linux-kernel - Re: [PATCH] MIPS: prctl: Disallow FRE without FR with PR_SET_FP

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Date:   Thu, 31 May 2018 22:29:57 +0100
From:   James Hogan <jhogan@...nel.org>
To:     "Maciej W. Rozycki" <macro@...s.com>
Cc:     Ralf Baechle <ralf@...ux-mips.org>, linux-mips@...ux-mips.org,
        linux-kernel@...r.kernel.org, stable@...r.kernel.org
Subject: Re: [PATCH] MIPS: prctl: Disallow FRE without FR with PR_SET_FP_MODE
 requests

On Tue, May 15, 2018 at 11:04:44PM +0100, Maciej W. Rozycki wrote:
> Having PR_FP_MODE_FRE (i.e. Config5.FRE) set without PR_FP_MODE_FR (i.e. 
> Status.FR) is not supported as the lone purpose of Config5.FRE is to 
> emulate Status.FR=0 handling on FPU hardware that has Status.FR=1 
> hardwired[1][2].  Also we do not handle this case elsewhere, and assume 
> throughout our code that TIF_HYBRID_FPREGS and TIF_32BIT_FPREGS cannot 
> be set both at once for a task, leading to inconsistent behaviour if 
> this does happen.
> 
> Return unsuccessfully then from prctl(2) PR_SET_FP_MODE calls requesting 
> PR_FP_MODE_FRE to be set with PR_FP_MODE_FR clear.  This corresponds to 
> modes allowed by `mips_set_personality_fp'.
> 
> References:
> 
> [1] "MIPS Architecture For Programmers, Vol. III: MIPS32 / microMIPS32
>     Privileged Resource Architecture", Imagination Technologies,
>     Document Number: MD00090, Revision 6.02, July 10, 2015, Table 9.69 
>     "Config5 Register Field Descriptions", p. 262
> 
> [2] "MIPS Architecture For Programmers, Volume III: MIPS64 / microMIPS64 
>     Privileged Resource Architecture", Imagination Technologies, 
>     Document Number: MD00091, Revision 6.03, December 22, 2015, Table 
>     9.72 "Config5 Register Field Descriptions", p. 288
> 
> Cc: stable@...r.kernel.org # 4.0+
> Fixes: 9791554b45a2 ("MIPS,prctl: add PR_[GS]ET_FP_MODE prctl options for MIPS")
> Signed-off-by: Maciej W. Rozycki <macro@...s.com>

Thanks, applied to mips-fixes, hopefully for 4.17 (but if it misses
tomorrows linux-next it may have to wait 'til 4.18).

Cheers
James

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