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

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Message-ID: <20150428034859.GI889@ZenIV.linux.org.uk>
Date:	Tue, 28 Apr 2015 04:48:59 +0100
From:	Al Viro <viro@...IV.linux.org.uk>
To:	Linus Torvalds <torvalds@...ux-foundation.org>
Cc:	Fabian Frederick <fabf@...net.be>, linux-kernel@...r.kernel.org
Subject: revert "fs/befs/linuxvfs.c: replace strncpy by strlcpy"

commit 39d7a29f867bd5a4a551fad6bb3812ceddb0bce1
Author: Fabian Frederick <fabf@...net.be>
Date:   Fri Jun 6 14:36:15 2014 -0700

    fs/befs/linuxvfs.c: replace strncpy by strlcpy

    strncpy + end of string assignment replaced by strlcpy

replaces perfectly safe code with undefined behaviour.  All in the name
of "security hardening", presumably.  Folks, seeing the words "designed to be
safer, more consistent, and less error prone replacement" in a manpage does
*NOT* mean "OK, quit reading it - no need to go further, not even to the end
of the paragraph".  Because in the end of that paragraph it says "This means
that for strlcpy() src must be NUL-terminated".  And sure enough, our
implementation relies on that - it starts with strlen().

strncpy() is guaranteed not to look further than size.  strlcpy() is *NOT*.
strncpy() on unvalidated source is safe, provided that you sanitize the copy;
strlcpy() on anything like that is an invitation for nasal daemons.

Sure, we can (and probably should) make strlcpy(dst, src, n) never access
memory beyond src + n - 1, but this kind of cargo-culting is a Bad Thing(tm);
mindless "security improvements" without so much as bothering to RTFM are
asking for trouble.  And in userland code anything like that _can't_ be
papered over afterwards - not unless you can patch every libc implementation
out there.

This particular code is completely pointless - if anything, it should've been
memcpy() + nd_terminate_link()...

Al, very unhappy about the prospect of looking through ~2000 calls of strlcpy()
we have in the tree...
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