linux-kernel - RE: [PATCH] mm: Return -EFAULT if copy_to

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Date:   Tue, 2 Mar 2021 14:59:17 +0000
From:   David Laight <David.Laight@...LAB.COM>
To:     'Wang Qing' <wangqing@...o.com>,
        Andrew Morton <akpm@...ux-foundation.org>,
        "linux-mm@...ck.org" <linux-mm@...ck.org>,
        "linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>
Subject: RE: [PATCH] mm: Return -EFAULT if copy_to_user() fails

From: Wang Qing <wangqing@...o.com>
> Sent: 01 March 2021 12:33
> 
> The copy_to_user() function returns the number of bytes remaining to be
> copied, but we want to return -EFAULT if the copy doesn't complete.
> 
> Signed-off-by: Wang Qing <wangqing@...o.com>
> ---
>  mm/mempolicy.c | 2 +-
>  1 file changed, 1 insertion(+), 1 deletion(-)
> 
> diff --git a/mm/mempolicy.c b/mm/mempolicy.c
> index ab51132..a116e56
> --- a/mm/mempolicy.c
> +++ b/mm/mempolicy.c
> @@ -1667,7 +1667,7 @@ COMPAT_SYSCALL_DEFINE5(get_mempolicy, int __user *, policy,
>  	if (!err && nmask) {
>  		unsigned long copy_size;
>  		copy_size = min_t(unsigned long, sizeof(bm), alloc_size);
> -		err = copy_from_user(bm, nm, copy_size);
> +		err = copy_from_user(bm, nm, copy_size) ? -EFAULT : 0;
>  		/* ensure entire bitmap is zeroed */
>  		err |= clear_user(nmask, ALIGN(maxnode-1, 8) / 8);
>  		err |= compat_put_bitmap(nmask, bm, nr_bits);

Given the 'err |= xxx' that follow I suspect the error code
doesn't matter at all.

	David

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