On 10/07/2020 17:03, Bartosz Golaszewski wrote:

On Fri, Jul 10, 2020 at 3:46 PM Jon Hunter [off-list ref] wrote:

quoted

Hi Bartosz,

On 29/06/2020 07:50, Bartosz Golaszewski wrote:

quoted

From: Bartosz Golaszewski <redacted>

Make devm_kmalloc() behave similarly to non-managed kmalloc(): return
ZERO_SIZE_PTR when requested size is 0. Update devm_kfree() to handle
this case.

Signed-off-by: Bartosz Golaszewski <redacted>
---
 drivers/base/devres.c | 9 ++++++---
 1 file changed, 6 insertions(+), 3 deletions(-)

diff --git a/drivers/base/devres.c b/drivers/base/devres.c
index 1df1fb10b2d9..ed615d3b9cf1 100644
--- a/drivers/base/devres.c
+++ b/drivers/base/devres.c

@@ -819,6 +819,9 @@ void *devm_kmalloc(struct device *dev, size_t size, gfp_t gfp)
 {
      struct devres *dr;

+     if (unlikely(!size))
+             return ZERO_SIZE_PTR;
+
      /* use raw alloc_dr for kmalloc caller tracing */
      dr = alloc_dr(devm_kmalloc_release, size, gfp, dev_to_node(dev));
      if (unlikely(!dr))

@@ -950,10 +953,10 @@ void devm_kfree(struct device *dev, const void *p)
      int rc;

      /*
-      * Special case: pointer to a string in .rodata returned by
-      * devm_kstrdup_const().
+      * Special cases: pointer to a string in .rodata returned by
+      * devm_kstrdup_const() or NULL/ZERO ptr.
       */
-     if (unlikely(is_kernel_rodata((unsigned long)p)))
+     if (unlikely(is_kernel_rodata((unsigned long)p) || ZERO_OR_NULL_PTR(p)))
              return;

      rc = devres_destroy(dev, devm_kmalloc_release,

This change caught a bug in one of our Tegra drivers, which I am in the
process of fixing. Once I bisected to this commit it was easy to track
down, but I am wondering if there is any reason why we don't add a
WARN_ON() if size is 0 in devm_kmalloc? It was essentially what I ended
up doing to find the bug.

Jon

--
nvpublic

Hi Jon,

this is in line with what the regular kmalloc() does. If size is zero,
it returns ZERO_SIZE_PTR. It's not an error condition. Actually in
user-space malloc() does a similar thing: for size == 0 it allocates
one-byte and returns a pointer to it (at least in glibc).

Yes that's fine, I was just wondering if there is any reason not to WARN
as well?

Cheers
Jon

-- 
nvpublic