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@xxxxxxxxxx>
Reviewed-by: Kees Cook <keescook@xxxxxxxxxxxx>
-Kees
> Reported-by: Nathan Chancellor <natechancellor@xxxxxxxxx>
> Reviewed-by: Stanislav Fomichev <sdf@xxxxxxxxxx>
> Tested-by: Stanislav Fomichev <sdf@xxxxxxxxxx>
> Cc: Alexei Starovoitov <ast@xxxxxxxxxx>
> Cc: Andrii Nakryiko <andrii.nakryiko@xxxxxxxxx>
> Cc: Daniel Borkmann <daniel@xxxxxxxxxxxxx>
> Cc: David S. Miller <davem@xxxxxxxxxxxxx>
> Cc: Kees Cook <keescook@xxxxxxxxxxxx>
> Cc: Michael Ellerman <mpe@xxxxxxxxxxxxxx>
> Cc: Naveen N. Rao <naveen.n.rao@xxxxxxxxxxxxxxxxxx>
> Cc: Nick Desaulniers <ndesaulniers@xxxxxxxxxx>
> Cc: clang-built-linux@xxxxxxxxxxxxxxxx
> ---
> 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
>
--
Kees Cook
