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From: Andy Lutomirski <luto@amacapital.net>
To: Linus Torvalds <torvalds@linux-foundation.org>
Cc: Nick Desaulniers <ndesaulniers@google.com>,
	Borislav Petkov <bp@suse.de>, Arnd Bergmann <arnd@arndb.de>,
	Arvind Sankar <nivedita@alum.mit.edu>,
	Kalle Valo <kvalo@codeaurora.org>,
	linux-wireless <linux-wireless@vger.kernel.org>,
	"linux-kernel@vger.kernel.org" <linux-kernel@vger.kernel.org>,
	"the arch/x86 maintainers" <x86@kernel.org>,
	Kees Cook <keescook@chromium.org>,
	Thomas Gleixner <tglx@linutronix.de>
Subject: Re: gcc-10: kernel stack is corrupted and fails to boot
Date: Wed, 13 May 2020 20:50:03 -0700	[thread overview]
Message-ID: <CALCETrUYA60fWu+=MviKx0NmW+_ppsOcv-ShUXdbyM4EjyfzHQ@mail.gmail.com> (raw)
In-Reply-To: <CAHk-=wg6G+p1RRjR6UZBEuSCDs9=iWBsxrDPTEwqh+y5RayqKA@mail.gmail.com>

> On May 13, 2020, at 7:20 PM, Linus Torvalds <torvalds@linux-foundation.org> wrote:
>
> On Wed, May 13, 2020 at 5:51 PM Nick Desaulniers
> <ndesaulniers@google.com> wrote:
>>
>> Are you sure LTO treats empty asm statements differently than full
>> memory barriers in regards to preventing tail calls?
>
> It had better.
>
> At link-time, there is nothing left of an empty asm statement. So by
> the time the linker runs, it only sees
>
>    call xyz
>    ret
>
> in the object code. At that point, it's somewhat reasonable for any
> link-time optimizer (or an optimizing assembler, for that matter) to
> say "I'll just turn that sequence into a simple 'jmp xyz' instead".
>

What, what?

LTO isn’t a linker taking regular .o files full of regular machine
code and optimizing it. That’s nuts.  LTO takes an intermediate
representation and optimizes *that*. This will contain actual
indications that something is inline asm.  If LTO starts rewriting
inline asm, then I bet all kinds of things will go wrong and this is
the least of our worries.  Also, trying to do the kinds of stuff LTO
does by looking at just machine code isn't going to work.

So the difference between:

asm volatile ("nop");

and

asm volatile ("");

will be, literally, the absence of the nop.  (And alignment changes, etc.)

  reply	other threads:[~2020-05-14  3:50 UTC|newest]

Thread overview: 38+ messages / expand[flat|nested]  mbox.gz  Atom feed  top
2020-05-09 12:06 [PATCH net-next 1/2] ath10k: fix gcc-10 zero-length-bounds warnings Arnd Bergmann
2020-05-09 12:06 ` [PATCH net-next 2/2] ath10k: fix ath10k_pci struct layout Arnd Bergmann
2020-05-11 12:05   ` Kalle Valo
2020-05-11 12:17     ` Kalle Valo
2020-05-11 12:39       ` Arnd Bergmann
2020-05-13  6:50       ` gcc-10: kernel stack is corrupted and fails to boot Kalle Valo
2020-05-13  8:49         ` Arnd Bergmann
2020-05-13 12:45           ` Kalle Valo
2020-05-13 13:45             ` Arnd Bergmann
2020-05-13 15:31               ` Kalle Valo
2020-05-13 16:00                 ` Arnd Bergmann
2020-05-13 16:07                   ` David Laight
2020-05-14  9:13                 ` Harald Arnesen
2020-05-13 15:48         ` Arvind Sankar
2020-05-13 21:28           ` Arnd Bergmann
2020-05-13 21:41             ` Borislav Petkov
2020-05-13 21:49               ` Arnd Bergmann
2020-05-13 22:20                 ` Borislav Petkov
2020-05-13 22:51                   ` Arvind Sankar
2020-05-13 23:13                   ` Linus Torvalds
2020-05-13 23:36                     ` Borislav Petkov
2020-05-14  0:11                       ` Linus Torvalds
2020-05-14  0:51                         ` Nick Desaulniers
2020-05-14  2:20                           ` Linus Torvalds
2020-05-14  3:50                             ` Andy Lutomirski [this message]
     [not found]                               ` <CAHk-=wgiGxRgJGS-zyer1C_x2MQUVo6iZn0=aJyuFTqJWk-mpA@mail.gmail.com>
2020-05-14  5:22                                 ` Arvind Sankar
2020-05-14  8:40                                   ` Arnd Bergmann
2020-05-14 13:27                                     ` [PATCH] x86: Fix early boot crash on gcc-10, third try Borislav Petkov
2020-05-14 14:45                                       ` Kalle Valo
2020-05-14 15:50                                     ` gcc-10: kernel stack is corrupted and fails to boot Arvind Sankar
2020-05-14  8:11                             ` David Laight
2020-05-13 23:07                 ` Linus Torvalds
2020-05-09 15:48 ` [PATCH net-next 1/2] ath10k: fix gcc-10 zero-length-bounds warnings Gustavo A. R. Silva
2020-05-11 12:02   ` Kalle Valo
2020-05-11 12:46     ` Arnd Bergmann
2020-05-11 13:09       ` Kalle Valo
2020-05-11 13:47         ` Arnd Bergmann
2020-05-12  7:33 ` Kalle Valo

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