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

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Message-ID: <5703e698-a92a-2026-e5d4-3c6340578918@intel.com>
Date:   Sat, 21 Jan 2023 08:46:58 -0800
From:   Dave Hansen <dave.hansen@...el.com>
To:     "H. Peter Anvin" <hpa@...or.com>, "Li, Xin3" <xin3.li@...el.com>,
        "tglx@...utronix.de" <tglx@...utronix.de>,
        "mingo@...hat.com" <mingo@...hat.com>,
        "bp@...en8.de" <bp@...en8.de>,
        "peterz@...radead.org" <peterz@...radead.org>,
        "dave.hansen@...ux.intel.com" <dave.hansen@...ux.intel.com>
Cc:     "x86@...nel.org" <x86@...nel.org>,
        "linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>
Subject: Re: the x86 sysret_rip test fails on the Intel FRED architecture

On 1/20/23 20:59, H. Peter Anvin wrote:
>> If not intentional, it might be something that can still be fixed.
>> If it is intentional and is going to be with us for a while we have
>> a few options.  If userspace is _really_ depending on this
>> behavior, we could just clobber r11 ourselves in the FRED entry
>> path.  If not, we can remove the assertion in the selftest.
> We can't clobber it in the FRED entry path, since it is common for
> all events, but we could do it in the syscall dispatch.
> 
> However, it doesn't seem to make sense to do so to me. The current
> behavior is much more of an artifact than desired behavior.
I guess the SDM statements really are for the kernel's benefit and not
for userspace.  Userspace _should_ be treating SYSCALL like a CALL and
r11 like any old register that can be clobbered.  Right now, the kernel
just happens to clobber it with RFLAGS.

I do the the odds of anyone relying on this behavior are pretty small.
Let's just zap the check from the selftest, document what we did in the
FRED docs and changelog and move on.

If someone screams later, we can fix in some SYSCALL-specific piece of
the FRED code.