linux-kernel - Re: [PATCH v2] char: hw_random: core.c: Changed from using strncat to strlcat

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Date:	Tue, 14 Oct 2014 11:36:06 -0400
From:	Jason Cooper <jason@...edaemon.net>
To:	Rickard Strandqvist <rickard_strandqvist@...ctrumdigital.se>
Cc:	Matt Mackall <mpm@...enic.com>,
	Herbert Xu <herbert@...dor.apana.org.au>,
	Torsten Duwe <duwe@....de>, Theodore Ts'o <tytso@....edu>,
	Amit Shah <amit.shah@...hat.com>,
	Stephen Boyd <sboyd@...eaurora.org>,
	Paul Gortmaker <paul.gortmaker@...driver.com>,
	Kees Cook <keescook@...omium.org>,
	Dan Carpenter <dan.carpenter@...cle.com>,
	"linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>
Subject: Re: [PATCH v2] char: hw_random: core.c: Changed from using strncat
 to strlcat

On Mon, Oct 13, 2014 at 11:20:35PM +0200, Rickard Strandqvist wrote:
> 2014-10-12 21:22 GMT+02:00 Jason Cooper <jason@...edaemon.net>:
> > Rickard,
> >
> > On Sun, Oct 12, 2014 at 12:49:31PM +0200, Rickard Strandqvist wrote:
> >> Changed from using strncat to strlcat to simplify the code
> >
> > I'd like to see a little more explicit discussion here.  As Guenter got
> > caught up in the mis-understanding, I doubt he'd be the only one.  I
> > think it's worth spelling out that the old code prevents overflowing the
> > buffer 'buf' of size PAGE_SIZE.  And that strlcat() does that internally
> > allowing this code to be more readable.
> >
> > It should also be mentioned that the final strlen(buf) is safe because
> > every operation on buf will insert a NULL terminator within the
> > buffers limit.
> >
> > thx,
> >
> > Jason.
> >
> >> Signed-off-by: Rickard Strandqvist <rickard_strandqvist@...ctrumdigital.se>
> >> ---

[1]

> >>  drivers/char/hw_random/core.c |   12 ++++--------
> >>  1 file changed, 4 insertions(+), 8 deletions(-)
> >>
> >> diff --git a/drivers/char/hw_random/core.c b/drivers/char/hw_random/core.c
> >> index aa30a25..1500cfd 100644
> >> --- a/drivers/char/hw_random/core.c
> >> +++ b/drivers/char/hw_random/core.c
> >> @@ -281,7 +281,6 @@ static ssize_t hwrng_attr_available_show(struct device *dev,
> >>                                        char *buf)
> >>  {
> >>       int err;
> >> -     ssize_t ret = 0;
> >>       struct hwrng *rng;
> >>
> >>       err = mutex_lock_interruptible(&rng_mutex);
> >> @@ -289,16 +288,13 @@ static ssize_t hwrng_attr_available_show(struct device *dev,
> >>               return -ERESTARTSYS;
> >>       buf[0] = '\0';
> >>       list_for_each_entry(rng, &rng_list, list) {
> >> -             strncat(buf, rng->name, PAGE_SIZE - ret - 1);
> >> -             ret += strlen(rng->name);
> >> -             strncat(buf, " ", PAGE_SIZE - ret - 1);
> >> -             ret++;
> >> +             strlcat(buf, rng->name, PAGE_SIZE);
> >> +             strlcat(buf, " ", PAGE_SIZE);
> >>       }
> >> -     strncat(buf, "\n", PAGE_SIZE - ret - 1);
> >> -     ret++;
> >> +     strlcat(buf, "\n", PAGE_SIZE);
> >>       mutex_unlock(&rng_mutex);
> >>
> >> -     return ret;
> >> +     return strlen(buf);
> >>  }
> >>
> >>  static DEVICE_ATTR(rng_current, S_IRUGO | S_IWUSR,
> >> --
> >> 1.7.10.4
> 
> 
> Hi
> 
> Do not know if I understand this right, you want to explain strlcat
> function better then ..?

I want to see that the submitter of the patch has thought this through
and isn't just blindly doing s/strn/strl/g and some cleanup.

Please keep in mind that the kernel community is *huge* and no one
person can see everything going on.  The type of fixes and cleanup
you're doing crosses many sub-systems.  As a result, you haven't popped
up on anyones radar as a regular contributor within a sub-system yet.

iow, I didn't have the thought in my head "Rickard, yeah, he's the guy
doing the cppcheck and strn/l cleanup properly" because none of your
patches have crossed my inbox until now.

> But while I think this is something you have to learn, rather than
> typing it in git comment.

Wether it's appropriate for the git comment or not is debatable, I'll
agree.  The point I'm trying to make is that reviewers aren't
super-human.  All I saw at first is a patch from someone I don't know
changing buffer handling in crypto/rng code.  I had no indication in the
email as to how carefully this had been done.  I'll call that out every
time. :)

A short explanation, here [1], would have let first-time reviewers of
your patches know that you had taken the time to grok the code and
wasn't blindly fulfilling a eudyptula challenge or similar.


thx,

Jason.
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