linux-kernel - Re: [PATCH 2/4] sparc: fix accurate exception reporting in copy_{from_to}

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Message-ID: <528a5d4b-1e07-4379-afd6-7e58d423e713@oracle.com>
Date: Fri, 29 Aug 2025 18:57:36 -0700
From: Anthony Yznaga <anthony.yznaga@...cle.com>
To: Michael Karcher <kernel@...rcher.dialup.fu-berlin.de>,
        linux-kernel@...r.kernel.org
Cc: sparclinux@...r.kernel.org, Andreas Larsson <andreas@...sler.com>,
        John Paul Adrian Glaubitz <glaubitz@...sik.fu-berlin.de>
Subject: Re: [PATCH 2/4] sparc: fix accurate exception reporting in
 copy_{from_to}_user for UltraSPARC III



On 8/26/25 9:03 AM, Michael Karcher wrote:
> Based on a finding by Anthony Yznaga that the UltraSPARC III copy_from_user
> returns invalid values breaking other parts of the kernel in case of a
> fault, while the generic implementation is correct.

I think there should be a little more text about the nature of the 
failure. Maybe:

He observed that a BUG_ON in ext4 code with large folios enabled 
resulted from copy_from_user() returning impossibly large values greater 
than the size to be copied. This lead to __copy_from_iter() returning 
impossible values instead of the actual number of bytes it was able to copy.

Otherwise, the fix looks good.

Reviewed-by: Anthony Yznaga <anthony.yznaga@...cle.com>


> 
> Fixes: ee841d0aff64 ("sparc64: Convert U3copy_{from,to}_user to accurate exception reporting.")
> Signed-off-by: Michael Karcher <kernel@...rcher.dialup.fu-berlin.de>
> ---
>   arch/sparc/lib/U3memcpy.S | 2 +-
>   1 file changed, 1 insertion(+), 1 deletion(-)
> 
> diff --git a/arch/sparc/lib/U3memcpy.S b/arch/sparc/lib/U3memcpy.S
> index 9248d59c734c..bace3a18f836 100644
> --- a/arch/sparc/lib/U3memcpy.S
> +++ b/arch/sparc/lib/U3memcpy.S
> @@ -267,6 +267,7 @@ FUNC_NAME:	/* %o0=dst, %o1=src, %o2=len */
>   	faligndata	%f10, %f12, %f26
>   	EX_LD_FP(LOAD(ldd, %o1 + 0x040, %f0), U3_retl_o2)
>   
> +	and		%o2, 0x3f, %o2
>   	subcc		GLOBAL_SPARE, 0x80, GLOBAL_SPARE
>   	add		%o1, 0x40, %o1
>   	bgu,pt		%XCC, 1f
> @@ -336,7 +337,6 @@ FUNC_NAME:	/* %o0=dst, %o1=src, %o2=len */
>   	 * Also notice how this code is careful not to perform a
>   	 * load past the end of the src buffer.
>   	 */
> -	and		%o2, 0x3f, %o2
>   	andcc		%o2, 0x38, %g2
>   	be,pn		%XCC, 2f
>   	 subcc		%g2, 0x8, %g2