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

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Message-ID: <alpine.DEB.2.00.1805241757321.10896@tp.orcam.me.uk>
Date:   Thu, 24 May 2018 18:06:05 +0100
From:   "Maciej W. Rozycki" <macro@...s.com>
To:     Paul Burton <paul.burton@...s.com>
CC:     James Hogan <jhogan@...nel.org>,
        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

Hi Paul,

> > 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.
> 
> Reviewing the code I think we should actually end up with FR=1 in this
> case, because neither __own_fpu() nor the FPU emulator depend on the
> value of TIF_32BIT_FPREGS if TIF_HYBRID_FPREGS is set. So it's not too
> awful & I don't see the kernel doing anything too crazy, but it
> definitely isn't what the user asked for.

 However `arch_ptrace' does check TIF_32BIT_FPREGS and gets things wrong 
if TIF_HYBRID_FPREGS is also set.  Which is actually how I discovered 
this issue.

> > 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'.
> 
> Looks good to me:
> 
>   Reviewed-by: Paul Burton <paul.burton@...s.com>

 Thanks for your review.

  Maciej