| Message ID | 20250401134408.37312-1-przemyslaw.kitszel@intel.com (mailing list archive) |
|---|---|
| State | New |
| Headers | show |
| Series | [RFC] slab: introduce auto_kfree macro | expand |
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. > > With new auto_kfree, we simply will start with > struct my_dev_foo *foo auto_kfree; > and be safe against future extensions. > > I believe this will open up way for broader adoption of Scope Based > Resource Management, say in networking. > I also believe that my proposed name is special enough that it will > be easy to know/spot that the assignement is hidden. I understand the issue and the problem it solves, but... > +#define auto_kfree __free(kfree) = NULL ...I do not like this syntax at all (note, you forgot to show the result in the code how it will look like). What would be better in my opinion is to have it something like DEFINE_*() type, which will look more naturally in the current kernel codebase (as we have tons of DEFINE_FOO(). DEFINE_AUTO_KFREE_VAR(name, struct foo); with equivalent to struct foo *name __free(kfree) = NULL
On Wed, Apr 02, 2025 at 01:32:52PM +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. > > > > With new auto_kfree, we simply will start with > > struct my_dev_foo *foo auto_kfree; > > and be safe against future extensions. > > > > I believe this will open up way for broader adoption of Scope Based > > Resource Management, say in networking. > > I also believe that my proposed name is special enough that it will > > be easy to know/spot that the assignement is hidden. > > > I understand the issue and the problem it solves, but... > > > +#define auto_kfree __free(kfree) = NULL > > ...I do not like this syntax at all (note, you forgot to show the result > in the code how it will look like). > > What would be better in my opinion is to have it something like DEFINE_*() > type, which will look more naturally in the current kernel codebase > (as we have tons of DEFINE_FOO(). > > DEFINE_AUTO_KFREE_VAR(name, struct foo); Maybe slightly better name is DEFINE_AUTO_KFREE_PTR() as we expect this to be a pointer. > with equivalent to > > struct foo *name __free(kfree) = NULL
Cc Kees and others from his related efforts: https://lore.kernel.org/all/20250321202620.work.175-kees@kernel.org/ On 4/1/25 15:44, 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. > > With new auto_kfree, we simply will start with > struct my_dev_foo *foo auto_kfree; > and be safe against future extensions. > > I believe this will open up way for broader adoption of Scope Based > Resource Management, say in networking. > I also believe that my proposed name is special enough that it will > be easy to know/spot that the assignement is hidden. > > Signed-off-by: Przemek Kitszel <przemyslaw.kitszel@intel.com> > --- > include/linux/slab.h | 1 + > 1 file changed, 1 insertion(+) > > diff --git a/include/linux/slab.h b/include/linux/slab.h > index 98e07e9e9e58..b943be0ce626 100644 > --- a/include/linux/slab.h > +++ b/include/linux/slab.h > @@ -471,6 +471,7 @@ void kfree_sensitive(const void *objp); > size_t __ksize(const void *objp); > > DEFINE_FREE(kfree, void *, if (!IS_ERR_OR_NULL(_T)) kfree(_T)) > +#define auto_kfree __free(kfree) = NULL > DEFINE_FREE(kfree_sensitive, void *, if (_T) kfree_sensitive(_T)) > > /**
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.
On Wed, Apr 02, 2025 at 01:32:51PM +0300, Andy Shevchenko wrote: > What would be better in my opinion is to have it something like DEFINE_*() > type, which will look more naturally in the current kernel codebase > (as we have tons of DEFINE_FOO(). > > DEFINE_AUTO_KFREE_VAR(name, struct foo); Still weird. Much better to have the compiler complain about the obvious use of uninitialized.
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.
On Wed, Apr 02, 2025 at 02:21:04PM +0200, Peter Zijlstra wrote: > On Wed, Apr 02, 2025 at 01:32:51PM +0300, Andy Shevchenko wrote: > > What would be better in my opinion is to have it something like DEFINE_*() > > type, which will look more naturally in the current kernel codebase > > (as we have tons of DEFINE_FOO(). > > > > DEFINE_AUTO_KFREE_VAR(name, struct foo); > > Still weird. Much better to have the compiler complain about the > obvious use of uninitialized. That would be ideal!
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++.
diff --git a/include/linux/slab.h b/include/linux/slab.h index 98e07e9e9e58..b943be0ce626 100644 --- a/include/linux/slab.h +++ b/include/linux/slab.h @@ -471,6 +471,7 @@ void kfree_sensitive(const void *objp); size_t __ksize(const void *objp); DEFINE_FREE(kfree, void *, if (!IS_ERR_OR_NULL(_T)) kfree(_T)) +#define auto_kfree __free(kfree) = NULL DEFINE_FREE(kfree_sensitive, void *, if (_T) kfree_sensitive(_T)) /**
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. With new auto_kfree, we simply will start with struct my_dev_foo *foo auto_kfree; and be safe against future extensions. I believe this will open up way for broader adoption of Scope Based Resource Management, say in networking. I also believe that my proposed name is special enough that it will be easy to know/spot that the assignement is hidden. Signed-off-by: Przemek Kitszel <przemyslaw.kitszel@intel.com> --- include/linux/slab.h | 1 + 1 file changed, 1 insertion(+)