[patch] Warn on memcmp(a, b, sizeof(a) != 0) & co

[Reid Kleckner]

Nice!  LGTM.  Looking forward to rolling this out over more code.  :)


On Fri, Dec 20, 2013 at 11:31 PM, Nico Weber <thakis at chromium.org> wrote:

> Hi,
>
> the attached patch adds a new warning that makes memcmp & co warn on
> misplaced parentheses such as
>
>   if (memcmp(a, b, sizeof(a) != 0))
>
> like so:
>
> test4.cc:5:30: warning: size argument in 'memcmp' call is a comparison
> [-Wmemsize-comparison]
>   if (memcmp(a, b, sizeof(a) != 0))
>                    ~~~~~~~~~~^~~~
> test4.cc:5:7: note: did you mean to compare the result of 'memcmp' instead?
>   if (memcmp(a, b, sizeof(a) != 0))
>       ^                          ~
>                             )
> test4.cc:5:20: note: explicitly cast the argument to size_t to silence
> this warning
>   if (memcmp(a, b, sizeof(a) != 0))
>                    ^
>                    (size_t)(     )
> 1 warning generated.
>
>
> This would have found one bug in NSS that we recently fixed [1] and found
> one more bug in chromium we didn't know about before [2]. It had 0 false
> positives on all of chromium.
>
> The idea of triggering this warning on a binop in the size argument is due
> to rnk.
>
> This warning can possibly be extended later on, but I feel this is a good
> start.
>
> Thoughts?
>
> Nico
>
> [1]:
> https://codereview.chromium.org/99423002/diff/1/net/third_party/nss/ssl/ssl3con.c
> [2]: https://codereview.chromium.org/8431007/#msg12
>
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