On 6/14/24 11:57 PM, Tony Ambardar wrote:
On Fri, Jun 14, 2024 at 11:47:19AM -0700, Yonghong Song wrote:
On 6/10/24 3:56 PM, Tony Ambardar wrote:
On Tue, Jun 04, 2024 at 10:55:39PM -0700, Yonghong Song wrote:
On 6/3/24 10:23 PM, Tony Ambardar wrote:
Some code includes the __used macro to prevent functions and data from being optimized out. This macro implements __attribute__((__used__)), which operates at the compiler and IR-level, and so still allows a linker to remove objects intended to be kept.
Compilers supporting __attribute__((__retain__)) can address this gap by setting the flag SHF_GNU_RETAIN on the section of a function/variable, indicating to the linker the object should be retained. This attribute is available since gcc 11, clang 13, and binutils 2.36.
Provide a __retain macro implementing __attribute__((__retain__)), whose first user will be the '__bpf_kfunc' tag.
Link: https://lore.kernel.org/bpf/ZlmGoT9KiYLZd91S@krava/T/ Cc: stable@vger.kernel.org # v6.6+ Signed-off-by: Tony Ambardar Tony.Ambardar@gmail.com
include/linux/compiler_types.h | 23 +++++++++++++++++++++++ 1 file changed, 23 insertions(+)diff --git a/include/linux/compiler_types.h b/include/linux/compiler_types.h index 93600de3800b..f14c275950b5 100644 --- a/include/linux/compiler_types.h +++ b/include/linux/compiler_types.h @@ -143,6 +143,29 @@ static inline void __chk_io_ptr(const volatile void __iomem *ptr) { } # define __preserve_most #endif +/*
- Annotating a function/variable with __retain tells the compiler to place
- the object in its own section and set the flag SHF_GNU_RETAIN. This flag
- instructs the linker to retain the object during garbage-cleanup or LTO
- phases.
- Note that the __used macro is also used to prevent functions or data
- being optimized out, but operates at the compiler/IR-level and may still
- allow unintended removal of objects during linking.
- Optional: only supported since gcc >= 11, clang >= 13
- */
+#if __has_attribute(__retain__) && \
- (defined(CONFIG_LD_DEAD_CODE_DATA_ELIMINATION) || \
defined(CONFIG_LTO_CLANG))Could you explain why CONFIG_LTO_CLANG is added here? IIUC, the __used macro permits garbage collection at section level, so CLANG_LTO_CLANG without CONFIG_LD_DEAD_CODE_DATA_ELIMINATION shuold not change final section dynamics, right?
Hi Yonghong,
I included the conditional guard to ensure consistent behaviour between __retain and other features forcing split sections. In particular, the same guard is used in vmlinux.lds.h to merge split sections where needed. For example, using __retain in llvm builds without CONFIG_LTO was failing CI tests on kernel-patches/bpf because the kernel didn't boot properly. And in further testing, the kernel had no issues loading BPF kfunc modules with such split sections, so I left the module (partial) linking scripts alone.
I tried with both bpf and bpf-next tree and I cannot make CONFIG_HAVE_LD_DEAD_CODE_DATA_ELIMINATION=y in .config file. The following are all occurances in Kconfig:
My understanding is one doesn't directly set HAVE_LD_DEAD_CODE_...; it's a per-arch capability flag which guards setting LD_DEAD_CODE_DATA_ELIMINATION but only targets "small systems" (i.e. embedded), so no surprise x86 isn't in the arch list below.
I see. Yes, mips should support it but not x86. No wonder why I cannot reproduce.
$ egrep -r HAVE_LD_DEAD_CODE_DATA_ELIMINATION arch/mips/Kconfig: select HAVE_LD_DEAD_CODE_DATA_ELIMINATION arch/powerpc/Kconfig: select HAVE_LD_DEAD_CODE_DATA_ELIMINATION if HAVE_OBJTOOL_MCOUNT && (!ARCH_USING_PATCHABLE_FUNCTION_ENTRY || (!CC_IS_GCC || GCC_VERSION >= 110100)) arch/riscv/Kconfig: select HAVE_LD_DEAD_CODE_DATA_ELIMINATION if !LD_IS_LLD init/Kconfig:config HAVE_LD_DEAD_CODE_DATA_ELIMINATION init/Kconfig: depends on HAVE_LD_DEAD_CODE_DATA_ELIMINATION
Are there some pending patches to enable HAVE_LD_DEAD_CODE_DATA_ELIMINATION for x86?
I doubt it given the target arches above, but curious what's the need for x86 support? Only x86_32? My patches were motivated seeing resolve_btfids and pahole errors for a couple years on MIPS routers. I don't recall seeing the same for x86 builds, so my testing focussed more on preserving x86 builds rather than adding/testing the arch flag for x86.
I could foce CONFIG_HAVE_LD_DEAD_CODE_DATA_ELIMINATION=y with the following hack: diff --git a/init/Kconfig b/init/Kconfig index 72404c1f2157..adf8718e2f5b 100644 --- a/init/Kconfig +++ b/init/Kconfig @@ -1402,7 +1402,7 @@ config CC_OPTIMIZE_FOR_SIZE endchoice config HAVE_LD_DEAD_CODE_DATA_ELIMINATION
bool
def_bool y help This requires that the arch annotates or otherwise protects its external entry points from being discarded. Linker scriptsBut with the above, I cannot boot the kernel.
OK, interesting exercise. Setting HAVE_LD_DEAD_CODE_DATA_ELIMINATION shouldn't change anything itself so I suppose you are also setting LD_DEAD_CODE_DATA_ELIMINATION? From previous testing on kernel-patches/CI, first guess would be vmlinux linker script doing section merges unaware of some x86 quirk. Or x86-specific linker script unhappy with split sections.
I guess x86 needs additional change to make HAVE_LD_DEAD_CODE_DATA_ELIMINATION work. I still curious about why CONFIG_LTO_CLANG is necessary.
In asm-generic/vmlinux.lds.h,
/* * LD_DEAD_CODE_DATA_ELIMINATION option enables -fdata-sections, which * generates .data.identifier sections, which need to be pulled in with * .data. We don't want to pull in .data..other sections, which Linux * has defined. Same for text and bss. * * With LTO_CLANG, the linker also splits sections by default, so we need * these macros to combine the sections during the final link. * * RODATA_MAIN is not used because existing code already defines .rodata.x * sections to be brought in with rodata. */ #if defined(CONFIG_LD_DEAD_CODE_DATA_ELIMINATION) || defined(CONFIG_LTO_CLANG) #define TEXT_MAIN .text .text.[0-9a-zA-Z_]* #define DATA_MAIN .data .data.[0-9a-zA-Z_]* .data..L* .data..compoundliteral* .data.$__unnamed_* .data.$L* #define SDATA_MAIN .sdata .sdata.[0-9a-zA-Z_]* #define RODATA_MAIN .rodata .rodata.[0-9a-zA-Z_]* .rodata..L* #define BSS_MAIN .bss .bss.[0-9a-zA-Z_]* .bss..compoundliteral* #define SBSS_MAIN .sbss .sbss.[0-9a-zA-Z_]* #else #define TEXT_MAIN .text #define DATA_MAIN .data #define SDATA_MAIN .sdata #define RODATA_MAIN .rodata #define BSS_MAIN .bss #define SBSS_MAIN .sbss #endif
If CONFIG_LTO_CLANG is defined and CONFIG_LD_DEAD_CODE_DATA_ELIMINATION is not defined, it is not clear whether __used functions will get eliminated or not. I tried with thinlto with a simple example on x86 with some unused function marked with __used, and that function survived in the final binary.
But your patch won't hurt, so I am okay with it.
Did I miss anything?
Maybe I misunderstand you question re: __used?
Thanks, Tony
+# define __retain __attribute__((__retain__)) +#else +# define __retain +#endif
- /* Compiler specific macros. */ #ifdef __clang__ #include <linux/compiler-clang.h>