[PATCH] ipc: msg, make msgrcv work with LONG_MIN

[Jiri Slaby]

When LONG_MIN is passed to msgrcv, one would expect to recieve any
message. But convert_mode does *msgtyp = -*msgtyp and -LONG_MIN is
undefined. In particular, with my gcc -LONG_MIN produces -LONG_MIN
again.

So handle this case properly by assigning LONG_MAX to *msgtyp if
LONG_MIN was specified as msgtyp to msgrcv.

This code:
  long msg[] = { 100, 200 };
  int m = msgget(IPC_PRIVATE, IPC_CREAT | 0644);
  msgsnd(m, &msg, sizeof(msg), 0);
  msgrcv(m, &msg, sizeof(msg), LONG_MIN, 0);
produces currently nothing:
  msgget(IPC_PRIVATE, IPC_CREAT|0644)     = 65538
  msgsnd(65538, {100, "\310\0\0\0\0\0\0\0\0\0\0\0\0\0\0\0"}, 16, 0) = 0
  msgrcv(65538, ...

Except a UBSAN warning:
UBSAN: Undefined behaviour in ipc/msg.c:745:13
negation of -9223372036854775808 cannot be represented in type 'long int':

With the patch, I see what I expect:
  msgget(IPC_PRIVATE, IPC_CREAT|0644)     = 0
  msgsnd(0, {100, "\310\0\0\0\0\0\0\0\0\0\0\0\0\0\0\0"}, 16, 0) = 0
  msgrcv(0, {100, "\310\0\0\0\0\0\0\0\0\0\0\0\0\0\0\0"}, 16, -9223372036854775808, 0) = 16

Signed-off-by: Jiri Slaby <jslaby@xxxxxxx>
---
 ipc/msg.c | 5 ++++-
 1 file changed, 4 insertions(+), 1 deletion(-)

diff --git a/ipc/msg.c b/ipc/msg.c
index e12307d0c920..ff10d43b5184 100644
--- a/ipc/msg.c
+++ b/ipc/msg.c
@@ -763,7 +763,10 @@ static inline int convert_mode(long *msgtyp, int msgflg)
 	if (*msgtyp == 0)
 		return SEARCH_ANY;
 	if (*msgtyp < 0) {
-		*msgtyp = -*msgtyp;
+		if (*msgtyp == LONG_MIN) /* -LONG_MIN is undefined */
+			*msgtyp = LONG_MAX;
+		else
+			*msgtyp = -*msgtyp;
 		return SEARCH_LESSEQUAL;
 	}
 	if (msgflg & MSG_EXCEPT)
-- 
2.10.1