linux-kernel - Re: revert "fs/befs/linuxvfs.c: replace strncpy by strlcpy"

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Date:	Tue, 28 Apr 2015 22:16:33 +0200 (CEST)
From:	Fabian Frederick <fabf@...net.be>
To:	Al Viro <viro@...IV.linux.org.uk>
Cc:	linux-kernel@...r.kernel.org,
	Linus Torvalds <torvalds@...ux-foundation.org>
Subject: Re: revert "fs/befs/linuxvfs.c: replace strncpy by strlcpy"



> On 28 April 2015 at 19:39 Al Viro <viro@...IV.linux.org.uk> wrote:
>
>
> On Tue, Apr 28, 2015 at 06:42:10PM +0200, Fabian Frederick wrote:
> >
> >
> > > On 28 April 2015 at 18:05 Al Viro <viro@...IV.linux.org.uk> wrote:
> > >
> > >
> > > On Tue, Apr 28, 2015 at 07:35:10AM +0200, Fabian Frederick wrote:
> > >
> > > > > Al, very unhappy about the prospect of looking through ~2000 calls of
> > > > > strlcpy()
> > > > > we have in the tree...
> > > >
> > > > Sorry Al, I thought it was more secure.
> > >
> > > It's not just you, unfortunately, and dumping all that annoyance on you
> > > as a proxy for everyone who does that kind of thing had been unfair.
> > > My apologies...
> >
> > No problem Al :) but why can't we harden strlcpy at first with
> > something like a strlen limited to max char.
> > (I don't know if it's already in kernel libs).
> >
> > size_t strlenl(const char *s, size_t maxlen)
>
> aka strnlen()
>
> >         const char *sc = s;
> >         size_t i = 0;
> >
> >         while (*sc != '\0' && (i < maxlen)) {
> >                 i++;
> >                 sc++;
> >         }
> >         return sc - s;
> > }
> >
> > Then we could solve problems downstream ...
>
> Can't.  Seriously, look what strlcpy() is supposed to return; it's pretty
> much a microoptimized snprintf(dst, size, "%s", src).  It's certainly
> been patterned after snprintf(3) - "don't exceed that size, NUL-terminate
> unless the size is zero, return the number of characters (excluding NUL)
> that would've been written if the size had been large enough".
>
> The following is a legitimate use of strlcpy():
>
> int foo(char *);      /* modifies string */
>
> int const_foo(const char *s)
> {
>       int res;
>       char buf[32], *p = buf;
>       size_t wanted = strlcpy(buf, sizeof(buf), s);
>       if (wanted >= sizeof(buf)) {
>               p = malloc(wanted + 1);
>               if (!p)
>                       return -ENOMEM;
>               memcpy(p, s, wanted + 1);
>       }
>       res = foo(p);
>       if (p != buf)
>               free(p);
>       return res;
> }
>
> None of the kernel callers are of exactly that form (and most ignore the
> return value completely), but if we make that sucker return something
> different from what strlcpy(3) would return, we'd damn better _not_ keep
> the name; there's enough confusion in that area as it is.
Of course with another function name. There's no other way to do it ...
strlncpy/strlncat ? :)

Regards,
Fabian
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