linux-kernel - Re: [RFC PATCH] string.h: Add stracpy/stracpy_pad (was: Re: [PATCH] checkpatch: Added warnings in favor of strscpy().)

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Message-ID: <b9bb5550b264d4b29b2b20f7ff8b1b40d20def6a.camel@perches.com>
Date:   Mon, 22 Jul 2019 10:43:50 -0700
From:   Joe Perches <joe@...ches.com>
To:     Kees Cook <keescook@...omium.org>
Cc:     Nitin Gote <nitin.r.gote@...el.com>, akpm@...ux-foundation.org,
        corbet@....net, apw@...onical.com, linux-doc@...r.kernel.org,
        linux-kernel@...r.kernel.org, kernel-hardening@...ts.openwall.com,
        Rasmus Villemoes <rasmus.villemoes@...vas.dk>
Subject: Re: [RFC PATCH] string.h: Add stracpy/stracpy_pad (was: Re: [PATCH]
 checkpatch: Added warnings in favor of strscpy().)

On Mon, 2019-07-22 at 10:33 -0700, Kees Cook wrote:
> On Thu, Jul 04, 2019 at 05:15:57PM -0700, Joe Perches wrote:
> > On Thu, 2019-07-04 at 13:46 -0700, Joe Perches wrote:
> > > On Thu, 2019-07-04 at 11:24 +0530, Nitin Gote wrote:
> > > > Added warnings in checkpatch.pl script to :
> > > > 
> > > > 1. Deprecate strcpy() in favor of strscpy().
> > > > 2. Deprecate strlcpy() in favor of strscpy().
> > > > 3. Deprecate strncpy() in favor of strscpy() or strscpy_pad().
> > > > 
> > > > Updated strncpy() section in Documentation/process/deprecated.rst
> > > > to cover strscpy_pad() case.
> > 
> > []
> > 
> > I sent a patch series for some strscpy/strlcpy misuses.
> > 
> > How about adding a macro helper to avoid the misuses like:
> > ---
> >  include/linux/string.h | 16 ++++++++++++++++
> >  1 file changed, 16 insertions(+)
> > 
> > diff --git a/include/linux/string.h b/include/linux/string.h
> > index 4deb11f7976b..ef01bd6f19df 100644
> > --- a/include/linux/string.h
> > +++ b/include/linux/string.h
> > @@ -35,6 +35,22 @@ ssize_t strscpy(char *, const char *, size_t);
> >  /* Wraps calls to strscpy()/memset(), no arch specific code required */
> >  ssize_t strscpy_pad(char *dest, const char *src, size_t count);
> >  
> > +#define stracpy(to, from)					\
> > +({								\
> > +	size_t size = ARRAY_SIZE(to);				\
> > +	BUILD_BUG_ON(!__same_type(typeof(*to), char));		\
> > +								\
> > +	strscpy(to, from, size);				\
> > +})
> > +
> > +#define stracpy_pad(to, from)					\
> > +({								\
> > +	size_t size = ARRAY_SIZE(to);				\
> > +	BUILD_BUG_ON(!__same_type(typeof(*to), char));		\
> > +								\
> > +	strscpy_pad(to, from, size);				\
> > +})
> > +
> >  #ifndef __HAVE_ARCH_STRCAT
> >  extern char * strcat(char *, const char *);
> >  #endif
> 
> This seems like a reasonable addition, yes. I think Coccinelle might
> actually be able to find all the existing strscpy(dst, src, sizeof(dst))
> cases to jump-start this conversion.

I did that.  It works.  It's a lot of conversions.

$ cat str.cpy.cocci
@@
expression e1;
expression e2;
@@

- strscpy(e1, e2, sizeof(e1))
+ stracpy(e1, e2)

@@
expression e1;
expression e2;
@@

- strlcpy(e1, e2, sizeof(e1))
+ stracpy(e1, e2)

> Devil's advocate: this adds yet more string handling functions... will
> this cause even more confusion?

Documentation is good.
Actual in-kernel use and examples better.