Messages in this thread Patch in this message | | | Date | Wed, 4 Jan 2012 17:29:02 +0100 | From | Oleg Nesterov <> | Subject | [PATCH 1/2] ptrace: partially fix the do_wait(WEXITED) vs EXIT_DEAD->EXIT_ZOMBIE race |
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Test-case:
int main(void) { int pid, status;
pid = fork(); if (!pid) { for (;;) { if (!fork()) return 0; if (waitpid(-1, &status, 0) < 0) { printf("ERR!! wait: %m\n"); return 0; } } }
assert(ptrace(PTRACE_ATTACH, pid, 0,0) == 0); assert(waitpid(-1, NULL, 0) == pid);
assert(ptrace(PTRACE_SETOPTIONS, pid, 0, PTRACE_O_TRACEFORK) == 0);
do { ptrace(PTRACE_CONT, pid, 0, 0); pid = waitpid(-1, NULL, 0); } while (pid > 0);
return 1; }
It fails because ->real_parent sees its child in EXIT_DEAD state while the tracer is going to change the state back to EXIT_ZOMBIE in wait_task_zombie().
The offending commit is 823b018e which moved the EXIT_DEAD check, but in fact we should not blame it. The original code was not correct as well because it didn't take ptrace_reparented() into account and because we can't really trust ->ptrace.
This patch adds the additional check to close this particular race but it doesn't solve the whole problem. We simply can't rely on ->ptrace in this case, it can be cleared if the tracer is multithreaded by the exiting ->parent.
I think we should kill EXIT_DEAD altogether, we should always remove the soon-to-be-reaped child from ->children or at least we should never do the DEAD->ZOMBIE transition. But this is too complex for 3.2.
Reported-and-tested-by: Denys Vlasenko <vda.linux@googlemail.com> Tested-by: Lukasz Michalik <lmi@ift.uni.wroc.pl> Acked-by: Tejun Heo <tj@kernel.org> Cc: <stable@kernel.org> [3.0+] Signed-off-by: Oleg Nesterov <oleg@redhat.com> --- kernel/exit.c | 9 ++++++++- 1 files changed, 8 insertions(+), 1 deletions(-)
diff --git a/kernel/exit.c b/kernel/exit.c index d0b7d98..e6e01b9 100644 --- a/kernel/exit.c +++ b/kernel/exit.c @@ -1540,8 +1540,15 @@ static int wait_consider_task(struct wait_opts *wo, int ptrace, } /* dead body doesn't have much to contribute */ - if (p->exit_state == EXIT_DEAD) + if (unlikely(p->exit_state == EXIT_DEAD)) { + /* + * But do not ignore this task until the tracer does + * wait_task_zombie()->do_notify_parent(). + */ + if (likely(!ptrace) && unlikely(ptrace_reparented(p))) + wo->notask_error = 0; return 0; + } /* slay zombie? */ if (p->exit_state == EXIT_ZOMBIE) { -- 1.5.5.1
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