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From:   David Laight <David.Laight@ACULAB.COM>
To:     'Daniel Xu' <dxu@dxuuu.xyz>,
        "bpf@vger.kernel.org" <bpf@vger.kernel.org>,
        "linux-kernel@vger.kernel.org" <linux-kernel@vger.kernel.org>,
        "ast@kernel.org" <ast@kernel.org>,
        "daniel@iogearbox.net" <daniel@iogearbox.net>
CC:     "kernel-team@fb.com" <kernel-team@fb.com>
Subject: RE: [PATCH bpf v2 1/2] lib/strncpy_from_user.c: Don't overcopy bytes
 after NUL terminator
Thread-Topic: [PATCH bpf v2 1/2] lib/strncpy_from_user.c: Don't overcopy bytes
 after NUL terminator
Thread-Index: AQHWsxsIE6fPG3mijk+FzhsUr3fzDam5OvTg
Date:   Thu, 5 Nov 2020 09:00:00 +0000
Message-ID: <cbe00e1421db4566a076102506b6d670@AcuMS.aculab.com>
References: <cover.1604542786.git.dxu@dxuuu.xyz>
 <487a07aa911b4e822a0b931f7b33a4f67fedb6bd.1604542786.git.dxu@dxuuu.xyz>
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From: Daniel Xu
> Sent: 05 November 2020 02:26
...
> --- a/lib/strncpy_from_user.c
> +++ b/lib/strncpy_from_user.c
> @@ -35,17 +35,22 @@ static inline long do_strncpy_from_user(char *dst, const char __user *src,
>  		goto byte_at_a_time;
> 
>  	while (max >= sizeof(unsigned long)) {
> -		unsigned long c, data;
> +		unsigned long c, data, mask, *out;
> 
>  		/* Fall back to byte-at-a-time if we get a page fault */
>  		unsafe_get_user(c, (unsigned long __user *)(src+res), byte_at_a_time);

It's not related to this change, but since both addresses
are aligned (checked earlier) a page fault on the word read
is fatal.

	David

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