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

[Yonghong Song]

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.

> perf tool uses build ids in .debug cache as file links, and we had
> few isues there
>
> jirka