Messages in this thread | | | Date | Mon, 20 Apr 2020 14:04:06 +0000 (UTC) | From | Michael Matz <> | Subject | Re: [PATCH v2] x86: fix early boot crash on gcc-10 |
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Hello,
On Fri, 17 Apr 2020, Nick Desaulniers wrote:
> Ah seems we do have __attribute__((no_selector)) > (https://reviews.llvm.org/D46300, > https://releases.llvm.org/7.0.0/tools/clang/docs/AttributeReference.html#no-stack-protector-clang-no-stack-protector-clang-no-stack-protector) > which differs from GCC attribute name.
As you will discover upthread that was tried with GCC and found insufficient, as GCC is a bit surprising with optimize attributes: it resets every -f option from the command line and applies only the ones from the attributes. Including a potential -fno-omit-frame-pointer, causing all kinds of itches :)
(The similar attribute in clang might work less surprising of course).
Ciao, Michael.
> > I'm still catching up on the thread (and my cat is insistent about > sleeping on my lap while I'm trying to use my laptop), but I like > https://lore.kernel.org/lkml/20200417190607.GY2424@tucnak/T/#m23d197d3a66a6c7d04c5444af4f51d940895b412 > if it additionally defined __no_stack_protector for compiler-clang.h. > > On Fri, Apr 17, 2020 at 12:06 PM Jakub Jelinek <jakub@redhat.com> wrote: > > > > On Fri, Apr 17, 2020 at 11:22:25AM -0700, Nick Desaulniers wrote: > > > > Sorry, I don't quite follow. The idea is that an empty asm statement > > > > in foo() should prevent foo() from being inlined into bar()? > > > > > > s/inlined/tail called/ > > > > Yeah. The thing is, the caller changes the stack protector guard base > > value, so at the start of the function it saves a different value then > > it compares at the end. But, the function that it calls at the end > > actually doesn't return, so this isn't a problem. > > If it is tail called though, the stack protector guard checking is done > > before the tail call and it crashes. > > If the called function is marked with noreturn attribute or _Noreturn, > > at least GCC will also not tail call it and all is fine, but not sure > > what LLVM does in that case. > > Seems fine? https://godbolt.org/z/VEoEfw > (try commenting out the __attribute__((noreturn)) to observe the tail calls. >
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