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Re: [RS6000] PR55341 linux unwind fixes
- From: David Edelsohn <dje dot gcc at gmail dot com>
- To: gcc-patches at gcc dot gnu dot org, Alan Modra <amodra at gmail dot com>
- Date: Thu, 14 Feb 2013 20:07:07 -0500
- Subject: Re: [RS6000] PR55341 linux unwind fixes
- References: <20130214223357.GA9150@bubble.grove.modra.org>
On Thu, Feb 14, 2013 at 5:33 PM, Alan Modra <email@example.com> wrote:
> PR55341 has two complaints, that powerpc gcc references a non-ABI
> symbol exported by glibc, and that the scheme used to find the aux
> vector can be broken by someone writing *environ = 0. Both true, but
> since this code is only exercised when running Linux kernels prior to
> 2.6.15 (7 years old!), I was inclined to "fix" the first problem by
> making __libc_stack_end weak and ignore the second problem entirely.
> However, since I first looked at this bug, I've been delving into
> unwinding code due to thinking we had a problem in the kernel vdso
> with VSX registers. We didn't, but the exercise taught me that my
> concern about blindly setting up locations for altivec registers is
> unfounded. If the kernel doesn't support altivec and those locations
> point well past the kernel sigcontext, perhaps even to unmapped
> memory, we still don't have a problem since the normal course of
> unwinding won't reference those locations. The unwinder only looks at
> few regs like cfa and ra during unwinding, and the rest of the regs
> when copying into _Unwind_* callee save locations using
> uw_install_context. That means you'll only reference the altivec save
> locations if you're using a libgcc with altivec support. Running such
> a libgcc on a kernel without altivec support is crazy, and will cause
> all sorts of problems before you even consider exception handling,
> eg. sigill if the hardware doesn't support altivec, no process context
> swapping of altivec regs etc.
> So we can do without the AT_HWCAP tests. There is also no need to
> set up locations for call used regs.
> Bootstrapped and regression tested powerpc64-linux using a ld.so hack
> to prevent registration of the kernel vdso. OK to apply?
> PR target/55431
> * config/rs6000/linux-unwind.h (ppc_linux_aux_vector): Delete.
> (ppc_fallback_frame_state): Always set up save locations for fp
> and altivec. Don't bother with non-callee-saved regs, r0-r13
> except for r2 on ppc64, fr0-fr13, v0-v19, vscr.