Replace the fd_sets in struct fdtable with an array of unsigned longs

Replace the fd_sets in struct fdtable with an array of unsigned longs and then
use the standard non-atomic bit operations rather than the FD_* macros.

This:

 (1) Removes the abuses of struct fd_set:

     (a) Since we don't want to allocate a full fd_set the vast majority of the
     	 time, we actually, in effect, just allocate a just-big-enough array of
     	 unsigned longs and cast it to an fd_set type - so why bother with the
     	 fd_set at all?

     (b) Some places outside of the core fdtable handling code (such as
     	 SELinux) want to look inside the array of unsigned longs hidden inside
     	 the fd_set struct for more efficient iteration over the entire set.

 (2) Eliminates the use of FD_*() macros in the kernel completely.

 (3) Permits the __FD_*() macros to be deleted entirely where not exposed to
     userspace.

Signed-off-by: David Howells <dhowells@redhat.com>
Link: http://lkml.kernel.org/r/20120216174954.23314.48147.stgit@warthog.procyon.org.uk
Signed-off-by: H. Peter Anvin <hpa@zytor.com>
Cc: Al Viro <viro@zeniv.linux.org.uk>

include/linux/fdtable.h

@@ -21,51 +21,43 @@
  */
 #define NR_OPEN_DEFAULT BITS_PER_LONG
 
-/*
- * The embedded_fd_set is a small fd_set,
- * suitable for most tasks (which open <= BITS_PER_LONG files)
- */
-struct embedded_fd_set {
-	unsigned long fds_bits[1];
-};
-
 struct fdtable {
 	unsigned int max_fds;
 	struct file __rcu **fd;      /* current fd array */
-	fd_set *close_on_exec;
-	fd_set *open_fds;
+	unsigned long *close_on_exec;
+	unsigned long *open_fds;
 	struct rcu_head rcu;
 	struct fdtable *next;
 };
 
 static inline void __set_close_on_exec(int fd, struct fdtable *fdt)
 {
-	FD_SET(fd, fdt->close_on_exec);
+	__set_bit(fd, fdt->close_on_exec);
 }
 
 static inline void __clear_close_on_exec(int fd, struct fdtable *fdt)
 {
-	FD_CLR(fd, fdt->close_on_exec);
+	__clear_bit(fd, fdt->close_on_exec);
 }
 
 static inline bool close_on_exec(int fd, const struct fdtable *fdt)
 {
-	return FD_ISSET(fd, fdt->close_on_exec);
+	return test_bit(fd, fdt->close_on_exec);
 }
 
 static inline void __set_open_fd(int fd, struct fdtable *fdt)
 {
-	FD_SET(fd, fdt->open_fds);
+	__set_bit(fd, fdt->open_fds);
 }
 
 static inline void __clear_open_fd(int fd, struct fdtable *fdt)
 {
-	FD_CLR(fd, fdt->open_fds);
+	__clear_bit(fd, fdt->open_fds);
 }
 
 static inline bool fd_is_open(int fd, const struct fdtable *fdt)
 {
-	return FD_ISSET(fd, fdt->open_fds);
+	return test_bit(fd, fdt->open_fds);
 }
 
 /*
@@ -83,8 +75,8 @@ struct files_struct {
    */
 	spinlock_t file_lock ____cacheline_aligned_in_smp;
 	int next_fd;
-	struct embedded_fd_set close_on_exec_init;
-	struct embedded_fd_set open_fds_init;
+	unsigned long close_on_exec_init[1];
+	unsigned long open_fds_init[1];
 	struct file __rcu * fd_array[NR_OPEN_DEFAULT];
 };