FAILED: patch "[PATCH] fbdev: udlfb: check for fb_deferred_io_init() error" failed to apply to 5.10-stable tree

[Sasha Levin <sashal@kernel.org>]

The patch below does not apply to the 5.10-stable tree.
If someone wants it applied there, or to any other stable or longterm
tree, then please email the backport, including the original git commit
id to <stable@vger.kernel.org>.

To reproduce the conflict and resubmit, you may use the following commands:

git fetch https://git.kernel.org/pub/scm/linux/kernel/git/stable/linux.git/ linux-5.10.y
git checkout FETCH_HEAD
git cherry-pick -x eae055f33e41e1d3e1a7e905bdcb1e9f196ed31d
# <resolve conflicts, build, test, etc.>
git commit -s
git send-email --to '<stable@vger.kernel.org>' --in-reply-to '20261107062159-failed-5.10-eae055f33e41@kernel.org' --subject-prefix 'PATCH 5.10.y' 'HEAD^..'

Thanks,
Sasha

------------------ original commit in Linus's tree ------------------

From eae055f33e41e1d3e1a7e905bdcb1e9f196ed31d Mon Sep 17 00:00:00 2001
From: "Lorenzo Stoakes (ARM)" <ljs@kernel.org>
Date: Sat, 26 Sep 2026 14:17:02 +0100
Subject: [PATCH] fbdev: udlfb: check for fb_deferred_io_init() error

fb_deferred_io_init() allocates deferred I/O state, populating
info->fbdefio_state, or leaving it NULL if an error occurs.

Currently dlfb_ops_open() ignores its return value.

Therefore if an error arises in fb_deferred_io_init() (for instance, due to
an allocation failure) info->fbdefio_state is left NULL.

fb_open() will then dereference a NULL pointer (calling
fb_deferred_io_open()) as soon as dlfb_ops_open() returns.

Additionally, if the allocation of info->fbdefio itself fails,
dlfb_ops_open() sets info->fbdefio to NULL and invokes
fb_deferred_io_init() regardless, hitting BUG_ON(!fbdefio).

Fix this by only invoking fb_deferred_io_init() if the allocation
succeeded, and checking for the error.

The driver already supports operating without deferred I/O, so in either
case fall back to that by setting info->fbdefio to NULL.

The ignored return value was introduced in commit 56c134f7f1b5 ("fbdev:
Track deferred-I/O pages in pageref struct").

However, the BUG_ON() issue originates from the earlier
commit 5bea1fbf9423 ("staging: udlfb: fix incorrect fb_defio
implementation for multiple framebuffers"), so target that for the fix.

Fixes: 5bea1fbf9423 ("staging: udlfb: fix incorrect fb_defio implementation for multiple framebuffers")
Cc: stable@vger.kernel.org
Signed-off-by: Lorenzo Stoakes (ARM) <ljs@kernel.org>
Signed-off-by: Helge Deller <deller@gmx.de>
---
 drivers/video/fbdev/udlfb.c | 9 ++++++---
 1 file changed, 6 insertions(+), 3 deletions(-)

diff --git a/drivers/video/fbdev/udlfb.c b/drivers/video/fbdev/udlfb.c
index 92b352bc7e969..8b7b4bdac4fbf 100644
--- a/drivers/video/fbdev/udlfb.c
+++ b/drivers/video/fbdev/udlfb.c
@@ -947,10 +947,13 @@ static int dlfb_ops_open(struct fb_info *info, int user)
 			fbdefio->delay = DL_DEFIO_WRITE_DELAY;
 			fbdefio->sort_pagereflist = true;
 			fbdefio->deferred_io = dlfb_dpy_deferred_io;
-		}
 
-		info->fbdefio = fbdefio;
-		fb_deferred_io_init(info);
+			info->fbdefio = fbdefio;
+			if (fb_deferred_io_init(info)) {
+				kfree(fbdefio);
+				info->fbdefio = NULL;
+			}
+		}
 	}
 
 	dev_dbg(info->dev, "open, user=%d fb_info=%p count=%d\n",
-- 
2.53.0