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

Alexei Starovoitov
2021-01-15T03:48:16Z

On Thu, Jan 14, 2021 at 3:44 PM Yonghong Song <yhs@fb.com> wrote:
>
>
>
> On 1/14/21 2:02 PM, Jiri Olsa wrote:
> > On Thu, Jan 14, 2021 at 01:05:33PM -0800, Yonghong Song wrote:
> >>
> >>
> >> 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.
> >
> > right, we can try make some MD5 build id binaries and check if it
> > explodes with some bcc tools, but I don't expect that.. I'll try
> > to find some time for that
>
> Thanks. We may have issues on bcc side. For build_id collected in
> kernel, bcc always generates a length-20 string. But for user
> binaries, the build_id string length is equal to actual size of
> the build_id. They may not match (MD5 length is 16).
> The fix is probably to append '0's (up to length 20) for user
> binary build_id's.
>
> I guess MD5 is very seldom used. I will wait if you can reproduce
> the issue and then we might fix it.

Indeed.
Jiri, please check whether md5 is really an issue.
Sounds like we have to do something on the kernel side.
Hopefully zero padding will be enough.
I would prefer to avoid extending uapi struct to cover rare case.

I've applied the series, since this issue sounds orthogonal.