linux-kernel - RE: [PATCH v2 3/4] fs: unicode: Use strscpy() instead of strncpy()

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Date:   Thu, 18 Mar 2021 15:40:53 +0000
From:   David Laight <David.Laight@...LAB.COM>
To:     'Shreeya Patel' <shreeya.patel@...labora.com>,
        "krisman@...labora.com" <krisman@...labora.com>,
        "jaegeuk@...nel.org" <jaegeuk@...nel.org>,
        "yuchao0@...wei.com" <yuchao0@...wei.com>,
        "tytso@....edu" <tytso@....edu>,
        "adilger.kernel@...ger.ca" <adilger.kernel@...ger.ca>,
        "drosen@...gle.com" <drosen@...gle.com>,
        "ebiggers@...gle.com" <ebiggers@...gle.com>
CC:     "linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>,
        "linux-fsdevel@...r.kernel.org" <linux-fsdevel@...r.kernel.org>,
        "kernel@...labora.com" <kernel@...labora.com>,
        "andre.almeida@...labora.com" <andre.almeida@...labora.com>,
        kernel test robot <lkp@...el.com>
Subject: RE: [PATCH v2 3/4] fs: unicode: Use strscpy() instead of strncpy()

From: Shreeya Patel
> Sent: 18 March 2021 14:13
> 
> On 18/03/21 7:03 pm, Shreeya Patel wrote:
> > Following warning was reported by Kernel Test Robot.
> >
> > In function 'utf8_parse_version',
> > inlined from 'utf8_load' at fs/unicode/utf8mod.c:195:7:
> >>> fs/unicode/utf8mod.c:175:2: warning: 'strncpy' specified bound 12 equals
> > destination size [-Wstringop-truncation]
> > 175 |  strncpy(version_string, version, sizeof(version_string));
> >      |  ^~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
> >
> > The -Wstringop-truncation warning highlights the unintended
> > uses of the strncpy function that truncate the terminating NULL
> > character from the source string.
> > Unlike strncpy(), strscpy() always null-terminates the destination string,
> > hence use strscpy() instead of strncpy().
> 
> 
> Not sure if strscpy is preferable. Just found this article
> https://lwn.net/Articles/659214/
> Should I go for memcpy instead?

Which length would you give memcpy() ?
The compiler will moan if you try to read beyond the end of the
input string.

strscpy() is about the best of a bad lot.

I think (I'm not sure!) that a good string copy function should
return the number of bytes copies or the buffer length is truncated.
Then you can do repeated:
	off += xxxcpy(buf + off, buflen - off, xxxxx);
without any danger of writing beyond the buffer end, always
getting a '\0' terminated string, and being able to detect overflow
right at the end.

	David

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