ipc,shm: document new limits in the uapi header

This is useful in the future and allows users to better understand the
reasoning behind the changes.

Also use UL as we're dealing with it anyways.

Signed-off-by: Davidlohr Bueso <davidlohr@hp.com>
Cc: Michael Kerrisk <mtk.manpages@gmail.com>
Cc: Manfred Spraul <manfred@colorfullife.com>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>

diff --git a/include/uapi/linux/shm.h b/include/uapi/linux/shm.h
index 74e786de6f4ea9605c23ce7c89b0d14e4c10cb78..e37fb08df9cddba9c98c43e10477ed3bfbedb503 100644 (file)
--- a/include/uapi/linux/shm.h
+++ b/include/uapi/linux/shm.h
@@ -8,17 +8,19 @@
 #endif
 
 /*
- * SHMMAX, SHMMNI and SHMALL are upper limits are defaults which can
- * be modified by sysctl.
+ * SHMMNI, SHMMAX and SHMALL are the default upper limits which can be
+ * modified by sysctl. Both SHMMAX and SHMALL have their default values
+ * to the maximum limit which is as large as it can be without helping
+ * userspace overflow the values. There is really nothing the kernel
+ * can do to avoid this any variables. It is therefore not advised to
+ * make them any larger. This is suitable for both 32 and 64-bit systems.
  */
-
 #define SHMMIN 1                        /* min shared seg size (bytes) */
 #define SHMMNI 4096                     /* max num of segs system wide */
-#define SHMMAX (ULONG_MAX - (1L<<24))   /* max shared seg size (bytes) */
-#define SHMALL (ULONG_MAX - (1L<<24))   /* max shm system wide (pages) */
+#define SHMMAX (ULONG_MAX - (1UL << 24)) /* max shared seg size (bytes) */
+#define SHMALL (ULONG_MAX - (1UL << 24)) /* max shm system wide (pages) */
 #define SHMSEG SHMMNI                   /* max shared segs per process */
 
-
 /* Obsolete, used only for backwards compatibility and libc5 compiles */
 struct shmid_ds {
        struct ipc_perm         shm_perm;       /* operation perms */