netdev - Re: [iproute PATCH 1/6] utils: Implement strlcpy() and strlcat()

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Date:   Wed, 6 Sep 2017 08:25:55 -0700
From:   Stephen Hemminger <stephen@...workplumber.org>
To:     David Laight <David.Laight@...LAB.COM>
Cc:     Phil Sutter <phil@....cc>,
        "netdev@...r.kernel.org" <netdev@...r.kernel.org>
Subject: Re: [iproute PATCH 1/6] utils: Implement strlcpy() and strlcat()

On Wed, 6 Sep 2017 13:59:27 +0000
David Laight <David.Laight@...LAB.COM> wrote:

> From: Stephen Hemminger [mailto:stephen@...workplumber.org]
> > Sent: 04 September 2017 19:25
> > On Mon, 4 Sep 2017 17:00:15 +0200
> > Phil Sutter <phil@....cc> wrote:
> >   
> > > On Mon, Sep 04, 2017 at 02:49:20PM +0000, David Laight wrote:  
> > > > From: Phil Sutter  
> > > > > Sent: 01 September 2017 17:53
> > > > > By making use of strncpy(), both implementations are really simple so
> > > > > there is no need to add libbsd as additional dependency.
> > > > >  
> > > > ...  
> > > > > +
> > > > > +size_t strlcpy(char *dst, const char *src, size_t size)
> > > > > +{
> > > > > +	if (size) {
> > > > > +		strncpy(dst, src, size - 1);
> > > > > +		dst[size - 1] = '\0';
> > > > > +	}
> > > > > +	return strlen(src);
> > > > > +}  
> > > >
> > > > Except that isn't really strlcpy().
> > > > Better would be:
> > > > 	len = strlen(src) + 1;
> > > > 	if (len <= size)
> > > > 		memcpy(dst, src, len);
> > > > 	else if (size) {
> > > > 		dst[size - 1] = 0;
> > > > 		memcpy(dst, src, size - 1);
> > > > 	}
> > > > 	return len - 1;  
> > >
> > > Please elaborate: Why isn't my version "really" strlcpy()? Why is your
> > > proposed version better?
> > >
> > > Thanks, Phil  
> > 
> > Linux kernel:
> > size_t strlcpy(char *dest, const char *src, size_t size)
> > {
> > 	size_t ret = strlen(src);
> > 
> > 	if (size) {
> > 		size_t len = (ret >= size) ? size - 1 : ret;
> > 		memcpy(dest, src, len);
> > 		dest[len] = '\0';
> > 	}
> > 	return ret;
> > }
> > 
> > FreeBSD:
> > size_t
> > strlcpy(char * __restrict dst, const char * __restrict src, size_t dsize)
> > {
> > 	const char *osrc = src;
> > 	size_t nleft = dsize;
> > 
> > 	/* Copy as many bytes as will fit. */
> > 	if (nleft != 0) {
> > 		while (--nleft != 0) {
> > 			if ((*dst++ = *src++) == '\0')
> > 				break;
> > 		}
> > 	}
> > 
> > 	/* Not enough room in dst, add NUL and traverse rest of src. */
> > 	if (nleft == 0) {
> > 		if (dsize != 0)
> > 			*dst = '\0';		/* NUL-terminate dst */
> > 		while (*src++)
> > 			;
> > 	}
> > 
> > 	return(src - osrc - 1);	/* count does not include NUL */
> > }
> > 
> > 
> > They all give the same results for some basic tests.
> > Test			FreeBSD		Linux		Iproute2
> > "",0:           	0 "JUNK"      	0 "JUNK"      	0 "JUNK"
> > "",1:           	0 ""          	0 ""          	0 ""
> > "",8:           	0 ""          	0 ""          	0 ""
> > "foo",0:        	3 "JUNK"      	3 "JUNK"      	3 "JUNK"
> > "foo",3:        	3 "fo"        	3 "fo"        	3 "fo"
> > "foo",4:        	3 "foo"       	3 "foo"       	3 "foo"
> > "foo",8:        	3 "foo"       	3 "foo"       	3 "foo"
> > "longstring",0: 	10 "JUNK"     	10 "JUNK"     	10 "JUNK"
> > "longstring",8: 	10 "longstr"  	10 "longstr"  	10 "longstr"  
> 
> You need to look at the contents of the destination buffer after the
> first '\0'.
> strlcpy() shouldn't change it.
> 
> 	David

Zeroing the bytes after the first null character should not be a big issue
other than a few nanoseconds extra work.