coredump: shift down_write(mmap_sem) into coredump_wait()

- move the cprm.mm_flags checks up, before we take mmap_sem

- move down_write(mmap_sem) and ->core_state check from do_coredump()
  to coredump_wait()

This simplifies the code and makes the locking symmetrical.

Signed-off-by: Oleg Nesterov <oleg@redhat.com>
Cc: David Howells <dhowells@redhat.com>
Cc: Neil Horman <nhorman@tuxdriver.com>
Cc: Roland McGrath <roland@redhat.com>
Cc: Andi Kleen <andi@firstfloor.org>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>

1 files changed, 7 insertions, 12 deletions

diff --git a/fs/exec.c b/fs/exec.c
index 6501823b7a1b..0c72d23ed0e2 100644
--- a/fs/exec.c
+++ b/fs/exec.c
@@ -1662,12 +1662,15 @@ static int coredump_wait(int exit_code, struct core_state *core_state)
         struct task_struct *tsk = current;
         struct mm_struct *mm = tsk->mm;
         struct completion *vfork_done;
-        int core_waiters;
+        int core_waiters = -EBUSY;
 
         init_completion(&core_state->startup);
         core_state->dumper.task = tsk;
         core_state->dumper.next = NULL;
-        core_waiters = zap_threads(tsk, mm, core_state, exit_code);
+
+        down_write(&mm->mmap_sem);
+        if (!mm->core_state)
+                core_waiters = zap_threads(tsk, mm, core_state, exit_code);
         up_write(&mm->mmap_sem);
 
         if (unlikely(core_waiters < 0))
@@ -1860,20 +1863,12 @@ void do_coredump(long signr, int exit_code, struct pt_regs *regs)
         binfmt = mm->binfmt;
         if (!binfmt || !binfmt->core_dump)
                 goto fail;
+        if (!__get_dumpable(cprm.mm_flags))
+                goto fail;
 
         cred = prepare_creds();
         if (!cred)
                 goto fail;
-
-        down_write(&mm->mmap_sem);
-        /*
-         * If another thread got here first, or we are not dumpable, bail out.
-         */
-        if (mm->core_state || !__get_dumpable(cprm.mm_flags)) {
-                up_write(&mm->mmap_sem);
-                goto fail_creds;
-        }
-
         /*
          *      We cannot trust fsuid as being the "true" uid of the
          *      process nor do we know its entire history. We only know it