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From: Namhyung Kim <namhyung@kernel.org>
Date: Wed, 7 Feb 2024 11:08:15 -0800
Message-ID: <CAM9d7cgV-c2bLE9KGSA2qTHNbzGriQo9NqYD5k177+46e51pJA@mail.gmail.com>
Subject: Re: [PATCH 14/14] perf annotate-data: Add stack canary type
To: Ian Rogers <irogers@google.com>
Cc: Arnaldo Carvalho de Melo <acme@kernel.org>, Jiri Olsa <jolsa@kernel.org>, 
	Adrian Hunter <adrian.hunter@intel.com>, Peter Zijlstra <peterz@infradead.org>, 
	Ingo Molnar <mingo@kernel.org>, LKML <linux-kernel@vger.kernel.org>, 
	linux-perf-users@vger.kernel.org, 
	Linus Torvalds <torvalds@linux-foundation.org>, Stephane Eranian <eranian@google.com>, 
	Masami Hiramatsu <mhiramat@kernel.org>, linux-toolchains@vger.kernel.org, 
	linux-trace-devel@vger.kernel.org
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On Tue, Feb 6, 2024 at 3:40=E2=80=AFPM Ian Rogers <irogers@google.com> wrot=
e:
>
> On Tue, Feb 6, 2024 at 3:19=E2=80=AFPM Namhyung Kim <namhyung@kernel.org>=
 wrote:
> >
> > On Fri, Feb 2, 2024 at 7:21=E2=80=AFPM Ian Rogers <irogers@google.com> =
wrote:
> > >
> > > On Fri, Feb 2, 2024 at 2:05=E2=80=AFPM Namhyung Kim <namhyung@kernel.=
org> wrote:
> > > >
> > > > When the stack protector is enabled, compiler would generate code t=
o
> > > > check stack overflow with a special value called 'stack carary' at
> > > > runtime.  On x86_64, GCC hard-codes the stack canary as %gs:40.
> > > >
> > > > While there's a definition of fixed_percpu_data in asm/processor.h,
> > > > it seems that the header is not included everywhere and many places
> > > > it cannot find the type info.  As it's in the well-known location (=
at
> > > > %gs:40), let's add a pseudo stack canary type to handle it speciall=
y.
> > >
> > > I wonder if cases like this can be handled by debug info rather than
> > > special cases in the tool. Special cases are fine too, but are
> > > potentially less portable.
> >
> > Agreed, but I couldn't find anything special in DWARF.
>
> The fs and gs selectors are commonly used for thread local storage, so
> could something like DW_OP_form_tls_address be used?
> https://dwarfstd.org/issues/110803.1.html

I'm not sure if it's the same thing.  Maybe it's for variables with
__thread annotation.  I don't know if compilers generate a
(pseudo) variable for stack canary.

Thanks,
Namhyung