[PATCH 1/2] dmaengine: mpc512x: fix dead empty check in mpc_dma_prep_slave_sg()

[Maoyi Xie <maoyixie.tju@gmail.com>]

mpc_dma_prep_slave_sg() reads mchan->free with list_first_entry()
and then tests the returned pointer against NULL. list_first_entry()
never returns NULL. On an empty free list it returns
container_of(&mchan->free, struct mpc_dma_desc, node), an aliased
pointer derived from the list head. The recovery path (drop lock,
scan completed list, return NULL) is dead code.

If the free list is ever empty here, the aliased mdesc points at
&mchan->free. The list_del(&mdesc->node) that follows then runs on
the head itself, corrupting mchan->free.next and mchan->free.prev.

The free list is reachable empty when the descriptor pool is
exhausted. The author intent was clear from the recovery path:
release the lock, scan the completed list to free descriptors, and
return NULL so the caller can retry.

Use list_first_entry_or_null() so the empty case returns NULL and
the existing recovery path runs as intended.

The same shape has been cleaned up elsewhere, for example in
commit fbb8bc408027 ("net: qed: Remove redundant NULL checks after list_first_entry()"),
commit c708d3fad421 ("crypto: atmel - use list_first_entry_or_null to simplify find_dev"),
and commit 10379171f346 ("ksmbd: use list_first_entry_or_null for opinfo_get_list()").
This site was missed by those cleanups.

Signed-off-by: Maoyi Xie <maoyixie.tju@gmail.com>
---
 drivers/dma/mpc512x_dma.c | 4 ++--
 1 file changed, 2 insertions(+), 2 deletions(-)

diff --git a/drivers/dma/mpc512x_dma.c b/drivers/dma/mpc512x_dma.c
index 0adc8e01057e..f5934136efc4 100644
--- a/drivers/dma/mpc512x_dma.c
+++ b/drivers/dma/mpc512x_dma.c
@@ -706,8 +706,8 @@ mpc_dma_prep_slave_sg(struct dma_chan *chan, struct scatterlist *sgl,
 	for_each_sg(sgl, sg, sg_len, i) {
 		spin_lock_irqsave(&mchan->lock, iflags);
 
-		mdesc = list_first_entry(&mchan->free,
-						struct mpc_dma_desc, node);
+		mdesc = list_first_entry_or_null(&mchan->free,
+						 struct mpc_dma_desc, node);
 		if (!mdesc) {
 			spin_unlock_irqrestore(&mchan->lock, iflags);
 			/* Try to free completed descriptors */
-- 
2.34.1