On Thu, Feb 14, 2013 at 5:33 PM, Alan Modra <amo...@gmail.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.
Okay. Thanks, David
