FAILED: patch "[PATCH] HID: core: Harden s32ton() against conversion to 0 bits" failed to apply to 6.1-stable tree

The patch below does not apply to the 6.1-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 <[email protected]>.

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-6.1.y
git checkout FETCH_HEAD
git cherry-pick -x a6b87bfc2ab5bccb7ad953693c85d9062aef3fdd
# <resolve conflicts, build, test, etc.>
git commit -s
git send-email --to '<[email protected]>' --in-reply-to '2025081233-observing-prepay-25da@gregkh' --subject-prefix 'PATCH 6.1.y' HEAD^..

Possible dependencies:

thanks,

greg k-h

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

From a6b87bfc2ab5bccb7ad953693c85d9062aef3fdd Mon Sep 17 00:00:00 2001
From: Alan Stern <[email protected]>
Date: Wed, 23 Jul 2025 10:37:04 -0400
Subject: [PATCH] HID: core: Harden s32ton() against conversion to 0 bits

Testing by the syzbot fuzzer showed that the HID core gets a shift-out-of-bounds exception when it tries to convert a 32-bit quantity to a 0-bit quantity. Ideally this should never occur, but there are buggy devices and some might have a report field with size set to zero; we shouldn't reject the report or the device just because of that.

Instead, harden the s32ton() routine so that it returns a reasonable result instead of crashing when it is called with the number of bits set to 0 -- the same as what snto32() does.

index ef1f79951d9b..f7d4efcae603 100644
@@ -66,8 +66,12 @@ static s32 snto32(__u32 value, unsigned int n)
 
 static u32 s32ton(__s32 value, unsigned int n)
 {
-	s32 a = value >> (n - 1);
+	s32 a;
 
+	if (!value || !n)
+		return 0;
+
+	a = value >> (n - 1);
 	if (a && a != -1)
 		return value < 0 ? 1 << (n - 1) : (1 << (n - 1)) - 1;
 	return value & ((1 << n) - 1);