Re: [RFC] slab: introduce auto_kfree macro

Re: [RFC] slab: introduce auto_kfree macro

[Peter Zijlstra]

On Wed, Apr 02, 2025 at 01:32:51PM +0300, Andy Shevchenko wrote:
> On Tue, Apr 01, 2025 at 03:44:08PM +0200, Przemek Kitszel wrote:
> > Add auto_kfree macro that acts as a higher level wrapper for manual
> > __free(kfree) invocation, and sets the pointer to NULL - to have both
> > well defined behavior also for the case code would lack other assignement.
> > 
> > Consider the following code:
> > int my_foo(int arg)
> > {
> > 	struct my_dev_foo *foo __free(kfree); /* no assignement */
> > 
> > 	foo = kzalloc(sizeof(*foo), GFP_KERNEL);
> > 	/* ... */
> > }
> > 
> > So far it is fine and even optimal in terms of not assigning when
> > not needed. But it is typical to don't touch (and sadly to don't
> > think about) code that is not related to the change, so let's consider
> > an extension to the above, namely an "early return" style to check
> > arg prior to allocation:
> > int my_foo(int arg)
> > {
> >         struct my_dev_foo *foo __free(kfree); /* no assignement */
> > +
> > +	if (!arg)
> > +		return -EINVAL;
> >         foo = kzalloc(sizeof(*foo), GFP_KERNEL);
> >         /* ... */
> > }
> > Now we have uninitialized foo passed to kfree, what likely will crash.
> > One could argue that `= NULL` should be added to this patch, but it is
> > easy to forgot, especially when the foo declaration is outside of the
> > default git context.

The compiler *should* complain. But neither GCC nor clang actually
appear to warn in this case.

I don't think we should be making dodgy macros like you propose to work
around this compiler deficiency. Instead I would argue we ought to get
both compilers fixed asap, and then none of this will be needed.

Re: [RFC] slab: introduce auto_kfree macro

[Peter Zijlstra]

On Wed, Apr 02, 2025 at 02:19:35PM +0200, Peter Zijlstra wrote:
> On Wed, Apr 02, 2025 at 01:32:51PM +0300, Andy Shevchenko wrote:
> > On Tue, Apr 01, 2025 at 03:44:08PM +0200, Przemek Kitszel wrote:
> > > Add auto_kfree macro that acts as a higher level wrapper for manual
> > > __free(kfree) invocation, and sets the pointer to NULL - to have both
> > > well defined behavior also for the case code would lack other assignement.
> > > 
> > > Consider the following code:
> > > int my_foo(int arg)
> > > {
> > > 	struct my_dev_foo *foo __free(kfree); /* no assignement */
> > > 
> > > 	foo = kzalloc(sizeof(*foo), GFP_KERNEL);
> > > 	/* ... */
> > > }
> > > 
> > > So far it is fine and even optimal in terms of not assigning when
> > > not needed. But it is typical to don't touch (and sadly to don't
> > > think about) code that is not related to the change, so let's consider
> > > an extension to the above, namely an "early return" style to check
> > > arg prior to allocation:
> > > int my_foo(int arg)
> > > {
> > >         struct my_dev_foo *foo __free(kfree); /* no assignement */
> > > +
> > > +	if (!arg)
> > > +		return -EINVAL;
> > >         foo = kzalloc(sizeof(*foo), GFP_KERNEL);
> > >         /* ... */
> > > }
> > > Now we have uninitialized foo passed to kfree, what likely will crash.
> > > One could argue that `= NULL` should be added to this patch, but it is
> > > easy to forgot, especially when the foo declaration is outside of the
> > > default git context.
> 
> The compiler *should* complain. But neither GCC nor clang actually
> appear to warn in this case.
> 
> I don't think we should be making dodgy macros like you propose to work
> around this compiler deficiency. Instead I would argue we ought to get
> both compilers fixed asap, and then none of this will be needed.

Ah, I think the problem is that the cleanup function takes a pointer to
the object, and pointers to uninitialized values are generally
considered okay.

The compilers would have to explicitly disallow this for the cleanup
functions.

Re: [RFC] slab: introduce auto_kfree macro

[Andy Shevchenko]

On Wed, Apr 02, 2025 at 02:22:24PM +0200, Peter Zijlstra wrote:
> On Wed, Apr 02, 2025 at 02:19:35PM +0200, Peter Zijlstra wrote:
> > On Wed, Apr 02, 2025 at 01:32:51PM +0300, Andy Shevchenko wrote:
> > > On Tue, Apr 01, 2025 at 03:44:08PM +0200, Przemek Kitszel wrote:
> > > > Add auto_kfree macro that acts as a higher level wrapper for manual
> > > > __free(kfree) invocation, and sets the pointer to NULL - to have both
> > > > well defined behavior also for the case code would lack other assignement.
> > > > 
> > > > Consider the following code:
> > > > int my_foo(int arg)
> > > > {
> > > > 	struct my_dev_foo *foo __free(kfree); /* no assignement */
> > > > 
> > > > 	foo = kzalloc(sizeof(*foo), GFP_KERNEL);
> > > > 	/* ... */
> > > > }
> > > > 
> > > > So far it is fine and even optimal in terms of not assigning when
> > > > not needed. But it is typical to don't touch (and sadly to don't
> > > > think about) code that is not related to the change, so let's consider
> > > > an extension to the above, namely an "early return" style to check
> > > > arg prior to allocation:
> > > > int my_foo(int arg)
> > > > {
> > > >         struct my_dev_foo *foo __free(kfree); /* no assignement */
> > > > +
> > > > +	if (!arg)
> > > > +		return -EINVAL;
> > > >         foo = kzalloc(sizeof(*foo), GFP_KERNEL);
> > > >         /* ... */
> > > > }
> > > > Now we have uninitialized foo passed to kfree, what likely will crash.
> > > > One could argue that `= NULL` should be added to this patch, but it is
> > > > easy to forgot, especially when the foo declaration is outside of the
> > > > default git context.
> > 
> > The compiler *should* complain. But neither GCC nor clang actually
> > appear to warn in this case.
> > 
> > I don't think we should be making dodgy macros like you propose to work
> > around this compiler deficiency. Instead I would argue we ought to get
> > both compilers fixed asap, and then none of this will be needed.
> 
> Ah, I think the problem is that the cleanup function takes a pointer to
> the object, and pointers to uninitialized values are generally
> considered okay.
> 
> The compilers would have to explicitly disallow this for the cleanup
> functions.

Hmm... What I have heard is that the cleanup is basically a port of
C++ destructor code to C, and it might be related to the virtual functions
that are may be absent for the basic classes. But not an expert here,
just speculating based on my poor knowledge of C++.

-- 
With Best Regards,
Andy Shevchenko

Re: [RFC] slab: introduce auto_kfree macro

[Herbert Xu]

Peter Zijlstra <peterz@infradead.org> wrote:
>
> The compiler *should* complain. But neither GCC nor clang actually
> appear to warn in this case.

Linus turned that warning off in 2020:

commit 78a5255ffb6a1af189a83e493d916ba1c54d8c75
Author: Linus Torvalds <torvalds@linux-foundation.org>
Date:   Sat May 9 13:57:10 2020 -0700

    Stop the ad-hoc games with -Wno-maybe-initialized

You need to enable it by hand to see the warning:

make KBUILD_CFLAGS_KERNEL=-Wmaybe-uninitialized CFLAGS_MODULE=-Wmaybe-uninitialized

W=2 enables it too but it also enables lots of other crap so it's
useless.

Cheers,
-- 
Email: Herbert Xu <herbert@gondor.apana.org.au>
Home Page: http://gondor.apana.org.au/~herbert/
PGP Key: http://gondor.apana.org.au/~herbert/pubkey.txt