linux-kernel - Re: [PATCH] locomo.c: convert strncpy(x, y, sizeof(x)) to strlcpy

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Message-ID: <47D07FDC.2010701@tiscali.nl>
Date:	Fri, 07 Mar 2008 00:35:56 +0100
From:	Roel Kluin <12o3l@...cali.nl>
To:	"H. Peter Anvin" <hpa@...or.com>
CC:	elf@...ci.com, Linux-arm <linux-arm-kernel@...ts.arm.linux.org.uk>,
	lkml <linux-kernel@...r.kernel.org>
Subject: Re: [PATCH] locomo.c: convert strncpy(x, y, sizeof(x)) to strlcpy

H. Peter Anvin wrote:
> Roel Kluin wrote:
>> This patch was not yet tested. Please confirm it's right.
>> ---
>> strncpy does not append '\0' if the length of the source string equals
>> the size parameter, strlcpy does.
>>
> 
> Are you sure it's safe to not zero out the contents of the buffer (no
> information leak)?
> 
>     -hpa

As I understand it, please correct me if I'm wrong:

Of the three variants: strcpy, strncpy and strlcpy.
- strcpy does not append \0 (unless the source string already contained it)
- strncpy appends \0's if the source string is smaller than the size
  parameter (for all remaining characters)
- strlcpy always appends a single \0 (unless size parameter was 0)

char *strcpy(char *dest, const char *src);
char *strncpy(char *dest, const char *src, size_t n);
size_t strlcpy(char *dst, const char *src, size_t n);

In the original code strncpy was used and the size parameter was equal
to the source string size:

strncpy(dev->dev.bus_id, info->name, sizeof(dev->dev.bus_id));

Since this the size was equal there was no \0 termination. To \0
terminate using strncpy we could write:

strncpy(dev->dev.bus_id, info->name, sizeof(dev->dev.bus_id) - 1);
dev->dev.bus_id[sizeof(dev->dev.bus_id) - 1] = '\0';

or using strlcpy, which does the same thing:

strlcpy(dev->dev.bus_id, info->name, sizeof(dev->dev.bus_id));

Roel
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