Re: [PATCH 2/2] MIPS: memset.S: Fix return of __clear_user from Lpartial_fixup

[James Hogan <jhogan@kernel.org>]

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On Thu, Mar 29, 2018 at 10:28:24AM +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. Currently it
> masks the remaining count of bytes (a2) with STORMASK, meaning that the
> least significant 2 (32bit) or 3 (64bit) bits of the remaining count are
> always clear.

Are you sure about that. It seems to do that *to ensure those bits are
set correctly*...

> Secondly, .Lpartial_fixup\@ expects t1 to contain the end address of the
> copy. This is set up by the initial block:
> 	PTR_ADDU	t1, a0			/* end address */
> However, the .Lmemset_partial\@ block then reuses register t1 to
> calculate a jump through a block of word copies. This leaves it no
> longer containing the end address of the copy operation if a page fault
> occurs, and the remaining bytes calculation is incorrect.
> 
> Fix these issues by removing the and of a2 with STORMASK, and replace t1
> with register t2 in the .Lmemset_partial\@ block.
> 
> Fixes: 1da177e4c3f4 ("Linux-2.6.12-rc2")
> Cc: stable@vger.kernel.org
> Signed-off-by: Matt Redfearn <matt.redfearn@mips.com>
> ---
> 
>  arch/mips/lib/memset.S | 9 ++++-----
>  1 file changed, 4 insertions(+), 5 deletions(-)
> 
> diff --git a/arch/mips/lib/memset.S b/arch/mips/lib/memset.S
> index 90bcdf1224ee..3257dca58cad 100644
> --- a/arch/mips/lib/memset.S
> +++ b/arch/mips/lib/memset.S
> @@ -161,19 +161,19 @@
>  
>  .Lmemset_partial\@:
>  	R10KCBARRIER(0(ra))
> -	PTR_LA		t1, 2f			/* where to start */
> +	PTR_LA		t2, 2f			/* where to start */
>  #ifdef CONFIG_CPU_MICROMIPS
>  	LONG_SRL	t7, t0, 1

Hmm, on microMIPS t7 isn't on the clobber list for __bzero, and nor is
t8...

>  #endif
>  #if LONGSIZE == 4
> -	PTR_SUBU	t1, FILLPTRG
> +	PTR_SUBU	t2, FILLPTRG
>  #else
>  	.set		noat
>  	LONG_SRL	AT, FILLPTRG, 1
> -	PTR_SUBU	t1, AT
> +	PTR_SUBU	t2, AT
>  	.set		at
>  #endif
> -	jr		t1
> +	jr		t2
>  	PTR_ADDU	a0, t0			/* dest ptr */

^^^ note this...

>  
>  	.set		push
> @@ -250,7 +250,6 @@
>  
>  .Lpartial_fixup\@:
>  	PTR_L		t0, TI_TASK($28)
> -	andi		a2, STORMASK

... this isn't right.

If I read correctly, t1 (after the above change stops clobbering it) is
the end of the full 64-byte blocks, i.e. the start address of the final
partial block.


The .Lfwd_fixup calculation (for full blocks) appears to be:

  a2 = ((len & 0x3f) + start_of_partial) - badvaddr

which is spot on. (len & 0x3f) is the partial block and remaining bytes
that haven't been set yet, add start_of_partial to get end of the full
range, subtract bad address to find how much didn't copy.


The calculation for .Lpartial_fixup however appears to (currently) do:

  a2 = ((len & STORMASK) + start_of_partial) - badvaddr

Which might make sense if start_of_partial (t1) was replaced with
end_of_partial, which does seem to be calculated as noted above, and put
in a0 ready for the final few bytes to be set.

>  	LONG_L		t0, THREAD_BUADDR(t0)
>  	LONG_ADDU	a2, t1

^^ So I think either it needs to just s/t1/a0/ here and not bother
preserving t1 above (smaller change and probably the original intent),
or preserve t1 and mask 0x3f instead of STORMASK like .Lfwd_fixup does
(which would work but seems needlessly complicated to me).

Does that make any sense or have I misunderstood some subtlety?

Cheers
James

>  	jr		ra
> -- 
> 2.7.4
> 

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