Re: [PATCH V2 1/2] string: Add stracpy and stracpy_pad mechanisms

From: Joe Perches <joe@perches.com>
To: Rasmus Villemoes <linux@rasmusvillemoes.dk>,
	Linus Torvalds <torvalds@linux-foundation.org>,
	linux-kernel@vger.kernel.org
Cc: Jonathan Corbet <corbet@lwn.net>, Stephen Kitt <steve@sk2.org>,
	Kees Cook <keescook@chromium.org>,
	Nitin Gote <nitin.r.gote@intel.com>,
	jannh@google.com, kernel-hardening@lists.openwall.com,
	Andrew Morton <akpm@linux-foundation.org>
Subject: Re: [PATCH V2 1/2] string: Add stracpy and stracpy_pad mechanisms
Date: Tue, 23 Jul 2019 08:39:49 -0700	[thread overview]
Message-ID: <c9ef2b56eaf36c8e5449b751ab6e5971b6b34311.camel@perches.com> (raw)
In-Reply-To: <ce1320d8-60df-7c54-2348-6aabac63c24d@rasmusvillemoes.dk>

On Tue, 2019-07-23 at 16:37 +0200, Rasmus Villemoes wrote:
> On 23/07/2019 15.51, Joe Perches wrote:
> > Several uses of strlcpy and strscpy have had defects because the
> > last argument of each function is misused or typoed.
> > 
> > Add macro mechanisms to avoid this defect.
> > 
> > stracpy (copy a string to a string array) must have a string
> > array as the first argument (dest) and uses sizeof(dest) as the
> > count of bytes to copy.
> > 
> > These mechanisms verify that the dest argument is an array of
> > char or other compatible types like u8 or s8 or equivalent.
> Sorry, but "compatible types" has a very specific meaning in C, so
> please don't use that word.

I think you are being overly pedantic here but
what wording do you actually suggest?

>  And yes, the kernel disables -Wpointer-sign,
> so passing an u8* or s8* when strscpy() expects a char* is silently
> accepted, but does such code exist?

u8 definitely, s8 I'm not sure.

I don't find via grep a use of s8 foo[] = "bar";
or "signed char foo[] = "bar";

I don't think it bad to allow it.

> > V2: Use __same_type testing char[], signed char[], and unsigned char[]
> >     Rename to, from, and size, dest, src and count
> 
> count is just as bad as size in terms of "the expression src might
> contain that identifier". But there's actually no reason to even declare
> a local variable, just use ARRAY_SIZE() directly as the third argument
> to strscpy().

I don't care about that myself.
It's a macro local identifier and shadowing in a macro
is common.  I'm not a big fan of useless underscores.

I think either works.