Re: [PATCH] Fix some sparse warnings

From: Stefan Beller <stefanbeller@googlemail.com>
Date: Jul 16, 2013, 21:18 UTC
Message-ID: <51E5B8BD.8090202@googlemail.com>
In-Reply-To: <6BDA2E3E7318418BBB2C19B475B2B118@PhilipOakley>

On 07/16/2013 10:53 PM, Philip Oakley wrote:

Show 60 quoted lines

> From: "Jeff King" <peff@peff.net>
> Sent: Tuesday, July 16, 2013 7:21 AM
>> On Tue, Jul 16, 2013 at 07:57:20AM +0200, Johannes Sixt wrote:
>>
>>> Am 7/15/2013 19:31, schrieb Ramsay Jones:
>>> > Sparse issues three "Using plain integer as NULL pointer" warnings.
>>> > Each warning relates to the use of an '{0}' initialiser expression
>>> > in the declaration of an 'struct object_info'.
>>>
>>> I question the value of this warning. Initialization with '= {0}' is
>>> a
>>> well-established idiom, and sparse should know about it. Also, plain
>>> 0
>>> *is* a null pointer constant.
>>
>> I agree with you. It's not a bug, and I think sparse is being overly
>> picky here; it is missing the forest for the trees in interpreting the
>> idiom.
>>
>> Still, it may be worth tweaking in the name of eliminating compiler
>> noise, since it does not cost us very much to do so (and I believe we
>> have done so in the past, too).
>>
>> We could also ask people with sparse to turn off the "use NULL instead
>> of 0" warning, but I think it _is_ a useful warning elsewhere (even
>> though it is never a bug, it violates our style guidelines and may be
>> an
>> indication of a bug). It would be nice if sparse learned to ignore the
>> warning in this particular idiom, but I am not going to hold my breath
>> for that.
>>
>> -Peff
>> -- 
> 
> On the subject of warnings and null pointers, yesterday's Code Project
> news linked to a blog on the problems of unexpected optimization bugs,
> such as dereferencing a null pointer. "Finding Undefined Behavior Bugs
> by Finding Dead Code" http://blog.regehr.org/archives/970 which links to
> the draft of an interesting paper
> [http://pdos.csail.mit.edu/~xi/papers/stack-sosp13.pdf]
> 
> Does anyone run the "new static checker called 'Stack' that precisely
> identifies unstable code"? [though the paper's conclusion says 'All
> Stack source code will be publicly available.' which suggests it's not
> yet available]
> 
> Or use the ‘-fno-delete-null-pointer-checks’ referred to in the blog
> comments (see also index : kernel/git/torvalds/linux.git "Add
> '-fno-delete-null-pointer-checks' to gcc CFLAGS"
> https://git.kernel.org/cgit/linux/kernel/git/torvalds/linux.git/commit/?id=a3ca86aea507904148870946d599e07a340b39bf
> 
> 
> You're probably already aware of these aspects but I thought it worth
> mentioning for the wider readership.
> 
> regards
> 
> Philip
> 
>

I recently started contributing and I used cppcheck found at https://github.com/danmar/cppcheck to submit the patches at origin/sb/misc-fixes As it is originally for C++, that tool throws lots of false-positives (i.e. warns about null pointer dereferencing when it's not possible to be a null pointer) unfortunately.

Also I hear llvm/clang comes with a good static code analyzer, which I tried today on a different project. Though I could not figure out how to use that on a pure C project such as git, as that tool seems to require a C++ compilation for doing its static code analysis.

The blog post you linked to seems very interesting and promising. :)

Regards, Stefan