Re: [PATCH] bpf: Support llvm-objcopy for vmlinux BTF

On Mon, Jun 08, 2020 at 01:39:59PM +0000, Android Kernel Team wrote:

From: Fangrui Song maskray@google.com

Simplify gen_btf logic to make it work with llvm-objcopy. The existing 'file format' and 'architecture' parsing logic is brittle and does not work with llvm-objcopy/llvm-objdump.

'file format' output of llvm-objdump>=11 will match GNU objdump, but 'architecture' (bfdarch) may not.

.BTF in .tmp_vmlinux.btf is non-SHF_ALLOC. Add the SHF_ALLOC flag because it is part of vmlinux image used for introspection. C code can reference the section via linker script defined __start_BTF and __stop_BTF. This fixes a small problem that previous .BTF had the SHF_WRITE flag (objcopy -I binary -O elf* synthesized .data).

Additionally, `objcopy -I binary` synthesized symbols _binary__btf_vmlinux_bin_start and _binary__btf_vmlinux_bin_stop (not used elsewhere) are replaced with more commonplace __start_BTF and __stop_BTF.

Add 2>/dev/null because GNU objcopy (but not llvm-objcopy) warns "empty loadable segment detected at vaddr=0xffffffff81000000, is this intentional?"

We use a dd command to change the e_type field in the ELF header from ET_EXEC to ET_REL so that lld will accept .btf.vmlinux.bin.o. Accepting ET_EXEC as an input file is an extremely rare GNU ld feature that lld does not intend to support, because this is error-prone.

The output section description .BTF in include/asm-generic/vmlinux.lds.h avoids potential subtle orphan section placement issues and suppresses --orphan-handling=warn warnings.

Fixes: df786c9b9476 ("bpf: Force .BTF section start to zero when dumping from vmlinux") Fixes: cb0cc635c7a9 ("powerpc: Include .BTF section") Reported-by: Nathan Chancellor natechancellor@gmail.com Signed-off-by: Fangrui Song maskray@google.com Signed-off-by: Daniel Borkmann daniel@iogearbox.net Tested-by: Stanislav Fomichev sdf@google.com Tested-by: Andrii Nakryiko andriin@fb.com Reviewed-by: Stanislav Fomichev sdf@google.com Reviewed-by: Kees Cook keescook@chromium.org Acked-by: Andrii Nakryiko andriin@fb.com Acked-by: Michael Ellerman mpe@ellerman.id.au (powerpc) Link: https://github.com/ClangBuiltLinux/linux/issues/871 Link: https://lore.kernel.org/bpf/20200318222746.173648-1-maskray@google.com (cherry picked from commit 90ceddcb495008ac8ba7a3dce297841efcd7d584) Cc: stable@vger.kernel.org # 5.4.x Signed-off-by: Maria Teguiani teguiani@google.com

Tested-by: Matthias Maennich maennich@google.com

Cheers, Matthias

---

arch/powerpc/kernel/vmlinux.lds.S | 6 ------ include/asm-generic/vmlinux.lds.h | 22 +++++++++++++++++++--- kernel/bpf/sysfs_btf.c | 11 +++++------ scripts/link-vmlinux.sh | 24 ++++++++++-------------- 4 files changed, 34 insertions(+), 29 deletions(-)

diff --git a/arch/powerpc/kernel/vmlinux.lds.S b/arch/powerpc/kernel/vmlinux.lds.S index 4638d2863388..060a1acd7c6d 100644 --- a/arch/powerpc/kernel/vmlinux.lds.S +++ b/arch/powerpc/kernel/vmlinux.lds.S @@ -326,12 +326,6 @@ SECTIONS *(.branch_lt) }

-#ifdef CONFIG_DEBUG_INFO_BTF

• .BTF : AT(ADDR(.BTF) - LOAD_OFFSET) {

• *(.BTF)
  

• }

-#endif

• .opd : AT(ADDR(.opd) - LOAD_OFFSET) { __start_opd = .; KEEP(*(.opd))

diff --git a/include/asm-generic/vmlinux.lds.h b/include/asm-generic/vmlinux.lds.h index dae64600ccbf..b6d7347ccda7 100644 --- a/include/asm-generic/vmlinux.lds.h +++ b/include/asm-generic/vmlinux.lds.h @@ -496,10 +496,12 @@ __start___modver = .; \ KEEP(*(__modver)) \ __stop___modver = .; \

• . = ALIGN((align));					\
  

• __end_rodata = .;					\
  

  } \
• . = ALIGN((align));

• 							\
  

• BTF \

• 							\
  

• . = ALIGN((align)); \
• __end_rodata = .;

/* RODATA & RO_DATA provided for backward compatibility.

• All archs are supposed to use RO_DATA() */

@@ -588,6 +590,20 @@ __stop___ex_table = .; \ }

+/*

• • .BTF
• */

+#ifdef CONFIG_DEBUG_INFO_BTF +#define BTF \

• .BTF : AT(ADDR(.BTF) - LOAD_OFFSET) { \

• __start_BTF = .;					\
  

• *(.BTF)							\
  

• __stop_BTF = .;						\
  

• }

+#else +#define BTF +#endif

/*

• Init task

*/ diff --git a/kernel/bpf/sysfs_btf.c b/kernel/bpf/sysfs_btf.c index 7ae5dddd1fe6..3b495773de5a 100644 --- a/kernel/bpf/sysfs_btf.c +++ b/kernel/bpf/sysfs_btf.c @@ -9,15 +9,15 @@ #include <linux/sysfs.h>

/* See scripts/link-vmlinux.sh, gen_btf() func for details */ -extern char __weak _binary__btf_vmlinux_bin_start[]; -extern char __weak _binary__btf_vmlinux_bin_end[]; +extern char __weak __start_BTF[]; +extern char __weak __stop_BTF[];

static ssize_t btf_vmlinux_read(struct file *file, struct kobject *kobj, struct bin_attribute *bin_attr, char *buf, loff_t off, size_t len) {

• memcpy(buf, _binary__btf_vmlinux_bin_start + off, len);

• memcpy(buf, __start_BTF + off, len); return len;

}

@@ -30,15 +30,14 @@ static struct kobject *btf_kobj;

static int __init btf_vmlinux_init(void) {

• if (!_binary__btf_vmlinux_bin_start)

• if (!__start_BTF) return 0;

  btf_kobj = kobject_create_and_add("btf", kernel_kobj); if (!btf_kobj) return -ENOMEM;

• bin_attr_btf_vmlinux.size = _binary__btf_vmlinux_bin_end -

• 		    _binary__btf_vmlinux_bin_start;
  

• bin_attr_btf_vmlinux.size = __stop_BTF - __start_BTF;

  return sysfs_create_bin_file(btf_kobj, &bin_attr_btf_vmlinux);

} diff --git a/scripts/link-vmlinux.sh b/scripts/link-vmlinux.sh index aa1386079f0c..8b6325c2dfc5 100755 --- a/scripts/link-vmlinux.sh +++ b/scripts/link-vmlinux.sh @@ -113,9 +113,6 @@ vmlinux_link() gen_btf() { local pahole_ver

• local bin_arch

• local bin_format

• local bin_file

  if ! [ -x "$(command -v ${PAHOLE})" ]; then echo >&2 "BTF: ${1}: pahole (${PAHOLE}) is not available"

@@ -133,17 +130,16 @@ gen_btf() info "BTF" ${2} LLVM_OBJCOPY=${OBJCOPY} ${PAHOLE} -J ${1}

• # dump .BTF section into raw binary file to link with final vmlinux
• bin_arch=$(LANG=C ${OBJDUMP} -f ${1} | grep architecture | \

• cut -d, -f1 | cut -d' ' -f2)
  

• bin_format=$(LANG=C ${OBJDUMP} -f ${1} | grep 'file format' | \

• awk '{print $4}')
  

• bin_file=.btf.vmlinux.bin
• ${OBJCOPY} --change-section-address .BTF=0 \

• --set-section-flags .BTF=alloc -O binary \
  

• --only-section=.BTF ${1} $bin_file
  

• ${OBJCOPY} -I binary -O ${bin_format} -B ${bin_arch} \

• --rename-section .data=.BTF $bin_file ${2}
  

• # Create ${2} which contains just .BTF section but no symbols. Add
• # SHF_ALLOC because .BTF will be part of the vmlinux image. --strip-all
• # deletes all symbols including __start_BTF and __stop_BTF, which will
• # be redefined in the linker script. Add 2>/dev/null to suppress GNU
• # objcopy warnings: "empty loadable segment detected at ..."
• ${OBJCOPY} --only-section=.BTF --set-section-flags .BTF=alloc,readonly \

• --strip-all ${1} ${2} 2>/dev/null
  

• # Change e_type to ET_REL so that it can be used to link final vmlinux.
• # Unlike GNU ld, lld does not allow an ET_EXEC input.
• printf '\1' | dd of=${2} conv=notrunc bs=1 seek=16 status=none

}

# Create ${2} .o file with all symbols from the ${1} object file

2.27.0.278.ge193c7cf3a9-goog

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