Re: [RFC/PATCH 3/3] btf_encoder: Func generation fix

[Arnaldo Carvalho de Melo <acme@kernel.org>]

Em Fri, Nov 13, 2020 at 11:59:23AM +0100, Jiri Olsa escreveu:
> On Thu, Nov 12, 2020 at 04:08:02PM -0800, Andrii Nakryiko wrote:
> > On Thu, Nov 12, 2020 at 1:14 PM Jiri Olsa <jolsa@redhat.com> wrote:
> > > On Thu, Nov 12, 2020 at 11:54:41AM -0800, Andrii Nakryiko wrote:
> > > > So I can't unfortunately reproduce that GCC bug with DWARF info. What
> > > > was exactly the symptom? Maybe you can also share readelf -wi dump for
> > > > your problematic vmlinux?

> > > hum, can't see -wi working for readelf, however I placed my vmlinux
> > > in here:
> > >   http://people.redhat.com/~jolsa/vmlinux.gz

> > > the symptom is that resolve_btfids will fail kernel build:

> > >   BTFIDS  vmlinux
> > > FAILED unresolved symbol vfs_getattr

> > > because BTF data contains multiple FUNC records for same function

> > > and the problem is that declaration tag itself is missing:
> > >   https://gcc.gnu.org/bugzilla/show_bug.cgi?id=97060
> > > so pahole won't skip them

> > > the first workaround was to workaround that and go for function
> > > records that have code address attached, but that's buggy as well:
> > >   https://bugzilla.redhat.com/show_bug.cgi?id=1890107

> > > then after some discussions we ended up using ftrace addresses
> > > as filter for what we want to display.. plus we got static functions
> > > as well

> > > however for this way we failed to get proper arguments ;-)

> > Right, I followed along overall, but forgot the details of the initial
> > problem. Thanks for the refresher. See below for my current thoughts
> > on dealing with all this.

I'll add this to the set of regression tests I use with pahole:

[acme@five vfs_gettattr.multiple.btf.entries.jolsa]$ bpftool btf dump file vmlinux | grep -w FUNC | cut -d\' -f2 | sort | uniq -c | sort -nr | head
      3 __x64_sys_userfaultfd
      3 __x64_sys_timerfd_settime
      3 __x64_sys_timerfd_gettime
      3 __x64_sys_timerfd_create
      3 __x64_sys_syslog
      3 __x64_sys_sysfs
      3 __x64_sys_swapon
      3 __x64_sys_swapoff
      3 __x64_sys_socketpair
      3 __x64_sys_socket
[acme@five vfs_gettattr.multiple.btf.entries.jolsa]$ pfunct -F btf vmlinux | sort | uniq -c | sort -nr | head
      3 __x64_sys_userfaultfd
      3 __x64_sys_timerfd_settime
      3 __x64_sys_timerfd_gettime
      3 __x64_sys_timerfd_create
      3 __x64_sys_syslog
      3 __x64_sys_sysfs
      3 __x64_sys_swapon
      3 __x64_sys_swapoff
      3 __x64_sys_socketpair
      3 __x64_sys_socket
[acme@five vfs_gettattr.multiple.btf.entries.jolsa]$

I.e. the output of those tools need to match and all functions need to
appear only once.

I'll also use pfunct with -F dwarf to get the same results, probably
will add these to the 'btfdiff' tool that is in the pahole git repo.

- Arnaldo