[PATCH] select: don't overflow if (SELECT_STACK_ALLOC % sizeof(long) != 0)

If SELECT_STACK_ALLOC is not a multiple of sizeof(long) then stack_fds[]
would be shorter than SELECT_STACK_ALLOC bytes and could overflow later in
the function.  Fixed by simply rearranging the test later to work on
sizeof(stack_fds) Currently SELECT_STACK_ALLOC is 256 so this doesn't
happen, but it's nasty to have things like this hidden in the code.  What
if later someone decides to change SELECT_STACK_ALLOC to 300?

Signed-off-by: Mitchell Blank Jr <mitch@sfgoth.com>
Signed-off-by: Andrew Morton <akpm@osdl.org>
Signed-off-by: Linus Torvalds <torvalds@osdl.org>

1 files changed, 9 insertions, 7 deletions

diff --git a/fs/select.c b/fs/select.c
index fce0fd1bb1d1..a8109baa5e46 100644
--- a/fs/select.c
+++ b/fs/select.c
@@ -311,7 +311,8 @@ static int core_sys_select(int n, fd_set __user *inp, fd_set __user *outp,
 {
         fd_set_bits fds;
         void *bits;
-        int ret, size, max_fdset;
+        int ret, max_fdset;
+        unsigned int size;
         struct fdtable *fdt;
         /* Allocate small arguments on the stack to save memory and be faster */
         long stack_fds[SELECT_STACK_ALLOC/sizeof(long)];
@@ -333,14 +334,15 @@ static int core_sys_select(int n, fd_set __user *inp, fd_set __user *outp,
          * since we used fdset we need to allocate memory in units of
          * long-words. 
          */
-        ret = -ENOMEM;
         size = FDS_BYTES(n);
-        if (6*size < SELECT_STACK_ALLOC)
-                bits = stack_fds;
-        else
+        bits = stack_fds;
+        if (size > sizeof(stack_fds) / 6) {
+                /* Not enough space in on-stack array; must use kmalloc */
+                ret = -ENOMEM;
                 bits = kmalloc(6 * size, GFP_KERNEL);
-        if (!bits)
-                goto out_nofds;
+                if (!bits)
+                        goto out_nofds;
+        }
         fds.in      = bits;
         fds.out     = bits +   size;
         fds.ex      = bits + 2*size;