linux-kernel - Re: [PATCH 1/3] perf symbol: Pass is_kallsyms to symbols__fixup

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Date:   Mon, 25 Apr 2022 12:14:38 -0700
From:   Namhyung Kim <namhyung@...nel.org>
To:     Ian Rogers <irogers@...gle.com>
Cc:     Arnaldo Carvalho de Melo <acme@...nel.org>,
        Jiri Olsa <jolsa@...hat.com>,
        Michael Petlan <mpetlan@...hat.com>,
        Peter Zijlstra <peterz@...radead.org>,
        Ingo Molnar <mingo@...nel.org>,
        Song Liu <songliubraving@...com>,
        Will Deacon <will@...nel.org>,
        Mark Rutland <mark.rutland@....com>,
        John Garry <john.garry@...wei.com>,
        Mathieu Poirier <mathieu.poirier@...aro.org>,
        Leo Yan <leo.yan@...aro.org>,
        Michael Ellerman <mpe@...erman.id.au>,
        Heiko Carstens <hca@...ux.ibm.com>,
        LKML <linux-kernel@...r.kernel.org>,
        linux-perf-users <linux-perf-users@...r.kernel.org>,
        linuxppc-dev@...ts.ozlabs.org, linux-s390@...r.kernel.org
Subject: Re: [PATCH 1/3] perf symbol: Pass is_kallsyms to symbols__fixup_end()

Hi Ian,

On Sat, Apr 16, 2022 at 7:59 AM Ian Rogers <irogers@...gle.com> wrote:
>
> On Fri, Apr 15, 2022 at 8:40 PM Namhyung Kim <namhyung@...nel.org> wrote:
> >
> > The symbol fixup is necessary for symbols in kallsyms since they don't
> > have size info.  So we use the next symbol's address to calculate the
> > size.  Now it's also used for user binaries because sometimes they
> > miss size for hand-written asm functions.
> >
> > There's a arch-specific function to handle kallsyms differently but
> > currently it cannot distinguish kallsyms from others.  Pass this
> > information explicitly to handle it properly.  Note that those arch
> > functions will be moved to the generic function so I didn't added it
> > to the arch-functions.
>
> Thanks Namhyung, in:
> https://lore.kernel.org/linux-perf-users/20220412154817.2728324-3-irogers@google.com/
> I used "dso->kernel != DSO_SPACE__USER" in symbol-elf to make this
> more than just kallsyms as presumably kernel code is the issue. Do we
> know elf kernel code has correctly sized symbols?

Yeah, IIUC the whole point of the symbol end fixup is because the
kallsyms doesn't have the symbol size info.  Every ELF binaries
should have the size except for some hand-written asm functions
which missed adding it manually.  I guess that's the reason it was
added to other DSO loading paths.

Also considering "[" (and "]") in the symbol name is specific to
kallsyms which has both kernel and module symbols together.

Thanks,
Namhyung