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Subject[PATCH 4.9 17/66] ipc: msg, make msgrcv work with LONG_MIN
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4.9-stable review patch.  If anyone has any objections, please let me know.

------------------

From: Jiri Slaby <jslaby@suse.cz>

commit 999898355e08ae3b92dfd0a08db706e0c6703d30 upstream.

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

Link: http://lkml.kernel.org/r/20161024082633.10148-1-jslaby@suse.cz
Signed-off-by: Jiri Slaby <jslaby@suse.cz>
Cc: Davidlohr Bueso <dave@stgolabs.net>
Cc: Manfred Spraul <manfred@colorfullife.com>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>

---
ipc/msg.c | 5 ++++-
1 file changed, 4 insertions(+), 1 deletion(-)

--- a/ipc/msg.c
+++ b/ipc/msg.c
@@ -763,7 +763,10 @@ static inline int convert_mode(long *msg
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)

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