linux-kernel - Re: [PATCH v2] x86: fix early boot crash on gcc-10

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Date:   Wed, 22 Apr 2020 16:16:53 +0200
From:   Martin Liška <mliska@...e.cz>
To:     Jakub Jelinek <jakub@...hat.com>, Borislav Petkov <bp@...en8.de>
Cc:     Peter Zijlstra <peterz@...radead.org>,
        Nick Desaulniers <ndesaulniers@...gle.com>,
        Michael Matz <matz@...e.de>,
        Sergei Trofimovich <slyfox@...too.org>,
        LKML <linux-kernel@...r.kernel.org>,
        Thomas Gleixner <tglx@...utronix.de>,
        Ingo Molnar <mingo@...hat.com>,
        "H. Peter Anvin" <hpa@...or.com>,
        Andy Lutomirski <luto@...nel.org>,
        "maintainer:X86 ARCHITECTURE (32-BIT AND 64-BIT)" <x86@...nel.org>,
        clang-built-linux <clang-built-linux@...glegroups.com>
Subject: Re: [PATCH v2] x86: fix early boot crash on gcc-10

On 4/22/20 3:55 PM, Jakub Jelinek wrote:
> On Wed, Apr 22, 2020 at 03:49:24PM +0200, Borislav Petkov wrote:
>> On Wed, Apr 22, 2020 at 01:40:07PM +0200, Peter Zijlstra wrote:
>>> You haz a whitespace issue there.
>>
>> Fixed.
>>
>>> Also, can we get this in writing, signed in blood, from the various
>>> compiler teams ;-)
>>
>> Yah, I wouldn't want to go fix this again in gcc11 or so. That's why I
>> wanted the explicit marking but let's try this first - it is too simple
>> to pass over without having tested it.
> 
> If virtual blood is enough, AFAIK GCC has never tried to accept volatile
> inline asm (asm ("") is such; non-volatile asm such as int x; asm ("" : "=r" (x));
> could be e.g. dead code eliminated) in the statements between function call and
> return when deciding about what function can be tail-called or can use
> tail-recursion and there are no plans to change that.
> 
> 	Jakub
> 
> 
> 

One possible solution can be usage of a GCC pragma that will disable the tail-call optimization:

$ cat tail.c
int foo(int);

#pragma GCC push_options
#pragma GCC optimize("-fno-optimize-sibling-calls")
int baz(int a)
{
   int r = foo(a);
   return r;
}
#pragma GCC pop_options

I'm not sure if clang provides something similar (the -foptimize-sibling-calls option
is supported as well).

And as I talked to Boris, I would recommend to come up with a "configure" check
that a compiler does not optimize the key code sequence:

$ cat asm-detect.c
int foo(int a);
int bar(int a)
{
   int r = foo(a);
   asm ("");
   return r;
}

$ gcc -O2 -c asm-detect.c -S -o/dev/stdout | grep jmp
[no output]

Martin