posix: shm: ensure addend is less than INTPTR_MAX

Although it's quite unlikely that we will see shared memory
objects in practice that are greater than 2GiB, there is a
possibility of integer overflow when comparing intptr_t and
size_t, since the former is signed and the latter is unsigned.

Explicitly check to ensure that the addend is less than
INTPTR_MAX before subtraction.

This fixes CID 487734

Signed-off-by: Chris Friedt <[email protected]>

Author: cfriedt

lib/posix/options/shm.c

@@ -159,7 +159,7 @@ static off_t shm_lseek(struct shm_obj *shm, off_t offset, int whence, size_t cur
 		return -1;
 	}
 
-	if ((INTPTR_MAX - addend) < offset) {
+	if ((addend > INTPTR_MAX) || ((INTPTR_MAX - addend) < offset)) {
 		errno = EOVERFLOW;
 		return -1;
 	}