prctl: avoid using mmap_sem for exe_file serialization

Oleg cleverly suggested using xchg() to set the new mm->exe_file instead
of calling set_mm_exe_file() which requires some form of serialization --
mmap_sem in this case.  For archs that do not have atomic rmw instructions
we still fallback to a spinlock alternative, so this should always be
safe.  As such, we only need the mmap_sem for looking up the backing
vm_file, which can be done sharing the lock.  Naturally, this means we
need to manually deal with both the new and old file reference counting,
and we need not worry about the MMF_EXE_FILE_CHANGED bits, which can
probably be deleted in the future anyway.

Signed-off-by: Davidlohr Bueso <dbueso@suse.de>
Suggested-by: Oleg Nesterov <oleg@redhat.com>
Acked-by: Oleg Nesterov <oleg@redhat.com>
Reviewed-by: Konstantin Khlebnikov <khlebnikov@yandex-team.ru>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>

1 files changed, 6 insertions, 0 deletions

diff --git a/fs/exec.c b/fs/exec.c
index c7f9b73..a5fef83 100644
--- a/fs/exec.c
+++ b/fs/exec.c
@@ -1078,7 +1078,13 @@ int flush_old_exec(struct linux_binprm * bprm)
 	if (retval)
 		goto out;
 
+	/*
+	 * Must be called _before_ exec_mmap() as bprm->mm is
+	 * not visibile until then. This also enables the update
+	 * to be lockless.
+	 */
 	set_mm_exe_file(bprm->mm, bprm->file);
+
 	/*
 	 * Release all of the old mmap stuff
 	 */