linux-kernel - Re: [PATCH] [v2] x86/fpu: Delay instruction pointer fixup until after warning

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Message-ID: <20250625085730.GD1613200@noisy.programming.kicks-ass.net>
Date: Wed, 25 Jun 2025 10:57:30 +0200
From: Peter Zijlstra <peterz@...radead.org>
To: Dave Hansen <dave.hansen@...ux.intel.com>
Cc: linux-kernel@...r.kernel.org, x86@...nel.org, tglx@...utronix.de,
	bp@...en8.de, mingo@...nel.org, chao.gao@...el.com,
	Alison Schofield <alison.schofield@...el.com>,
	"Chang S. Bae" <chang.seok.bae@...el.com>,
	Eric Biggers <ebiggers@...gle.com>, Rik van Riel <riel@...hat.com>,
	stable@...r.kernel.org
Subject: Re: [PATCH] [v2] x86/fpu: Delay instruction pointer fixup until
 after warning

On Tue, Jun 24, 2025 at 02:01:48PM -0700, Dave Hansen wrote:
> 
> Changes from v1:
>  * Fix minor typos
>  * Use the more generic and standard ex_handler_default(). Had the
>    original code used this helper, the bug would not have been there
>    in the first place.

Doesn't this here typically go under the --- with the diffstat etc?

> --
> 
> From: Dave Hansen <dave.hansen@...ux.intel.com>
> 
> Right now, if XRSTOR fails a console message like this is be printed:
> 
> 	Bad FPU state detected at restore_fpregs_from_fpstate+0x9a/0x170, reinitializing FPU registers.
> 
> However, the text location (...+0x9a in this case) is the instruction
> *AFTER* the XRSTOR. The highlighted instruction in the "Code:" dump
> also points one instruction late.
> 
> The reason is that the "fixup" moves RIP up to pass the bad XRSTOR and
> keep on running after returning from the #GP handler. But it does this
> fixup before warning.
> 
> The resulting warning output is nonsensical because it looks like the
> non-FPU-related instruction is #GP'ing.
> 
> Do not fix up RIP until after printing the warning. Do this by using
> the more generic and standard ex_handler_default().
> 
> Signed-off-by: Dave Hansen <dave.hansen@...ux.intel.com>
> Fixes: d5c8028b4788 ("x86/fpu: Reinitialize FPU registers if restoring FPU state fails")
> Acked-by: Alison Schofield <alison.schofield@...el.com>
> Cc: stable@...r.kernel.org
> Cc: Eric Biggers <ebiggers@...gle.com>
> Cc: Rik van Riel <riel@...hat.com>
> Cc: Borislav Petkov <bp@...en8.de>
> Cc: Chang S. Bae <chang.seok.bae@...el.com>

Acked-by: Peter Zijlstra (Intel) <peterz@...radead.org>