include/linux/hidden.h
to KBUILD_CFLAGS_KERNEL, so that the compiler understands that all external references are resolved at link time, not at load/run time.
The problem is the linker failed to handle a direct access to undefined weak symbol on LoongArch.
...
With Binutils trunk, an error is emitted instead of silently producing buggy executable. Still I don't think emitting an error is correct when linking a static PIE (our vmlinux is a static PIE). Instead the linker should just rewrite
pcalau12i rd, %pc_hi20(undef_weak)to
move rd, $zero
Is that transformation even possible at link time? Isn't pc_hi20 part of a pair?
Also the "recompile with -fPIE" suggestion in the error message is completely misleading. We are *already* compiling relocatable kernel with -fPIE.
And this is the most important difference between LoongArch and the other arches - LoongArch already uses PIC code explicitly. Other architectures use ordinary position dependent codegen and linking, or -in the case of arm64- use position dependent codegen and PIE linking, where the fact that this is even possible is a happy accident.
...
What happens if it is enabled for other arches? Why doesn't it break them?
The other arches have copy relocation, so their -mdirect-extern-access is intended to work with dynamically linked executable, thus it's the default and not as strong as ours. On them -mdirect-extern-access still uses GOT to address weak symbols.
We don't have copy relocation, thus our default is -mno-direct-extern- access, and -mdirect-extern-access is only intended for static executables (including OS kernel, embedded firmware, etc). So it's designed to be stronger, unfortunately the toolchain failed to implement it correctly.
This has nothing to do with copy relocations - those are only relevant when shared libraries come into play.
Other architectures don't break because they either a) use position dependent codegen with absolute addressing, and simply resolve undefined weak references as 0x0, or b) use GOT indirection, where the reference is a GOT load and the address in the GOT is set to 0x0.
So the issue here appears to be that the compiler fails to emit a GOT entry for this reference, even though it is performing PIC codegen. This is probably due to -mdirect-extern-access being taken into account too strictly. The upshot is that a relative reference is emitted to an undefined symbol, and it is impossible for a relative reference to [reliably] yield NULL, and so the reference produces a bogus non-NULL address.
As these patches deal with symbols that are only undefined in the preliminary first linker pass, and are guaranteed to exist afterwards, silently emitting a bogus relative reference was not a problem in these cases. Obviously, throwing an error is.
The patches should be rather harmless in practice, but I know Masahiro did not like the approach for the kallsyms markers, and made some subsequent modifications to it.
Given that this is relatively new toolchain behavior, I'd suggest fixing the compiler to emit weak external references via GOT entries even when -mdirect-extern-access is in effect.
On Mon, 2024-12-09 at 09:31 +0100, Ard Biesheuvel wrote:
Given that this is relatively new toolchain behavior, I'd suggest fixing the compiler to emit weak external references via GOT entries even when -mdirect-extern-access is in effect.
I'm working on an approach in the linker instead. A PC-relative address in +/- 2GiB range is
pcalau12i.d $a0, %pc_hi20(sym + addend) addi.d $a0, $a0, %pc_lo12(sym + addend)
If doing a static linking, when sym is weak undefined, we should just load addend. The compiler already guarantees addend is in [-2**31, 2**31) range, so we just need to rewrite the pair to
lu12i.w $a0, ((addend + 0x800) & ~0x7ff) addi.d $a0, $a0, (addend & 0x7ff)
OTOH if not doing a static linking, the user shouldn't use -mdirect- extern-access at all [this rule is the thing related to copy relocation: if copy relocation was available it would be possibly valid to use - mdirect-extern-access w/o static linking] and the linker is correct to report an error (but the error message is unclear and I need to fix it anyway).
Hi, Greg,
It's rather unfortunate that currently, almost all Linux distributions supporting LoongArch are using LTS kernels version v6.6 or older, such as openEuler and deepin. [1][2]
If this bugfix isn't merged into linux-stable, then every single distro kernel team will have to waste time fixing the same darn bug over and over, even though it's already fixed in later kernels.
This would really make LTS look like it's failing to serve its intended purpose. And I'm sure all of us do not want to see something so terrible happen.
On Fri, Dec 6, 2024 at 9:04 PM Greg Kroah-Hartman wrote:
Odd, why doesn't this affect other arches as well using new binutils? I hate to have to backport all of this just for one arch, as that feels odd.
Could you help me understand why you expressed that you "hate" to have to backport something for only one arch? Given that we've historically done quite a bit of similar backporting for architectures such as arm, powerpc, and x86...It's not exactly unprecedented. I just want to grasp the rationale, as it all seems perfectly justified and necessary.
Moreover, with all the active and strict code reviews by all developers, such occurrences are not frequent on LoongArch. You could be not exactly "always" backporting something like this just for LoongArch, so perhaps that might make you and your colleagues feel a little less "hate" :-)
As for your questions on the root cause of the issue and the effectiveness of this fix, I reckon Xi Ruoyao's explanation and Ard Biesheuvel's supplementary points have already provided ample details. [3][4][5]
If, after your feedback, you still have any lingering doubts regarding the issue itself or the LoongArch architecture, I believe that Xi Ruoyao, Ard Biesheuvel, and Huacai Chen would all be more than willing to elaborate further.
I'm bringing this up because we've encountered concrete issues in the process of maintaining distributions. Furthermore, as an upstream resource, linux-stable can help us more effectively drive forward community development efforts. Plus, we realize this benefits all Linux community developers just the same.
Hoping you could spare a moment from your busy schedule to take another look at this patch series and perhaps reconsider the LTS inclusion of this bugfix.
[1]. https://gitee.com/openeuler/kernel/blob/openEuler-25.03/Makefile#L3 [2]. https://github.com/deepin-community/kernel/blob/linux-6.6.y/Makefile#L3 [3]. https://lore.kernel.org/all/ccb1fa9034b177042db8fcbe7a95a2a5b466dc30.camel@x... [4]. https://lore.kernel.org/all/CAMj1kXEV+HC+2HMLhDaLfAufQLrXRs2J7akMNr1mjejDYc7... [5]. https://lore.kernel.org/all/c9a43e5da01ee2215393c0f3c50956171fe660ab.camel@x...
Best Regards, -- WangYuli
On Thu, 6 Feb 2025 at 09:53, WangYuli wangyuli@uniontech.com wrote:
Hi, Greg,
It's rather unfortunate that currently, almost all Linux distributions supporting LoongArch are using LTS kernels version v6.6 or older, such as openEuler and deepin. [1][2]
If this bugfix isn't merged into linux-stable, then every single distro kernel team will have to waste time fixing the same darn bug over and over, even though it's already fixed in later kernels.
This would really make LTS look like it's failing to serve its intended purpose. And I'm sure all of us do not want to see something so terrible happen.
On Fri, Dec 6, 2024 at 9:04 PM Greg Kroah-Hartman wrote:
Odd, why doesn't this affect other arches as well using new binutils? I hate to have to backport all of this just for one arch, as that feels odd.
Could you help me understand why you expressed that you "hate" to have to backport something for only one arch? Given that we've historically done quite a bit of similar backporting for architectures such as arm, powerpc, and x86...It's not exactly unprecedented. I just want to grasp the rationale, as it all seems perfectly justified and necessary.
Moreover, with all the active and strict code reviews by all developers, such occurrences are not frequent on LoongArch. You could be not exactly "always" backporting something like this just for LoongArch, so perhaps that might make you and your colleagues feel a little less "hate" :-)
As for your questions on the root cause of the issue and the effectiveness of this fix, I reckon Xi Ruoyao's explanation and Ard Biesheuvel's supplementary points have already provided ample details. [3][4][5]
If, after your feedback, you still have any lingering doubts regarding the issue itself or the LoongArch architecture, I believe that Xi Ruoyao, Ard Biesheuvel, and Huacai Chen would all be more than willing to elaborate further.
I'm bringing this up because we've encountered concrete issues in the process of maintaining distributions. Furthermore, as an upstream resource, linux-stable can help us more effectively drive forward community development efforts. Plus, we realize this benefits all Linux community developers just the same.
Hoping you could spare a moment from your busy schedule to take another look at this patch series and perhaps reconsider the LTS inclusion of this bugfix.
[1]. https://gitee.com/openeuler/kernel/blob/openEuler-25.03/Makefile#L3 [2]. https://github.com/deepin-community/kernel/blob/linux-6.6.y/Makefile#L3 [3]. https://lore.kernel.org/all/ccb1fa9034b177042db8fcbe7a95a2a5b466dc30.camel@x... [4]. https://lore.kernel.org/all/CAMj1kXEV+HC+2HMLhDaLfAufQLrXRs2J7akMNr1mjejDYc7... [5]. https://lore.kernel.org/all/c9a43e5da01ee2215393c0f3c50956171fe660ab.camel@x...
You might consider sending a Loongarch-only patch for mainline that adds weak definitions of these symbols, and backport that to -stable once it hits Linus's tree. That way, the weak references are always satisfied, even during the first linker pass.
On Thu, Feb 06, 2025 at 04:37:02PM +0800, WangYuli wrote:
Hi, Greg,
It's rather unfortunate that currently, almost all Linux distributions supporting LoongArch are using LTS kernels version v6.6 or older, such as openEuler and deepin. [1][2]
If this bugfix isn't merged into linux-stable, then every single distro kernel team will have to waste time fixing the same darn bug over and over, even though it's already fixed in later kernels.
This would really make LTS look like it's failing to serve its intended purpose. And I'm sure all of us do not want to see something so terrible happen.
LTS is here to ensure that the original release of these branches, keeps working for that branch. Adding support for newer toolchains sometimes happens, but is not a requirement or a normal thing to do as that really isn't a "regression", right?
Most of the time, fixing things up for newer compilers is simple. Sometimes it is not simple. The "not simple" ones we usually just do not backport as that causes extra work for everyone over time.
As for the distros like openEuler, and deepin, they are free to add these patches there, on top of their other non-LTS patches, right?
thanks,
greg k-h
linux-stable-mirror@lists.linaro.org
-
Ard Biesheuvel -
Greg KH -
Greg Kroah-Hartman
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Huacai Chen -
Huacai Chen -
Sasha Levin -
WangYuli -
Xi Ruoyao