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

[Dan Carpenter]

On Tue, Apr 30, 2024 at 02:09:10PM +0200, Vlastimil Babka wrote:
> 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?
> 

I wondered why Peter Z wrote it like this as well...  I think he did
it so the compiler can figure out which calls to kfree() are unnecessary
and remove them.  These functions are inline and kfree() is not.  I
haven't measured to see if it actually results in a space savings but
the theory is sound.

regards,
dan carpenter