[Ksummit-discuss] uninitialized variables bugs

[Ksummit-discuss] uninitialized variables bugs

[Dan Carpenter]

Ever since commit 78a5255ffb6a ("Stop the ad-hoc games with
-Wno-maybe-initialized"), GCC's uninitialized variable warnings have
been disabled by default.  Now, you have to turn on W=1 or W=2 to see
the warnings which nobody except Arnd does.

Disabling that has lead to a bunch of embarrassing bugs where variables
are *never* initialized.  Very unsubtle bugs.  The bugs doesn't reach
users because Nathan Chancellor and I review Clang and Smatch warnings
respectively.  Also the kbuild-bot reports uninitialized variables.

It's a lot to deal with.  Uninitialized variable bugs are probably the
most common bug I have to deal with.

It's frustrating.  Sometimes the false positives are hard to analyse
because I have to read through multiple functions.  A lot of times
when I write a patch and a commit message Nathan has already fixed it
so it's just a waste of time.

It's risky as well.  The Smatch check for uninitialized variables was
broken for most of 2021.  Nathan sometimes goes on vacation.

I guess I would hope that one day we can turn on the GCC uninitialized
variable warnings again.  That would mean silencing false positives
which a lot of people don't want to do...  Maybe Clang has fewer false
positives than GCC?

The Smatch check for uninitialized variable was deliberately written to
be more strict than GCC because GCC was missing bugs.  So I think
leaving Smatch false positives is fine.  There is a trade off between
fewer false positives and missing bugs and Smatch is meant to err on the
side of finding bugs but with the cost of false positives.

Most of the Smatch uninitialized false positives are caused by loops:

	int i, ret;

	for (i = 0; i < bytes; i++) { // <-- what if bytes is zero?
		if (...)
			continue; // <-- can every iteration hit continue?
		ret = frob();
	}

	return ret;

There is also stuff like this which is harmless:

	uint val;

	ret = read(&val);
	*p = val;  // <-- uninitialized variable if read() fails
	return ret;

Btw, here is how to run Smatch on your code:
https://staticthinking.wordpress.com/2022/04/25/how-to-run-smatch-on-your-code/

regards,
dan carpenter

[Ksummit-discuss] uninitialized variables bugs

[Julia Lawall]

On Fri, 6 May 2022, Dan Carpenter wrote:

> Ever since commit 78a5255ffb6a ("Stop the ad-hoc games with
> -Wno-maybe-initialized"), GCC's uninitialized variable warnings have
> been disabled by default.  Now, you have to turn on W=1 or W=2 to see
> the warnings which nobody except Arnd does.
>
> Disabling that has lead to a bunch of embarrassing bugs where variables
> are *never* initialized.  Very unsubtle bugs.  The bugs doesn't reach
> users because Nathan Chancellor and I review Clang and Smatch warnings
> respectively.  Also the kbuild-bot reports uninitialized variables.
>
> It's a lot to deal with.  Uninitialized variable bugs are probably the
> most common bug I have to deal with.
>
> It's frustrating.  Sometimes the false positives are hard to analyse
> because I have to read through multiple functions.  A lot of times
> when I write a patch and a commit message Nathan has already fixed it
> so it's just a waste of time.
>
> It's risky as well.  The Smatch check for uninitialized variables was
> broken for most of 2021.  Nathan sometimes goes on vacation.
>
> I guess I would hope that one day we can turn on the GCC uninitialized
> variable warnings again.  That would mean silencing false positives
> which a lot of people don't want to do...  Maybe Clang has fewer false
> positives than GCC?
>
> The Smatch check for uninitialized variable was deliberately written to
> be more strict than GCC because GCC was missing bugs.  So I think
> leaving Smatch false positives is fine.  There is a trade off between
> fewer false positives and missing bugs and Smatch is meant to err on the
> side of finding bugs but with the cost of false positives.
>
> Most of the Smatch uninitialized false positives are caused by loops:
>
> 	int i, ret;
>
> 	for (i = 0; i < bytes; i++) { // <-- what if bytes is zero?
> 		if (...)
> 			continue; // <-- can every iteration hit continue?
> 		ret = frob();
> 	}
>
> 	return ret;
>
> There is also stuff like this which is harmless:
>
> 	uint val;
>
> 	ret = read(&val);
> 	*p = val;  // <-- uninitialized variable if read() fails
> 	return ret;
>
> Btw, here is how to run Smatch on your code:
> https://staticthinking.wordpress.com/2022/04/25/how-to-run-smatch-on-your-code/

Could smatch inform the user that some results are likely false positives,
or even order the results according to their likely true positiveness?

julia

[Ksummit-discuss] uninitialized variables bugs

[Arnd Bergmann]

On Fri, May 6, 2022 at 11:13 AM Dan Carpenter <dan.carpenter@oracle.com> wrote:

>
> It's frustrating.  Sometimes the false positives are hard to analyse
> because I have to read through multiple functions.  A lot of times
> when I write a patch and a commit message Nathan has already fixed it
> so it's just a waste of time.

Agreed. I'm not actually checking for those warnings on gcc any more,
but just the clang warnings point to a bigger problem.

> It's risky as well.  The Smatch check for uninitialized variables was
> broken for most of 2021.  Nathan sometimes goes on vacation.
>
> I guess I would hope that one day we can turn on the GCC uninitialized
> variable warnings again.  That would mean silencing false positives
> which a lot of people don't want to do...  Maybe Clang has fewer false
> positives than GCC?

I think for the gcc warnings to become useful again, we may have to
wait for a future compiler release. I have not checked gcc-12 for this,
but it's a very old topic.

Fundamentally, it's impossible for any compiler to do this correctly,
because of the halting problem. gcc apparently has some heuristics
that worked well enough in the past, but it misses some obvious
cases and causes false positives in unexpected places, often
depending on optimization flags.

Recent gcc versions are much worse than older ones, since the
inlining changed in a way that caused a ton of false-positives.

clang is generally better at catching the simple cases reliably,
and it does this independent of optimization flags. However, it
stops at the function boundary, so it never catches some of the
cases that gcc was good at.

The gcc static analyzer apparently gained an option[1] that
works similarly to what you have in smatch. I have not tried
using this, but this may be something we can do in CI
systems that may not want to run smatch for some reason.

        Arnd

[1] https://gcc.gnu.org/onlinedocs/gcc-12.1.0/gcc/Static-Analyzer-Options.html#index-Wanalyzer-use-of-uninitialized-value

[Ksummit-discuss] uninitialized variables bugs

[Shuah Khan]

On 5/6/22 5:56 AM, Arnd Bergmann wrote:
> On Fri, May 6, 2022 at 11:13 AM Dan Carpenter <dan.carpenter@oracle.com> wrote:
> 
>>
>> It's frustrating.  Sometimes the false positives are hard to analyse
>> because I have to read through multiple functions.  A lot of times
>> when I write a patch and a commit message Nathan has already fixed it
>> so it's just a waste of time.
> 
> Agreed. I'm not actually checking for those warnings on gcc any more,
> but just the clang warnings point to a bigger problem.
> 
>> It's risky as well.  The Smatch check for uninitialized variables was
>> broken for most of 2021.  Nathan sometimes goes on vacation.
>>
>> I guess I would hope that one day we can turn on the GCC uninitialized
>> variable warnings again.  That would mean silencing false positives
>> which a lot of people don't want to do...  Maybe Clang has fewer false
>> positives than GCC?
> 

I would like to throw resource leak bugs in the mix. I am finding cppcheck
has been effective in finding them.

I am seeing a lot of file pointer leaks  in error legs in kselftest code error
paths. I have a few fixes in the works to send out.

We could discuss this topic at the LPC Kernel Testing and Dependability mini-conf
as well.

thanks,
-- Shuah

[Ksummit-discuss] uninitialized variables bugs

[Krzysztof Kozlowski]

On 06/05/2022 11:13, Dan Carpenter wrote:
> There is also stuff like this which is harmless:
> 
> 	uint val;
> 
> 	ret = read(&val);
> 	*p = val;  // <-- uninitialized variable if read() fails
> 	return ret;
> 
> Btw, here is how to run Smatch on your code:
> https://staticthinking.wordpress.com/2022/04/25/how-to-run-smatch-on-your-code/

In the topic of suppressing false positives we also have several
"fixes", sometimes pointed out incorrectly by Coverity, for missing
check for of_device_get_match_data().

Compare:
https://elixir.bootlin.com/linux/v5.18-rc7/source/drivers/clk/clk-aspeed.c#L415
https://elixir.bootlin.com/linux/v5.18-rc7/source/drivers/clk/clk-oxnas.c#L216

Although in theory the of_device_get_match_data() can return NULL, in
practice it is not possible because driver matches via OF thus there
will be always of_device_id->driver data.

Coverity screams about it, people fix it by adding checks for NULL,
which is pointless. Half of drivers add the !NULL check, half do not...

Best regards,
Krzysztof