| Message ID | 20200318222746.173648-1-maskray@google.com |
|---|---|
| State | Accepted |
| Delegated to: | BPF Maintainers |
| Headers | show |
| Series | [bpf-next,v6] bpf: Support llvm-objcopy for vmlinux BTF | expand |
On Wed, Mar 18, 2020 at 3:27 PM Fangrui Song <maskray@google.com> wrote: > > 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. > > v6: > - drop llvm-objdump from the title. We don't run objdump now > - delete unused local variables: bin_arch, bin_format and bin_file > - mention in the comment that lld does not allow an ET_EXEC input > - rename BTF back to .BTF . The section name is assumed by bpftool > - add output section description to include/asm-generic/vmlinux.lds.h > - mention cb0cc635c7a9 ("powerpc: Include .BTF section") > > v5: > - rebase on top of bpf-next/master > - rename .BTF to BTF > > Fixes: df786c9b9476 ("bpf: Force .BTF section start to zero when dumping from vmlinux") > Fixes: cb0cc635c7a9 ("powerpc: Include .BTF section") > Link: https://github.com/ClangBuiltLinux/linux/issues/871 > Signed-off-by: Fangrui Song <maskray@google.com> > Reported-by: Nathan Chancellor <natechancellor@gmail.com> > Reviewed-by: Stanislav Fomichev <sdf@google.com> > Tested-by: Stanislav Fomichev <sdf@google.com> > Cc: Alexei Starovoitov <ast@kernel.org> > Cc: Andrii Nakryiko <andrii.nakryiko@gmail.com> > Cc: Daniel Borkmann <daniel@iogearbox.net> > Cc: David S. Miller <davem@davemloft.net> > Cc: Kees Cook <keescook@chromium.org> > Cc: Michael Ellerman <mpe@ellerman.id.au> > Cc: Naveen N. Rao <naveen.n.rao@linux.vnet.ibm.com> > Cc: Nick Desaulniers <ndesaulniers@google.com> > Cc: clang-built-linux@googlegroups.com > --- Thanks for detailed commit message and comments in the script, that's very helpful. Looks good to me, I've tested with my local setup and everything works across bpftool, selftests and my private BTF tool, which doesn't use libbpf. Acked-by: Andrii Nakryiko <andriin@fb.com> Tested-by: Andrii Nakryiko <andriin@fb.com> > arch/powerpc/kernel/vmlinux.lds.S | 6 ------ > include/asm-generic/vmlinux.lds.h | 15 +++++++++++++++ > kernel/bpf/btf.c | 9 ++++----- > kernel/bpf/sysfs_btf.c | 11 +++++------ > scripts/link-vmlinux.sh | 24 ++++++++++-------------- > 5 files changed, 34 insertions(+), 31 deletions(-) [...]
On Wed, Mar 18, 2020 at 03:27:46PM -0700, Fangrui Song wrote: > 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. > > v6: > - drop llvm-objdump from the title. We don't run objdump now > - delete unused local variables: bin_arch, bin_format and bin_file > - mention in the comment that lld does not allow an ET_EXEC input > - rename BTF back to .BTF . The section name is assumed by bpftool > - add output section description to include/asm-generic/vmlinux.lds.h > - mention cb0cc635c7a9 ("powerpc: Include .BTF section") > > v5: > - rebase on top of bpf-next/master > - rename .BTF to BTF > > Fixes: df786c9b9476 ("bpf: Force .BTF section start to zero when dumping from vmlinux") > Fixes: cb0cc635c7a9 ("powerpc: Include .BTF section") > Link: https://github.com/ClangBuiltLinux/linux/issues/871 > Signed-off-by: Fangrui Song <maskray@google.com> Reviewed-by: Kees Cook <keescook@chromium.org> -Kees > Reported-by: Nathan Chancellor <natechancellor@gmail.com> > Reviewed-by: Stanislav Fomichev <sdf@google.com> > Tested-by: Stanislav Fomichev <sdf@google.com> > Cc: Alexei Starovoitov <ast@kernel.org> > Cc: Andrii Nakryiko <andrii.nakryiko@gmail.com> > Cc: Daniel Borkmann <daniel@iogearbox.net> > Cc: David S. Miller <davem@davemloft.net> > Cc: Kees Cook <keescook@chromium.org> > Cc: Michael Ellerman <mpe@ellerman.id.au> > Cc: Naveen N. Rao <naveen.n.rao@linux.vnet.ibm.com> > Cc: Nick Desaulniers <ndesaulniers@google.com> > Cc: clang-built-linux@googlegroups.com > --- > arch/powerpc/kernel/vmlinux.lds.S | 6 ------ > include/asm-generic/vmlinux.lds.h | 15 +++++++++++++++ > kernel/bpf/btf.c | 9 ++++----- > kernel/bpf/sysfs_btf.c | 11 +++++------ > scripts/link-vmlinux.sh | 24 ++++++++++-------------- > 5 files changed, 34 insertions(+), 31 deletions(-) > > diff --git a/arch/powerpc/kernel/vmlinux.lds.S b/arch/powerpc/kernel/vmlinux.lds.S > index a32d478a7f41..b4c89a1acebb 100644 > --- a/arch/powerpc/kernel/vmlinux.lds.S > +++ b/arch/powerpc/kernel/vmlinux.lds.S > @@ -303,12 +303,6 @@ 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 e00f41aa8ec4..39da8d8b561d 100644 > --- a/include/asm-generic/vmlinux.lds.h > +++ b/include/asm-generic/vmlinux.lds.h > @@ -535,6 +535,7 @@ > \ > RO_EXCEPTION_TABLE \ > NOTES \ > + BTF \ > \ > . = ALIGN((align)); \ > __end_rodata = .; > @@ -621,6 +622,20 @@ > __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/btf.c b/kernel/bpf/btf.c > index 50080add2ab9..6f397c4da05e 100644 > --- a/kernel/bpf/btf.c > +++ b/kernel/bpf/btf.c > @@ -3477,8 +3477,8 @@ static struct btf *btf_parse(void __user *btf_data, u32 btf_data_size, > return ERR_PTR(err); > } > > -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[]; > extern struct btf *btf_vmlinux; > > #define BPF_MAP_TYPE(_id, _ops) > @@ -3605,9 +3605,8 @@ struct btf *btf_parse_vmlinux(void) > } > env->btf = btf; > > - btf->data = _binary__btf_vmlinux_bin_start; > - btf->data_size = _binary__btf_vmlinux_bin_end - > - _binary__btf_vmlinux_bin_start; > + btf->data = __start_BTF; > + btf->data_size = __stop_BTF - __start_BTF; > > err = btf_parse_hdr(env); > if (err) > 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 ac569e197bfa..d09ab4afbda4 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 > -- > 2.25.1.481.gfbce0eb801-goog >
Fangrui Song <maskray@google.com> writes: > 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. > > v6: > - drop llvm-objdump from the title. We don't run objdump now > - delete unused local variables: bin_arch, bin_format and bin_file > - mention in the comment that lld does not allow an ET_EXEC input > - rename BTF back to .BTF . The section name is assumed by bpftool > - add output section description to include/asm-generic/vmlinux.lds.h > - mention cb0cc635c7a9 ("powerpc: Include .BTF section") > > v5: > - rebase on top of bpf-next/master > - rename .BTF to BTF > > Fixes: df786c9b9476 ("bpf: Force .BTF section start to zero when dumping from vmlinux") > Fixes: cb0cc635c7a9 ("powerpc: Include .BTF section") > Link: https://github.com/ClangBuiltLinux/linux/issues/871 > Signed-off-by: Fangrui Song <maskray@google.com> > Reported-by: Nathan Chancellor <natechancellor@gmail.com> > Reviewed-by: Stanislav Fomichev <sdf@google.com> > Tested-by: Stanislav Fomichev <sdf@google.com> > Cc: Alexei Starovoitov <ast@kernel.org> > Cc: Andrii Nakryiko <andrii.nakryiko@gmail.com> > Cc: Daniel Borkmann <daniel@iogearbox.net> > Cc: David S. Miller <davem@davemloft.net> > Cc: Kees Cook <keescook@chromium.org> > Cc: Michael Ellerman <mpe@ellerman.id.au> > Cc: Naveen N. Rao <naveen.n.rao@linux.vnet.ibm.com> > Cc: Nick Desaulniers <ndesaulniers@google.com> > Cc: clang-built-linux@googlegroups.com > --- > arch/powerpc/kernel/vmlinux.lds.S | 6 ------ > include/asm-generic/vmlinux.lds.h | 15 +++++++++++++++ > kernel/bpf/btf.c | 9 ++++----- > kernel/bpf/sysfs_btf.c | 11 +++++------ > scripts/link-vmlinux.sh | 24 ++++++++++-------------- > 5 files changed, 34 insertions(+), 31 deletions(-) > > diff --git a/arch/powerpc/kernel/vmlinux.lds.S b/arch/powerpc/kernel/vmlinux.lds.S > index a32d478a7f41..b4c89a1acebb 100644 > --- a/arch/powerpc/kernel/vmlinux.lds.S > +++ b/arch/powerpc/kernel/vmlinux.lds.S > @@ -303,12 +303,6 @@ SECTIONS > *(.branch_lt) > } > > -#ifdef CONFIG_DEBUG_INFO_BTF > - .BTF : AT(ADDR(.BTF) - LOAD_OFFSET) { > - *(.BTF) > - } > -#endif > - I wasn't able to easily test as I don't have bpf-next/master handy. But this part of the change looks fine to me. Acked-by: Michael Ellerman <mpe@ellerman.id.au> (powerpc) cheers > diff --git a/include/asm-generic/vmlinux.lds.h b/include/asm-generic/vmlinux.lds.h > index e00f41aa8ec4..39da8d8b561d 100644 > --- a/include/asm-generic/vmlinux.lds.h > +++ b/include/asm-generic/vmlinux.lds.h > @@ -535,6 +535,7 @@ > \ > RO_EXCEPTION_TABLE \ > NOTES \ > + BTF \ > \ > . = ALIGN((align)); \ > __end_rodata = .; > @@ -621,6 +622,20 @@ > __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/btf.c b/kernel/bpf/btf.c > index 50080add2ab9..6f397c4da05e 100644 > --- a/kernel/bpf/btf.c > +++ b/kernel/bpf/btf.c > @@ -3477,8 +3477,8 @@ static struct btf *btf_parse(void __user *btf_data, u32 btf_data_size, > return ERR_PTR(err); > } > > -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[]; > extern struct btf *btf_vmlinux; > > #define BPF_MAP_TYPE(_id, _ops) > @@ -3605,9 +3605,8 @@ struct btf *btf_parse_vmlinux(void) > } > env->btf = btf; > > - btf->data = _binary__btf_vmlinux_bin_start; > - btf->data_size = _binary__btf_vmlinux_bin_end - > - _binary__btf_vmlinux_bin_start; > + btf->data = __start_BTF; > + btf->data_size = __stop_BTF - __start_BTF; > > err = btf_parse_hdr(env); > if (err) > 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 ac569e197bfa..d09ab4afbda4 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 > -- > 2.25.1.481.gfbce0eb801-goog
On 3/19/20 11:27 AM, Michael Ellerman wrote: > Fangrui Song <maskray@google.com> writes: >> 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. >> >> v6: >> - drop llvm-objdump from the title. We don't run objdump now >> - delete unused local variables: bin_arch, bin_format and bin_file >> - mention in the comment that lld does not allow an ET_EXEC input >> - rename BTF back to .BTF . The section name is assumed by bpftool >> - add output section description to include/asm-generic/vmlinux.lds.h >> - mention cb0cc635c7a9 ("powerpc: Include .BTF section") >> >> v5: >> - rebase on top of bpf-next/master >> - rename .BTF to BTF >> >> Fixes: df786c9b9476 ("bpf: Force .BTF section start to zero when dumping from vmlinux") >> Fixes: cb0cc635c7a9 ("powerpc: Include .BTF section") >> Link: https://github.com/ClangBuiltLinux/linux/issues/871 >> Signed-off-by: Fangrui Song <maskray@google.com> >> Reported-by: Nathan Chancellor <natechancellor@gmail.com> >> Reviewed-by: Stanislav Fomichev <sdf@google.com> >> Tested-by: Stanislav Fomichev <sdf@google.com> >> Cc: Alexei Starovoitov <ast@kernel.org> >> Cc: Andrii Nakryiko <andrii.nakryiko@gmail.com> >> Cc: Daniel Borkmann <daniel@iogearbox.net> >> Cc: David S. Miller <davem@davemloft.net> >> Cc: Kees Cook <keescook@chromium.org> >> Cc: Michael Ellerman <mpe@ellerman.id.au> >> Cc: Naveen N. Rao <naveen.n.rao@linux.vnet.ibm.com> >> Cc: Nick Desaulniers <ndesaulniers@google.com> >> Cc: clang-built-linux@googlegroups.com Applied, thanks everyone!
diff --git a/arch/powerpc/kernel/vmlinux.lds.S b/arch/powerpc/kernel/vmlinux.lds.S index a32d478a7f41..b4c89a1acebb 100644 --- a/arch/powerpc/kernel/vmlinux.lds.S +++ b/arch/powerpc/kernel/vmlinux.lds.S @@ -303,12 +303,6 @@ 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 e00f41aa8ec4..39da8d8b561d 100644 --- a/include/asm-generic/vmlinux.lds.h +++ b/include/asm-generic/vmlinux.lds.h @@ -535,6 +535,7 @@ \ RO_EXCEPTION_TABLE \ NOTES \ + BTF \ \ . = ALIGN((align)); \ __end_rodata = .; @@ -621,6 +622,20 @@ __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/btf.c b/kernel/bpf/btf.c index 50080add2ab9..6f397c4da05e 100644 --- a/kernel/bpf/btf.c +++ b/kernel/bpf/btf.c @@ -3477,8 +3477,8 @@ static struct btf *btf_parse(void __user *btf_data, u32 btf_data_size, return ERR_PTR(err); } -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[]; extern struct btf *btf_vmlinux; #define BPF_MAP_TYPE(_id, _ops) @@ -3605,9 +3605,8 @@ struct btf *btf_parse_vmlinux(void) } env->btf = btf; - btf->data = _binary__btf_vmlinux_bin_start; - btf->data_size = _binary__btf_vmlinux_bin_end - - _binary__btf_vmlinux_bin_start; + btf->data = __start_BTF; + btf->data_size = __stop_BTF - __start_BTF; err = btf_parse_hdr(env); if (err) 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 ac569e197bfa..d09ab4afbda4 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
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. v6: - drop llvm-objdump from the title. We don't run objdump now - delete unused local variables: bin_arch, bin_format and bin_file - mention in the comment that lld does not allow an ET_EXEC input - rename BTF back to .BTF . The section name is assumed by bpftool - add output section description to include/asm-generic/vmlinux.lds.h - mention cb0cc635c7a9 ("powerpc: Include .BTF section") v5: - rebase on top of bpf-next/master - rename .BTF to BTF Fixes: df786c9b9476 ("bpf: Force .BTF section start to zero when dumping from vmlinux") Fixes: cb0cc635c7a9 ("powerpc: Include .BTF section") Link: https://github.com/ClangBuiltLinux/linux/issues/871 Signed-off-by: Fangrui Song <maskray@google.com> Reported-by: Nathan Chancellor <natechancellor@gmail.com> Reviewed-by: Stanislav Fomichev <sdf@google.com> Tested-by: Stanislav Fomichev <sdf@google.com> Cc: Alexei Starovoitov <ast@kernel.org> Cc: Andrii Nakryiko <andrii.nakryiko@gmail.com> Cc: Daniel Borkmann <daniel@iogearbox.net> Cc: David S. Miller <davem@davemloft.net> Cc: Kees Cook <keescook@chromium.org> Cc: Michael Ellerman <mpe@ellerman.id.au> Cc: Naveen N. Rao <naveen.n.rao@linux.vnet.ibm.com> Cc: Nick Desaulniers <ndesaulniers@google.com> Cc: clang-built-linux@googlegroups.com --- arch/powerpc/kernel/vmlinux.lds.S | 6 ------ include/asm-generic/vmlinux.lds.h | 15 +++++++++++++++ kernel/bpf/btf.c | 9 ++++----- kernel/bpf/sysfs_btf.c | 11 +++++------ scripts/link-vmlinux.sh | 24 ++++++++++-------------- 5 files changed, 34 insertions(+), 31 deletions(-)