Re: [PATCH] mm/slab: make __free(kfree) accept error pointers

[Vlastimil Babka]

On 4/29/24 5:03 AM, Matthew Wilcox wrote:
> On Sun, Apr 28, 2024 at 05:26:44PM +0300, Dan Carpenter wrote:
>> Currently, if an automatically freed allocation is an error pointer that
>> will lead to a crash.  An example of this is in wm831x_gpio_dbg_show().
>> 
>>    171	char *label __free(kfree) = gpiochip_dup_line_label(chip, i);
>>    172	if (IS_ERR(label)) {
>>    173		dev_err(wm831x->dev, "Failed to duplicate label\n");
>>    174		continue;
>>    175  }
>> 
>> The auto clean up function should check for error pointers as well,
>> otherwise we're going to keep hitting issues like this.
>> 
>> Fixes: 54da6a092431 ("locking: Introduce __cleanup() based infrastructure")
>> Cc: <stable@xxxxxxxxxxxxxxx>
>> Signed-off-by: Dan Carpenter <dan.carpenter@xxxxxxxxxx>
>> ---
>> Obviously, the fixes tag isn't very fair but it will tell the -stable
>> tools how far to backport this.
>> 
>>  include/linux/slab.h  | 4 ++--
>>  1 files changed, 2 insertions(+), 2 deletions(-)
>> 
>> diff --git a/include/linux/slab.h b/include/linux/slab.h
>> index 4cc37ef22aae..5f5766219375 100644
>> --- a/include/linux/slab.h
>> +++ b/include/linux/slab.h
>> @@ -279,7 +279,7 @@ void kfree(const void *objp);
>>  void kfree_sensitive(const void *objp);
>>  size_t __ksize(const void *objp);
>>  
>> -DEFINE_FREE(kfree, void *, if (_T) kfree(_T))
>> +DEFINE_FREE(kfree, void *, if (!IS_ERR_OR_NULL(_T)) kfree(_T))
> 
> Wait, why do we check 'if (_T)' at all?  kfree() already handles NULL
> pointers just fine.  I wouldn't be averse to making it handle error
> pointers either.

Making kfree() handle IS_ERR() is perhaps a discussion for something else
than a stable fix. But Christoph has a point that kfree() checks
ZERO_OR_NULL_PTR. Here we check IS_ERR_OR_NULL. How about we checked only
IS_ERR here so it makes some sense?

>> -DEFINE_FREE(kvfree, void *, if (_T) kvfree(_T))
>> +DEFINE_FREE(kvfree, void *, if (!IS_ERR_OR_NULL(_T)) kvfree(_T))
> 
> Ditto kvfree().  Fixing kfree() would fix both of these.

ZERO and NULL should be both false for is_vmalloc_addr() so ultimately
kfree() will handle those, so we could also do only IS_ERR here?