Re: [PATCH] ext4: remove superfluous check that pointer is not NULL

Re: [PATCH] ext4: remove superfluous check that pointer is not NULL

[Theodore Ts'o]

On Mon, May 08, 2023 at 03:13:37PM +0000, Tudor Ambarus wrote:
> If @buffer is NULL, no operation is performed for kvfree(buffer),
> remove superfluous check.
> 
> Signed-off-by: Tudor Ambarus <tudor.ambarus@linaro.org>

I was looking at this just a few weeks ago, and I couldn't find any
actual *documentation* that it was safe to call vfree(NIILL) or
kvfree(NULL).  The problem is there are a lot of architecture-specific
functions, and unlike with kfree() there is no top-level "if (ptr ==
NULL) return;" in the top-level vfree() and kvfree().

So I thought about removing the NULL check for kvfree(), and
ultimately chickened out, since I was afraid that there might be
crashes for some obscure architecture or kernel CONFIG setup.

I've added linux-mm@ for their comments, and for a plea that if it
is safe to pass NULL to vfree, kvfree, kvfree_rcu, etc. that it
actually be *documented* somewhere.

						- Ted

Re: [PATCH] ext4: remove superfluous check that pointer is not NULL

[Matthew Wilcox]

On Mon, May 08, 2023 at 12:14:54PM -0400, Theodore Ts'o wrote:
> On Mon, May 08, 2023 at 03:13:37PM +0000, Tudor Ambarus wrote:
> > If @buffer is NULL, no operation is performed for kvfree(buffer),
> > remove superfluous check.
> > 
> > Signed-off-by: Tudor Ambarus <tudor.ambarus@linaro.org>
> 
> I was looking at this just a few weeks ago, and I couldn't find any
> actual *documentation* that it was safe to call vfree(NIILL) or
> kvfree(NULL).  The problem is there are a lot of architecture-specific
> functions, and unlike with kfree() there is no top-level "if (ptr ==
> NULL) return;" in the top-level vfree() and kvfree().

There doesn't need to be in kvfree().  is_vmalloc_addr() returns 'false'
for NULL, so it calls kfree(), which as you note has an explicit check
for ZERO_OR_NULL_PTR().  is_vmalloc_addr() also returns false for the
ZERO pointer, fwiw.

I agree that this should be explicitly documented as allowed, since it's
not reasonable to expect users to dig through these functions to verify
that such a change is safe.

Re: [PATCH] ext4: remove superfluous check that pointer is not NULL

[Theodore Ts'o]

On Mon, May 08, 2023 at 10:13:27PM +0100, Matthew Wilcox wrote:
> > 
> > I was looking at this just a few weeks ago, and I couldn't find any
> > actual *documentation* that it was safe to call vfree(NIILL) or
> > kvfree(NULL).  The problem is there are a lot of architecture-specific
> > functions, and unlike with kfree() there is no top-level "if (ptr ==
> > NULL) return;" in the top-level vfree() and kvfree().
> 
> There doesn't need to be in kvfree().  is_vmalloc_addr() returns 'false'
> for NULL, so it calls kfree(), which as you note has an explicit check
> for ZERO_OR_NULL_PTR().  is_vmalloc_addr() also returns false for the
> ZERO pointer, fwiw.
> 
> I agree that this should be explicitly documented as allowed, since it's
> not reasonable to expect users to dig through these functions to verify
> that such a change is safe.

I seem to recall at one point looking at kvfree_rcu (at least the one
argument variant), and I *thought* it would unconditionally allocate
memory so it could be put on a linked list to be freed after an RCU
grace period had elapsed.  But I tried tracing through the huge
numbers of cpp macros and other layers of #ifdef's and other
abstractions, and in my conference-induced sleep depreviation, it
caused my head to spin, and I gave up trying to trace it down so I had
100% confidence.

So if someone could document *all* of the k[v]free_* variants whether
it is safe/optimal to pass NULL to them, that would be great, thanks.

						- Ted