Messages in this thread | | | Subject | Re: [PATCH bpf-next 2/3] bpf: Add size arg to build_id_parse function | From | Yonghong Song <> | Date | Thu, 14 Jan 2021 15:43:56 -0800 |
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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 >
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