linux-kernel - Re: the x86 sysret_rip test fails on the Intel FRED architecture

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Message-ID: <5ecc383c-621b-57d9-7f6d-d63496fca3b3@zytor.com>
Date:   Mon, 23 Jan 2023 16:01:11 -0800
From:   "H. Peter Anvin" <hpa@...or.com>
To:     Andrew Cooper <Andrew.Cooper3@...rix.com>,
        Ammar Faizi <ammarfaizi2@...weeb.org>
Cc:     Dave Hansen <dave.hansen@...el.com>,
        Dave Hansen <dave.hansen@...ux.intel.com>,
        "Li, Xin3" <xin3.li@...el.com>,
        Thomas Gleixner <tglx@...utronix.de>,
        Ingo Molnar <mingo@...hat.com>, Borislav Petkov <bp@...en8.de>,
        Peter Zijlstra <peterz@...radead.org>,
        "x86@...nel.org" <x86@...nel.org>,
        Linux Kernel Mailing List <linux-kernel@...r.kernel.org>
Subject: Re: the x86 sysret_rip test fails on the Intel FRED architecture

On 1/23/23 15:53, Andrew Cooper wrote:
>>
>> A simple workaround is: just compile it with "-mno-red-zone" flag.
> 
> You can't compile userspace like that, unless you recompile and
> statically link everything including libc.
> 
> The proper way to fix this is to add a "+r" constraint on the stack
> pointer, at which point the compiler will arrange for nothing in the
> redzone to be live around the asm block.
> 

So you are of course right that that is The Right Way[TM] to fix this.

However, to be picky: since the redzone is not in use across function 
calls, it is perfectly legitimate to inhibit the redzone for one 
function only.

	-hpa