Messages in this thread | | | From | Michael Ellerman <> | Subject | Re: [PATCH] powerpc: avoid broken GCC __attribute__((optimize)) | Date | Thu, 29 Oct 2020 22:35:48 +1100 |
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Ard Biesheuvel <ardb@kernel.org> writes: > On Wed, 28 Oct 2020 at 09:04, Ard Biesheuvel <ardb@kernel.org> wrote: >> >> Commit 7053f80d9696 ("powerpc/64: Prevent stack protection in early boot") >> introduced a couple of uses of __attribute__((optimize)) with function >> scope, to disable the stack protector in some early boot code. >> >> Unfortunately, and this is documented in the GCC man pages [0], overriding >> function attributes for optimization is broken, and is only supported for >> debug scenarios, not for production: the problem appears to be that >> setting GCC -f flags using this method will cause it to forget about some >> or all other optimization settings that have been applied. >> >> So the only safe way to disable the stack protector is to disable it for >> the entire source file. >> >> [0] https://gcc.gnu.org/onlinedocs/gcc/Common-Function-Attributes.html >> >> Cc: Michael Ellerman <mpe@ellerman.id.au> >> Cc: Benjamin Herrenschmidt <benh@kernel.crashing.org> >> Cc: Paul Mackerras <paulus@samba.org> >> Cc: Nick Desaulniers <ndesaulniers@google.com> >> Cc: Arvind Sankar <nivedita@alum.mit.edu> >> Cc: Randy Dunlap <rdunlap@infradead.org> >> Cc: Josh Poimboeuf <jpoimboe@redhat.com> >> Cc: Thomas Gleixner <tglx@linutronix.de> >> Cc: Alexei Starovoitov <ast@kernel.org> >> Cc: Daniel Borkmann <daniel@iogearbox.net> >> Cc: Peter Zijlstra (Intel) <peterz@infradead.org> >> Cc: Geert Uytterhoeven <geert@linux-m68k.org> >> Cc: Kees Cook <keescook@chromium.org> >> Fixes: 7053f80d9696 ("powerpc/64: Prevent stack protection in early boot") >> Signed-off-by: Ard Biesheuvel <ardb@kernel.org> >> --- >> Related discussion here: >> https://lore.kernel.org/lkml/CAMuHMdUg0WJHEcq6to0-eODpXPOywLot6UD2=GFHpzoj_hCoBQ@mail.gmail.com/ >> >> TL;DR using __attribute__((optimize("-fno-gcse"))) in the BPF interpreter >> causes the compiler to forget about -fno-asynchronous-unwind-tables passed >> on the command line, resulting in unexpected .eh_frame sections in vmlinux. >> >> arch/powerpc/kernel/Makefile | 3 +++ >> arch/powerpc/kernel/paca.c | 2 +- >> arch/powerpc/kernel/setup.h | 6 ------ >> arch/powerpc/kernel/setup_64.c | 2 +- >> 4 files changed, 5 insertions(+), 8 deletions(-)
Thanks for the patch.
> FYI i was notified by one of the robots that I missed one occurrence > of __nostackprotector in arch/powerpc/kernel/paca.c > > Let me know if I need to resend.
That's fine I'll fix it up when applying.
With the existing code, with STACKPROTECTOR_STRONG=y, I see two functions in setup_64.c that are triggering stack protection. One is __init, and the other takes no parameters and is not easily reachable from userspace, so I don't think losing the stack canary on either of those is a concern.
I don't see anything in paca.c triggering stack protection.
I don't think there's any evidence this is causing a bug for us, so I'll plan to put this in next for v5.11.
cheers
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