linux-kernel - Re: [PATCH] MIPS: R6: memcpy bugfix

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Date:	Tue, 28 Apr 2015 15:40:55 +0000
From:	Leonid Yegoshin <Leonid.Yegoshin@...tec.com>
To:	Jonas Gorski <jogo@...nwrt.org>
CC:	MIPS Mailing List <linux-mips@...ux-mips.org>,
	Markos Chandras <Markos.Chandras@...tec.com>,
	Ralf Baechle <ralf@...ux-mips.org>,
	"linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>
Subject: Re: [PATCH] MIPS: R6: memcpy bugfix - zero length overwrites memory

You right,

I am debugging new core and got a wrong backtrace.

Please cancel it, sorry for noise.

- Leonid.


Jonas Gorski <jogo@...nwrt.org> wrote:


Hi,

On Tue, Apr 28, 2015 at 1:35 AM, Leonid Yegoshin
<Leonid.Yegoshin@...tec.com> wrote:
> MIPS R6 version of memcpy has bug - then length to copy is zero
> and addresses are not aligned then it can overwrite a whole memory.
>
> Signed-off-by: Leonid Yegoshin <Leonid.Yegoshin@...tec.com>
> ---
>  arch/mips/lib/memcpy.S |    2 ++
>  1 file changed, 2 insertions(+)
>
> diff --git a/arch/mips/lib/memcpy.S b/arch/mips/lib/memcpy.S
> index 9245e1705e69..7e0250f3aec8 100644
> --- a/arch/mips/lib/memcpy.S
> +++ b/arch/mips/lib/memcpy.S
> @@ -514,6 +514,8 @@
>
>  #ifdef CONFIG_CPU_MIPSR6
>  .Lcopy_unaligned_bytes\@:
> +       beqz    len, .Ldone\@
> +        nop
>  1:
>         COPY_BYTE(0)
>         COPY_BYTE(1)

AFAICT it should never reach that if the amount to copy is zero bytes,
so the check seems to be superfluous:

        sltu    t2, len, NBYTES <- check for < NBYTES (4/8 bit
depending on 32/64 bit)
        and     t1, dst, ADDRMASK
        PREFS(  0, 1*32(src) )
        PREFD(  1, 1*32(dst) )
        bnez    t2, .Lcopy_bytes_checklen\@ <- skip to
copy_bytes_checklen if < NBYTES
         and    t0, src, ADDRMASK
        PREFS(  0, 2*32(src) )
        PREFD(  1, 2*32(dst) )
#ifndef CONFIG_CPU_MIPSR6
        bnez    t1, .Ldst_unaligned\@
         nop
        bnez    t0, .Lsrc_unaligned_dst_aligned\@
#else
        or      t0, t0, t1
        bnez    t0, .Lcopy_unaligned_bytes\@ <- only outside place to
branch to it, and only reachable if len >= NBYTES bytes.
#endif


And in the loop itself each COPY_BYTE() will already break out if len
becomes zero, so the unconditional b 1b should also never be reached
with len == 0 in that case..

But maybe I overlooked something.


Regards
Jonas
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