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Date: Mon, 5 Oct 2015 18:28:02 +0200
From: Ingo Molnar <mingo@kernel.org>
To: Linus Torvalds <torvalds@linux-foundation.org>
Cc: Alexey Dobriyan <adobriyan@gmail.com>, Chris Metcalf <cmetcalf@ezchip.com>,
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        Thomas Gleixner <tglx@linutronix.de>, "H. Peter Anvin" <hpa@zytor.com>,
        Borislav Petkov <bp@alien8.de>
Subject: Re: [PATCH] string: Improve the generic strlcpy() implementation
Message-ID: <20151005162802.GA11474@gmail.com>
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* Ingo Molnar <mingo@kernel.org> wrote:

> We could do something like:
> 
>                 c = *(unsigned long *)(src+res);
>                 *(unsigned long *)(dest+res) = c;
> 
>                 if (has_zero(c, &data, &constants)) {
> 			unsigned int zero_pos;
> 
>                         data = prep_zero_mask(c, data, &constants);
>                         data = create_zero_mask(data);
> 
> 			zero_pos = find_zero(data);
> 			res += zero_pos;
> 
> 			memset(dest+res, 0, sizeof(long)-zero_pos);
> 
>                         return res;
>                 }
> 
> I.e. the extra memset() clears out the partial word (if any) after the NUL.

A slightly more paranoid version would be:

		c = *(unsigned long *)(src+res);
 
		if (has_zero(c, &data, &constants)) {
 			unsigned int zero_pos;
 
			data = prep_zero_mask(c, data, &constants);
			data = create_zero_mask(data);
 
 			zero_pos = find_zero(data);

			/* Clear out undefined data within the final word after the NUL: */ 
 			memset((void *)&c + zero_pos, 0, sizeof(long)-zero_pos);

			*(unsigned long *)(dest+res) = c;
 
			return res+zero_pos;
		}
		*(unsigned long *)(dest+res) = c;

This would solve any theoretical races in the _target_ buffer: if the target 
buffer may be copied to user-space in a racy fashion and we don't ever want it to 
have undefined data, then this variant does the tail-zeroing of the final word in 
the temporary copy, not in the target buffer.

Still untested.

Thanks,

	Ingo