linux-kernel - Re: eh_frame confusion

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Message-Id: <1583169014.zvau4om8mi.naveen@linux.ibm.com>
Date:   Mon, 02 Mar 2020 22:47:18 +0530
From:   "Naveen N. Rao" <naveen.n.rao@...ux.ibm.com>
To:     Rasmus Villemoes <linux@...musvillemoes.dk>,
        Segher Boessenkool <segher@...nel.crashing.org>,
        Michael Ellerman <mpe@...erman.id.au>
Cc:     Linux Kbuild mailing list <linux-kbuild@...r.kernel.org>,
        LKML <linux-kernel@...r.kernel.org>,
        "linuxppc-dev@...ts.ozlabs.org" <linuxppc-dev@...ts.ozlabs.org>
Subject: Re: eh_frame confusion

Segher Boessenkool wrote:
> On Mon, Mar 02, 2020 at 11:56:05AM +0100, Rasmus Villemoes wrote:
>> I'm building a ppc32 kernel, and noticed that after upgrading from gcc-7
>> to gcc-8 all object files now end up having .eh_frame section.
> 
> Since GCC 8, we enable -fasynchronous-unwind-tables by default for
> PowerPC.  See https://gcc.gnu.org/r259298 .
> 
>> For
>> vmlinux, that's not a problem, because they all get discarded in
>> arch/powerpc/kernel/vmlinux.lds.S . However, they stick around in
>> modules, which doesn't seem to be useful - given that everything worked
>> just fine with gcc-7, and I don't see anything in the module loader that
>> handles .eh_frame.
> 
> It is useful for debugging.  Not many people debug the kernel like this,
> of course.

I'm trying to understand if we need that. Other architectures seems to 
pass -fasynchronous-unwind-tables only for the vdso, but disable it for 
the kernel build. I suppose we can do the same.

If using -fno-asynchronous-unwind-tables, would crash/perf have 
problems?

- Naveen