Re: [PATCH bpf-next 2/3] bpf: Add size arg to build_id_parse function

[Yonghong Song]

On 1/14/21 5:40 AM, Jiri Olsa wrote:
> It's possible to have other build id types (other than default SHA1).
> Currently there's also ld support for MD5 build id.

Currently, bpf build_id based stackmap does not returns the size of
the build_id. Did you see an issue here? I guess user space can check
the length of non-zero bits of the build id to decide what kind of
type it is, right?

> Adding size argument to build_id_parse function, that returns (if defined)
> size of the parsed build id, so we can recognize the build id type.
>
> Cc: Alexei Starovoitov <ast@xxxxxxxxxx>
> Cc: Song Liu <songliubraving@xxxxxx>
> Signed-off-by: Jiri Olsa <jolsa@xxxxxxxxxx>
> ---
>   include/linux/buildid.h |  3 ++-
>   kernel/bpf/stackmap.c   |  2 +-
>   lib/buildid.c           | 29 +++++++++++++++++++++--------
>   3 files changed, 24 insertions(+), 10 deletions(-)
>
> diff --git a/include/linux/buildid.h b/include/linux/buildid.h
> index 08028a212589..40232f90db6e 100644
> --- a/include/linux/buildid.h
> +++ b/include/linux/buildid.h
> @@ -6,6 +6,7 @@
>   
>   #define BUILD_ID_SIZE_MAX 20
>   
> -int build_id_parse(struct vm_area_struct *vma, unsigned char *build_id);
> +int build_id_parse(struct vm_area_struct *vma, unsigned char *build_id,
> +		   __u32 *size);
>   
>   #endif
> diff --git a/kernel/bpf/stackmap.c b/kernel/bpf/stackmap.c
> index 55d254a59f07..cabaf7db8efc 100644
> --- a/kernel/bpf/stackmap.c
> +++ b/kernel/bpf/stackmap.c
> @@ -189,7 +189,7 @@ static void stack_map_get_build_id_offset(struct bpf_stack_build_id *id_offs,
>   
>   	for (i = 0; i < trace_nr; i++) {
>   		vma = find_vma(current->mm, ips[i]);
> -		if (!vma || build_id_parse(vma, id_offs[i].build_id)) {
> +		if (!vma || build_id_parse(vma, id_offs[i].build_id, NULL)) {
>   			/* per entry fall back to ips */
>   			id_offs[i].status = BPF_STACK_BUILD_ID_IP;
>   			id_offs[i].ip = ips[i];
> diff --git a/lib/buildid.c b/lib/buildid.c
> index 4a4f520c0e29..6156997c3895 100644
> --- a/lib/buildid.c
> +++ b/lib/buildid.c
> @@ -12,6 +12,7 @@
>    */
>   static inline int parse_build_id(void *page_addr,
>   				 unsigned char *build_id,
> +				 __u32 *size,
>   				 void *note_start,
>   				 Elf32_Word note_size)
>   {
> @@ -38,6 +39,8 @@ static inline int parse_build_id(void *page_addr,
>   			       nhdr->n_descsz);
>   			memset(build_id + nhdr->n_descsz, 0,
>   			       BUILD_ID_SIZE_MAX - nhdr->n_descsz);
> +			if (size)
> +				*size = nhdr->n_descsz;
>   			return 0;
>   		}
>   		new_offs = note_offs + sizeof(Elf32_Nhdr) +
> @@ -50,7 +53,8 @@ static inline int parse_build_id(void *page_addr,
>   }
>   
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