printk: Fix signed integer overflow when defining LOG_BUF_LEN_MAX

From: Kuan-Wei Chiu <[email protected]>

[ Upstream commit 3d6f83df8ff2d5de84b50377e4f0d45e25311c7a ]

Shifting 1 << 31 on a 32-bit int causes signed integer overflow, which leads to undefined behavior. To prevent this, cast 1 to u32 before performing the shift, ensuring well-defined behavior.

This change explicitly avoids any potential overflow by ensuring that the shift occurs on an unsigned 32-bit integer.

---
 kernel/printk/printk.c | 2 +-
 1 file changed, 1 insertion(+), 1 deletion(-)

index a8af93cbc293..3a7fd61c0e7b 100644
@@ -420,7 +420,7 @@ static u64 clear_seq;
 /* record buffer */
 #define LOG_ALIGN __alignof__(unsigned long)
 #define __LOG_BUF_LEN (1 << CONFIG_LOG_BUF_SHIFT)
-#define LOG_BUF_LEN_MAX (u32)(1 << 31)
+#define LOG_BUF_LEN_MAX ((u32)1 << 31)
 static char __log_buf[__LOG_BUF_LEN] __aligned(LOG_ALIGN);
 static char *log_buf = __log_buf;
 static u32 log_buf_len = __LOG_BUF_LEN;

2.47.1

[ Sasha's backport helper bot ]

Hi,

Summary of potential issues:
❌ Build failures detected

The upstream commit SHA1 provided is correct: 3d6f83df8ff2d5de84b50377e4f0d45e25311c7a

WARNING: Author mismatch between patch and upstream commit:
Backport author: Abdelkareem Abdelsaamad<[email protected]> Commit author: Kuan-Wei Chiu<[email protected]>

Status in newer kernel trees:
6.13.y | Present (different SHA1: 4a2c4e7265b8)
6.12.y | Present (different SHA1: 404e5fd918a0)
6.6.y | Present (different SHA1: 4acf6bab775d)
6.1.y | Present (different SHA1: 9a6d43844de2)
5.15.y | Present (different SHA1: 131e6c9d16e2)

Note: The patch differs from the upstream commit:
---
1: 3d6f83df8ff2d ! 1: eb76eb68e0427 printk: Fix signed integer overflow when defining LOG_BUF_LEN_MAX
@@ Metadata
## Commit message ##
printk: Fix signed integer overflow when defining LOG_BUF_LEN_MAX

+ [ Upstream commit 3d6f83df8ff2d5de84b50377e4f0d45e25311c7a ]
+
Shifting 1 << 31 on a 32-bit int causes signed integer overflow, which leads to undefined behavior. To prevent this, cast 1 to u32 before performing the shift, ensuring well-defined behavior.
@@ Commit message

+ Signed-off-by: Sasha Levin <[email protected]>
+ (cherry picked from commit 9a6d43844de2479a3ff8d674c3e2a16172e01598)
+ Signed-off-by: Abdelkareem Abdelsaamad <[email protected]>

## kernel/printk/printk.c ##
-@@ kernel/printk/printk.c: static struct latched_seq clear_seq = { +@@ kernel/printk/printk.c: static u64 clear_seq;
/* record buffer */
#define LOG_ALIGN __alignof__(unsigned long)
#define __LOG_BUF_LEN (1 << CONFIG_LOG_BUF_SHIFT)
---

Results of testing on various branches:

| Branch                    | Patch Apply | Build Test |
|---------------------------|-------------|------------|
| stable/linux-5.10.y       |  Success    |  Success   |
| stable/linux-5.15.y       |  Failed     |  N/A       |

Build Errors:
Patch failed to apply on stable/linux-5.15.y. Reject:

diff a/kernel/printk/printk.c b/kernel/printk/printk.c (rejected hunks)
@@ -420,7 +420,7 @@ static u64 clear_seq;
/* record buffer */
#define LOG_ALIGN __alignof__(unsigned long) #define __LOG_BUF_LEN (1 << CONFIG_LOG_BUF_SHIFT)
-#define LOG_BUF_LEN_MAX (u32)(1 << 31)
+#define LOG_BUF_LEN_MAX ((u32)1 << 31)
static char __log_buf[__LOG_BUF_LEN] __aligned(LOG_ALIGN); static char *log_buf = __log_buf;
static u32 log_buf_len = __LOG_BUF_LEN;