linux-kernel - Re: [RFC PATCH v1 1/2] selftests/x86: sysret

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Date:   Wed, 15 Feb 2023 09:17:23 +0000
From:   Andrew Cooper <Andrew.Cooper3@...rix.com>
To:     Ammar Faizi <ammarfaizi2@...weeb.org>,
        "H. Peter Anvin" <hpa@...or.com>
Cc:     Dave Hansen <dave.hansen@...el.com>,
        Dave Hansen <dave.hansen@...ux.intel.com>,
        "Kirill A. Shutemov" <kirill.shutemov@...ux.intel.com>,
        Thomas Gleixner <tglx@...utronix.de>,
        Peter Zijlstra <peterz@...radead.org>,
        Brian Gerst <brgerst@...il.com>,
        Borislav Petkov <bp@...en8.de>, Shuah Khan <shuah@...nel.org>,
        Ingo Molnar <mingo@...nel.org>,
        Andy Lutomirski <luto@...nel.org>,
        x86 Mailing List <x86@...nel.org>,
        Linux Kselftest Mailing List 
        <linux-kselftest@...r.kernel.org>,
        Linux Kernel Mailing List <linux-kernel@...r.kernel.org>
Subject: Re: [RFC PATCH v1 1/2] selftests/x86: sysret_rip: Handle syscall in a
 FRED system

On 26/01/2023 8:08 pm, Ammar Faizi wrote:
> On Mon, Jan 23, 2023 at 05:40:23PM -0800, H. Peter Anvin wrote:
>> So as per Andrew's comment, add:
>>
>> register void * rsp asm("%rsp");
>>
>> ...
>>
>> "+r" (rsp)	/* clobber the redzone */
>>
>> ... as the right way to avoid redzone problems.
> I played with this more. I found something wrong with this. This doesn't
> work for me. The compiler still uses red zone despite I use "+r" (rsp).
>
> What did I do wrong?

Well this is a fine mess...

https://godbolt.org/z/MaPM7s8qr does the right thing, but is now
contrary to the prior discussion regarding calls in asm, which concluded
that the "+r"(rsp) was the way to go.

Furthermore GCC regressed in 9.0 and emits:

  warning: listing the stack pointer register 'rsp' in a clobber list is
deprecated [-Wdeprecated]

which might be the intention of the developers, but is wrong seeing as
this is the only way to say "I modify the redzone" to the compiler...

~Andrew