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

[Yonghong Song <yhs@fb.com>]

On 1/14/21 12:01 PM, Jiri Olsa wrote:
> On Thu, Jan 14, 2021 at 10:56:33AM -0800, Yonghong Song wrote:
>>
>>
>> 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?
> 
> you can have zero bytes in the build id hash, so you need to get the size
> 
> I never saw MD5 being used in practise just SHA1, but we added the
> size to be complete and make sure we'll fit with build id, because
> there's only limited space in mmap2 event

I am asking to check whether we should extend uapi struct
bpf_stack_build_id to include build_id_size as well. I guess
we can delay this until a real use case.


> 
> jirka
> 
>>
>>>
>>> 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@kernel.org>
>>> Cc: Song Liu <songliubraving@fb.com>
>>> Signed-off-by: Jiri Olsa <jolsa@kernel.org>
>>> ---
>>>    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,
>>>    }
>> [...]
>>
>