[PATCH] wait4 PTRACE_ATTACH race fix

Back about a year ago when I last fiddled heavily with the do_wait code, I
was thinking too hard about the wrong thing and I now think I introduced a
bug whose inverse thought I was fixing.

Apparently noone was looking too hard over much shoulder, so as to cite my
bogus reasoning at the time.  In the race condition when PTRACE_ATTACH is
about to steal a child and then the child hits a tracing event (what
my_ptrace_child checks for), the real parent does need to set its flag
noting it has some eligible live children.  Otherwise a spurious ECHILD
error is possible, since the child in question is not yet on the
ptrace_children list.

Signed-off-by: Roland McGrath <roland@redhat.com>
Signed-off-by: Andrew Morton <akpm@osdl.org>
Signed-off-by: Linus Torvalds <torvalds@osdl.org>

1 files changed, 9 insertions, 0 deletions

diff --git a/kernel/exit.c b/kernel/exit.c
index 6ef8f7356a74..2d39ccc367e6 100644
--- a/kernel/exit.c
+++ b/kernel/exit.c
@@ -1383,6 +1383,15 @@ repeat:
 
                         switch (p->state) {
                         case TASK_TRACED:
+                                /*
+                                 * When we hit the race with PTRACE_ATTACH,
+                                 * we will not report this child.  But the
+                                 * race means it has not yet been moved to
+                                 * our ptrace_children list, so we need to
+                                 * set the flag here to avoid a spurious ECHILD
+                                 * when the race happens with the only child.
+                                 */
+                                flag = 1;
                                 if (!my_ptrace_child(p))
                                         continue;
                                 /*FALLTHROUGH*/