[PATCH] tmon: Fix undefined behavior in left shift

[PATCH] tmon: Fix undefined behavior in left shift

[Kuan-Wei Chiu]

Using 1 << j when j reaches 31 triggers undefined behavior because
the constant 1 is of type int, and shifting it left by 31 exceeds
the range of signed int. UBSAN reports:

tmon.c:174:54: runtime error: left shift of 1 by 31 places cannot be represented in type 'int'

According to the C11 standard:

"If E1 has a signed type and E1 x 2^E2 is not representable in the
result type, the behavior is undefined."

Fix this by using 1U << j, ensuring the shift is performed on an
unsigned type where all 32 bits are representable.

Fixes: 94f69966faf8 ("tools/thermal: Introduce tmon, a tool for thermal subsystem")
Signed-off-by: Kuan-Wei Chiu <visitorckw@gmail.com>
---
 tools/thermal/tmon/tmon.c | 2 +-
 1 file changed, 1 insertion(+), 1 deletion(-)

diff --git a/tools/thermal/tmon/tmon.c b/tools/thermal/tmon/tmon.c
index 7eb3216a27f4..ef67cd1a4861 100644
--- a/tools/thermal/tmon/tmon.c
+++ b/tools/thermal/tmon/tmon.c
@@ -171,7 +171,7 @@ static void prepare_logging(void)
 
 		memset(binding_str, 0, sizeof(binding_str));
 		for (j = 0; j < 32; j++)
-			binding_str[j] = (ptdata.tzi[i].cdev_binding & (1 << j)) ?
+			binding_str[j] = (ptdata.tzi[i].cdev_binding & (1U << j)) ?
 				'1' : '0';
 
 		fprintf(tmon_log, "#thermal zone %s%02d cdevs binding: %32s\n",
-- 
2.34.1

Re: [PATCH] tmon: Fix undefined behavior in left shift

[Kuan-Wei Chiu]

On Mon, Sep 01, 2025 at 10:47:56PM +0800, Kuan-Wei Chiu wrote:
> Using 1 << j when j reaches 31 triggers undefined behavior because
> the constant 1 is of type int, and shifting it left by 31 exceeds
> the range of signed int. UBSAN reports:
> 
> tmon.c:174:54: runtime error: left shift of 1 by 31 places cannot be represented in type 'int'
> 

I forgot to mention in the commit message that the UBSAN report was
triggered when running sudo ./tmon -l. Let me know if you'd like me
to include that detail, and I can update and resend as v2.

Regards,
Kuan-Wei

Re: [PATCH] tmon: Fix undefined behavior in left shift

[Kuan-Wei Chiu]

On Mon, Sep 01, 2025 at 10:47:56PM +0800, Kuan-Wei Chiu wrote:
> Using 1 << j when j reaches 31 triggers undefined behavior because
> the constant 1 is of type int, and shifting it left by 31 exceeds
> the range of signed int. UBSAN reports:
> 
> tmon.c:174:54: runtime error: left shift of 1 by 31 places cannot be represented in type 'int'
> 
> According to the C11 standard:
> 
> "If E1 has a signed type and E1 x 2^E2 is not representable in the
> result type, the behavior is undefined."
> 
> Fix this by using 1U << j, ensuring the shift is performed on an
> unsigned type where all 32 bits are representable.
> 
> Fixes: 94f69966faf8 ("tools/thermal: Introduce tmon, a tool for thermal subsystem")
> Signed-off-by: Kuan-Wei Chiu <visitorckw@gmail.com>

I know maintainers are busy people, so I usually try to avoid sending
ping messages. However, this patch was submitted about two months ago
and hasn't received any feedback yet. It looks like it might have
somehow got lost?

Regards,
Kuan-Wei