[PATCH net] sctp: Fix undefined behavior in left shift operation

[PATCH net] sctp: Fix undefined behavior in left shift operation

[Yu-Chun Lin]

According to the C11 standard (ISO/IEC 9899:2011, 6.5.7):
"If E1 has a signed type and E1 x 2^E2 is not representable in the result
type, the behavior is undefined."

Shifting 1 << 31 causes signed integer overflow, which leads to undefined
behavior.

Fix this by explicitly using '1U << 31' to ensure the shift operates on
an unsigned type, avoiding undefined behavior.

Signed-off-by: Yu-Chun Lin <eleanor15x@gmail.com>
---
 net/sctp/stream.c | 2 +-
 1 file changed, 1 insertion(+), 1 deletion(-)

diff --git a/net/sctp/stream.c b/net/sctp/stream.c
index c241cc552e8d..bfcff6d6a438 100644
--- a/net/sctp/stream.c
+++ b/net/sctp/stream.c
@@ -735,7 +735,7 @@ struct sctp_chunk *sctp_process_strreset_tsnreq(
 	 *     value SHOULD be the smallest TSN not acknowledged by the
 	 *     receiver of the request plus 2^31.
 	 */
-	init_tsn = sctp_tsnmap_get_ctsn(&asoc->peer.tsn_map) + (1 << 31);
+	init_tsn = sctp_tsnmap_get_ctsn(&asoc->peer.tsn_map) + (1U << 31);
 	sctp_tsnmap_init(&asoc->peer.tsn_map, SCTP_TSN_MAP_INITIAL,
 			 init_tsn, GFP_ATOMIC);
 
-- 
2.43.0

Re: [PATCH net] sctp: Fix undefined behavior in left shift operation

[patchwork-bot+netdevbpf]

Hello:

This patch was applied to netdev/net.git (main)
by Jakub Kicinski <kuba@kernel.org>:

On Tue, 18 Feb 2025 16:12:16 +0800 you wrote:
> According to the C11 standard (ISO/IEC 9899:2011, 6.5.7):
> "If E1 has a signed type and E1 x 2^E2 is not representable in the result
> type, the behavior is undefined."
> 
> Shifting 1 << 31 causes signed integer overflow, which leads to undefined
> behavior.
> 
> [...]

Here is the summary with links:
  - [net] sctp: Fix undefined behavior in left shift operation
    https://git.kernel.org/netdev/net/c/606572eb22c1

You are awesome, thank you!
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