From: Nick Desaulniers <ndesaulniers@google.com> To: Borislav Petkov <bp@alien8.de> Cc: Jakub Jelinek <jakub@redhat.com>, Sergei Trofimovich <slyfox@gentoo.org>, Michael Matz <matz@suse.de>, LKML <linux-kernel@vger.kernel.org>, Thomas Gleixner <tglx@linutronix.de>, Ingo Molnar <mingo@redhat.com>, "H. Peter Anvin" <hpa@zytor.com>, Andy Lutomirski <luto@kernel.org>, Peter Zijlstra <peterz@infradead.org>, "maintainer:X86 ARCHITECTURE (32-BIT AND 64-BIT)" <x86@kernel.org>, clang-built-linux <clang-built-linux@googlegroups.com> Subject: Re: [PATCH v2] x86: fix early boot crash on gcc-10 Date: Fri, 17 Apr 2020 11:15:56 -0700 Message-ID: <CAKwvOdnFXPBJsAUD++HtYS5JiR2KmX73M5GAUe-tvX-JYV7DaA@mail.gmail.com> (raw) In-Reply-To: <20200417090909.GC7322@zn.tnic> 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
next prev parent reply index Thread overview: 79+ messages / expand[flat|nested] mbox.gz Atom feed top 2020-03-14 16:44 [PATCH] " Sergei Trofimovich 2020-03-16 13:04 ` Peter Zijlstra 2020-03-16 13:26 ` Jakub Jelinek 2020-03-16 13:42 ` Peter Zijlstra 2020-03-16 17:54 ` Borislav Petkov 2020-03-16 18:03 ` Jakub Jelinek 2020-03-17 14:36 ` Borislav Petkov 2020-03-17 14:39 ` Jakub Jelinek 2020-03-17 14:49 ` Borislav Petkov 2020-03-17 16:35 ` David Laight 2020-03-25 13:31 ` Borislav Petkov 2020-03-26 21:54 ` Sergei Trofimovich 2020-03-26 22:35 ` Borislav Petkov 2020-03-28 8:48 ` [PATCH v2] " Sergei Trofimovich 2020-04-13 14:15 ` [tip: x86/urgent] x86: Fix " tip-bot2 for Sergei Trofimovich 2020-04-13 16:35 ` [PATCH v2] x86: fix " Borislav Petkov 2020-04-14 13:50 ` Michael Matz 2020-04-15 7:48 ` Borislav Petkov 2020-04-15 14:53 ` Michael Matz 2020-04-15 22:19 ` Sergei Trofimovich 2020-04-17 7:57 ` Borislav Petkov 2020-04-17 8:07 ` Jakub Jelinek 2020-04-17 8:42 ` Borislav Petkov 2020-04-17 8:58 ` Jakub Jelinek 2020-04-17 9:09 ` Borislav Petkov 2020-04-17 18:15 ` Nick Desaulniers [this message] 2020-04-17 18:22 ` Nick Desaulniers 2020-04-17 19:06 ` Jakub Jelinek 2020-04-17 19:49 ` Nick Desaulniers 2020-04-17 19:53 ` Nick Desaulniers 2020-04-20 14:04 ` Michael Matz 2020-04-22 10:23 ` Borislav Petkov 2020-04-22 11:40 ` Peter Zijlstra 2020-04-22 13:49 ` Borislav Petkov 2020-04-22 13:55 ` Jakub Jelinek 2020-04-22 14:16 ` Martin Liška 2020-04-22 15:06 ` Michael Matz 2020-04-22 16:53 ` Borislav Petkov 2020-04-22 17:02 ` Jakub Jelinek 2020-04-22 18:47 ` Nick Desaulniers 2020-04-22 18:55 ` Nick Desaulniers 2020-04-22 19:21 ` Borislav Petkov 2020-04-22 21:05 ` Nick Desaulniers 2020-04-22 21:26 ` Borislav Petkov 2020-04-22 22:57 ` Nick Desaulniers 2020-04-23 12:53 ` Borislav Petkov 2020-04-23 16:12 ` [PATCH] x86: Fix early boot crash on gcc-10, next try Borislav Petkov 2020-04-23 17:30 ` Borislav Petkov 2020-04-23 18:02 ` Nick Desaulniers 2020-04-23 18:27 ` Borislav Petkov 2020-04-27 11:37 ` [tip: x86/build] x86/build: Check whether the compiler is sane tip-bot2 for Borislav Petkov 2020-04-23 19:40 ` [PATCH] x86: Fix early boot crash on gcc-10, next try Kees Cook 2020-04-25 1:46 ` Arvind Sankar 2020-04-25 8:57 ` Borislav Petkov 2020-04-25 11:09 ` Jürgen Groß 2020-04-25 15:04 ` Arvind Sankar 2020-04-25 17:31 ` Borislav Petkov 2020-04-25 17:52 ` Borislav Petkov 2020-04-27 17:07 ` David Laight 2020-04-25 18:37 ` Segher Boessenkool 2020-04-25 18:53 ` Borislav Petkov 2020-04-25 19:15 ` Segher Boessenkool 2020-04-25 22:17 ` Borislav Petkov 2020-04-25 22:25 ` Arvind Sankar 2020-04-17 10:38 ` [PATCH v2] x86: fix early boot crash on gcc-10 Peter Zijlstra 2020-04-18 13:12 ` David Laight 2020-04-17 10:41 ` Peter Zijlstra 2020-03-16 18:20 ` [PATCH] " Arvind Sankar 2020-03-16 18:54 ` Arvind Sankar 2020-03-16 19:53 ` Arvind Sankar 2020-03-16 20:08 ` Jakub Jelinek 2020-03-16 20:40 ` Arvind Sankar 2020-03-16 22:12 ` Sergei Trofimovich 2020-03-17 11:46 ` Jakub Jelinek 2020-03-17 18:10 ` Sergei Trofimovich 2020-03-16 18:22 ` Arvind Sankar 2020-03-26 23:16 ` [PATCH v2] " Sergei Trofimovich 2020-04-27 11:37 ` [tip: x86/build] x86: Fix early boot crash on gcc-10, next try tip-bot2 for Borislav Petkov 2020-05-15 11:20 ` [tip: x86/urgent] x86: Fix early boot crash on gcc-10, third try tip-bot2 for Borislav Petkov
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