On Thu, Jan 14, 2021 at 07:47:20PM -0800, Alexei Starovoitov wrote: > On Thu, Jan 14, 2021 at 3:44 PM Yonghong Song <yhs@xxxxxx> 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. build_id_parse is already doing the zero padding, so we are ok I tried several bcc tools over perf bench with md5 buildid and the results looked ok jirka
