linux-kernel - Re: [PATCH v2 1/4] MIPS: memset.S: Fix clobber of v1 in last

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Date:   Wed, 18 Apr 2018 23:02:37 +0100
From:   James Hogan <jhogan@...nel.org>
To:     Matt Redfearn <matt.redfearn@...s.com>
Cc:     Ralf Baechle <ralf@...ux-mips.org>, linux-mips@...ux-mips.org,
        stable@...r.kernel.org, linux-kernel@...r.kernel.org
Subject: Re: [PATCH v2 1/4] MIPS: memset.S: Fix clobber of v1 in last_fixup

On Tue, Apr 17, 2018 at 04:40:00PM +0100, Matt Redfearn wrote:
> The label .Llast_fixup\@ is jumped to on page fault within the final
> byte set loop of memset (on < MIPSR6 architectures). For some reason, in
> this fault handler, the v1 register is randomly set to a2 & STORMASK.
> This clobbers v1 for the calling function. This can be observed with the
> following test code:
> 
> static int __init __attribute__((optimize("O0"))) test_clear_user(void)
> {
>   register int t asm("v1");
>   char *test;
>   int j, k;
> 
>   pr_info("\n\n\nTesting clear_user\n");
>   test = vmalloc(PAGE_SIZE);
> 
>   for (j = 256; j < 512; j++) {
>     t = 0xa5a5a5a5;
>     if ((k = clear_user(test + PAGE_SIZE - 256, j)) != j - 256) {
>         pr_err("clear_user (%px %d) returned %d\n", test + PAGE_SIZE - 256, j, k);
>     }
>     if (t != 0xa5a5a5a5) {
>        pr_err("v1 was clobbered to 0x%x!\n", t);
>     }
>   }
> 
>   return 0;
> }
> late_initcall(test_clear_user);
> 
> Which demonstrates that v1 is indeed clobbered (MIPS64):
> 
> Testing clear_user
> v1 was clobbered to 0x1!
> v1 was clobbered to 0x2!
> v1 was clobbered to 0x3!
> v1 was clobbered to 0x4!
> v1 was clobbered to 0x5!
> v1 was clobbered to 0x6!
> v1 was clobbered to 0x7!
> 
> Since the number of bytes that could not be set is already contained in
> a2, the andi placing a value in v1 is not necessary and actively
> harmful in clobbering v1.
> 
> Fixes: 1da177e4c3f4 ("Linux-2.6.12-rc2")
> Cc: stable@...r.kernel.org
> Reported-by: James Hogan <jhogan@...nel.org>
> Signed-off-by: Matt Redfearn <matt.redfearn@...s.com>

Thanks, Patches 1 & 2 applied to my fixes branch.

Cheers
James

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