[PATCH 2/2] dmaengine: rz-dmac: fix dead empty check in rz_dmac_chan_get_residue()

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

rz_dmac_chan_get_residue() reads channel->ld_active with
list_first_entry() and then tests the returned pointer against
NULL. list_first_entry() never returns NULL. On an empty list it
returns container_of(&channel->ld_active, struct rz_dmac_desc,
node), an aliased pointer derived from the list head. The "return
0" shortcut is dead code.

If ld_active is ever empty here, current_desc points at
&channel->ld_active. The subsequent cookie and status processing
then reads bogus values from the head's neighbouring memory.

ld_active can be empty when a residue query races with descriptor
completion on another path. The author intent was clear from the
existing comment on the next-following check, which already
acknowledges that the descriptor "could now be complete". The
empty case is the limit of that race.

Use list_first_entry_or_null() so the empty case returns NULL and
the existing "return 0" path runs.

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/sh/rz-dmac.c | 4 ++--
 1 file changed, 2 insertions(+), 2 deletions(-)

diff --git a/drivers/dma/sh/rz-dmac.c b/drivers/dma/sh/rz-dmac.c
index 625ff29024de..3dd76615881f 100644
--- a/drivers/dma/sh/rz-dmac.c
+++ b/drivers/dma/sh/rz-dmac.c
@@ -723,8 +723,8 @@ static u32 rz_dmac_chan_get_residue(struct rz_dmac_chan *channel,
 	u32 crla, crtb, i;
 
 	/* Get current processing virtual descriptor */
-	current_desc = list_first_entry(&channel->ld_active,
-					struct rz_dmac_desc, node);
+	current_desc = list_first_entry_or_null(&channel->ld_active,
+						struct rz_dmac_desc, node);
 	if (!current_desc)
 		return 0;
 
-- 
2.34.1