linux-kernel - Re: [PATCH v3] MIPS: memset.S: Fix return of __clear_user from Lpartial

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Message-ID: <20180417154259.GA21386@saruman>
Date:   Tue, 17 Apr 2018 16:43:00 +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 v3] MIPS: memset.S: Fix return of __clear_user from
 Lpartial_fixup

On Tue, Apr 17, 2018 at 03:52:21PM +0100, Matt Redfearn wrote:
> The __clear_user function is defined to return the number of bytes that
> could not be cleared. From the underlying memset / bzero implementation
> this means setting register a2 to that number on return. Currently if a
> page fault is triggered within the memset_partial block, the value
> loaded into a2 on return is meaningless.
> 
> The label .Lpartial_fixup\@ is jumped to on page fault. In order to work
> out how many bytes failed to copy, the exception handler should find how
> many bytes left in the partial block (andi a2, STORMASK), add that to
> the partial block end address (a2), and subtract the faulting address to
> get the remainder. Currently it incorrectly subtracts the partial block
> start address (t1), which has additionally has been clobbered to
> generate a jump target in memset_partial. Fix this by adding the block
> end address instead.
> 
> This issue was found with the following test code:
>       int j, k;
>       for (j = 0; j < 512; j++) {
>         if ((k = clear_user(NULL, j)) != j) {
>            pr_err("clear_user (NULL %d) returned %d\n", j, k);
>         }
>       }
> Which now passes on Creator Ci40 (MIPS32) and Cavium Octeon II (MIPS64).
> 
> Fixes: 1da177e4c3f4 ("Linux-2.6.12-rc2")
> Cc: stable@...r.kernel.org
> Suggested-by: James Hogan <jhogan@...nel.org>
> Signed-off-by: Matt Redfearn <matt.redfearn@...s.com>

Applied, thanks
James

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