Messages in this thread | | | From | Nick Desaulniers <> | Date | Fri, 17 Apr 2020 11:15:56 -0700 | Subject | Re: [PATCH v2] x86: fix early boot crash on gcc-10 |
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On Fri, Apr 17, 2020 at 2:09 AM Borislav Petkov <bp@alien8.de> wrote: > > On Fri, Apr 17, 2020 at 10:58:59AM +0200, Jakub Jelinek wrote: > > On Fri, Apr 17, 2020 at 10:42:24AM +0200, Borislav Petkov wrote: > > > On Fri, Apr 17, 2020 at 10:07:26AM +0200, Jakub Jelinek wrote: > > > > If you want minimal changes, you can as I said earlier either > > > > mark cpu_startup_entry noreturn (in the declaration in some header so that > > > > smpboot.c sees it), or you could add something after the cpu_startup_entry > > > > call to ensure it is not tail call optimized (e.g. just > > > > /* Prevent tail call to cpu_startup_entry because the stack > > > > protector guard has been changed in the middle of this function > > > > and must not be checked before tail calling another function. */ > > > > asm (""); > > > > > > That sounds ok-ish to me too. > > > > > > I know you probably can't tell the future :) but what stops gcc from > > > doing the tail-call optimization in the future? > > > > > > Or are optimization decisions behind an inline asm a no-no and will > > > pretty much always stay that way? > > > > GCC intentionally treats asm as a black box, the only thing which it does
Yep, that's how I would describe how LLVM see's inline asm, too.
> > with it is: non-volatile asm (but asm without outputs is implicitly > > volatile) can be CSEd, and if the compiler needs to estimate size, it > > uses some heuristics by counting ; and newlines. > > And it will stay this way.
I recently implemented parsing support for `asm inline` in Clang; I could have sworn I saw code in LLVM parsing newlines for a size estimate years ago, but when implementing `asm inline`, I couldn't find it. And test cases I wrote that used the C preprocessor to create thousand+ line inline asm strings would always be inlined, regardless of the `inline` asm qualifier.
Not sure about implied volatility (...inner stock trader had a laugh at that...) for output-less asm statements.
> > > > > And I hope the clang folks don't come around and say, err, nope, we're > > > much more aggressive here. > > > > Unlike GCC, I think clang uses the builtin assembler to parse the string, > > but don't know if it still treats the asms more like black boxes or not. > > Certainly there is a lot of code in the wild that uses inline asm > > as optimization barriers, so if it doesn't, then it would cause a lot of > > problems. > > > > Or go with the for (;;);, I don't think any compiler optimizes those away; > > GCC 10 for C++ can optimize away infinite loops that have some conditional > > exit because the language guarantees forward progress, but the C language > > rules are different and for unconditional infinite loops GCC doesn't > > optimize them away even if explicitly asked to -ffinite-loops. > > Lemme add Nick for clang for an opinion: > > Nick, we're discussing what would be the cleanest and future-proof > way to disable stack protector for the function in the kernel which
Oh, this reminds me of commit d0a8d9378d16 ("x86/paravirt: Make native_save_fl() extern inline"), where the insertion of stack guards was also causing some pain.
The cleanest solution would be to have function attributes that say "yes, I know I said -fstack-protector*, but for this one lone function I really need -fno-stack-protector. I know what I'm doing and accept whatever the consequences are." But maybe the attribute would be shorter than all that? :P
Compared to playing games with each other's inlining heuristics, that would be the cleanest and future-proof solution. (Then we can even revert d0a8d9378d16, and use such a function attribute. I somehow prefer gnu_inline's semantics to ISO C99's extern inline semantics, and simultaneously hate the problems for which it's used.)
> generates the canary value as gcc10 ends up checking that value due to > tail-call optimizing the last function called by start_secondary()... > upthread are all the details. > > And question is, can Jakub's suggestions above prevent tail-call > optimization on clang too and how reliable and future proof would that > be if we end up going that way?
Sorry, I don't quite follow. The idea is that an empty asm statement in foo() should prevent foo() from being inlined into bar()? https://godbolt.org/z/7xBRGY -- Thanks, ~Nick Desaulniers
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