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SubjectRe: 'perf probe' and symbols from .text.<something>
On Tue, 23 Feb 2021 10:23:31 +0900
Masami Hiramatsu <mhiramat@kernel.org> wrote:

> On Mon, 22 Feb 2021 11:51:50 -0600
> Josh Poimboeuf <jpoimboe@redhat.com> wrote:
>
> > On Tue, Feb 23, 2021 at 12:05:08AM +0900, Masami Hiramatsu wrote:
> > > > Of course, one could place probes using absolute addresses of the
> > > > functions but that would be less convenient.
> > > >
> > > > This also affects many livepatch modules where the kernel code can be
> > > > compiled with -ffunction-sections and each function may end up in a
> > > > separate section .text.<function_name>. 'perf probe' cannot be used
> > > > there, except with the absolute addresses.
> > > >
> > > > Moreover, if FGKASLR patches are merged
> > > > (https://lwn.net/Articles/832434/) and the kernel is built with FGKASLR
> > > > enabled, -ffunction-sections will be used too. 'perf probe' will be
> > > > unable to see the kernel functions then.
> > >
> > > Hmm, if the FGKASLAR really randomizes the symbol address, perf-probe
> > > should give up "_text-relative" probe for that kernel, and must fallback
> > > to the "symbol-based" probe. (Are there any way to check the FGKASLR is on?)
> > > The problem of "symbol-based" probe is that local (static) symbols
> > > may share a same name sometimes. In that case, it can not find correct
> > > symbol. (Maybe I can find a candidate from its size.)
> > > Anyway, sometimes the security and usability are trade-off.
> >
> > We had a similar issue with FGKASLR and live patching. The proposed
> > solution is a new linker flag which eliminates duplicates: -z
> > unique-symbol.
> >
> > https://sourceware.org/bugzilla/show_bug.cgi?id=26391
>
> Interesting, but it might not be enough for perf-probe.
> Since the perf-probe has to handle both dwarf and elf, both must be
> changed. I think the problem is that the dwarf is generated while
> compiling, but this -z seems converting elf symbols in linkage.
> As far as I can see, this appends ".COUNT" suffix to the non-unique
> symbols in the linkage phase. Is that also applied to dwarf too?

Ah, OK. If there is an offline elf binary with symbol map, I can convert
DWARF symbol -> address -> offline elf symbol (unique name)-> kallsyms.
Currently, it directly converts address by kallsyms, so I will change it
to find elf-symbol and solve address by kallsyms in post processing.

Thank you,

--
Masami Hiramatsu <mhiramat@kernel.org>

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