tree 3f5eb4b4390ea71031eb2c93261052e6698cbdc1
parent 3f024c1a4bc8ef9a149879351ce8b3aa749e0c2f
author David Meybohm <[EMAIL PROTECTED]> Tue, 23 Aug 2005 03:11:08 -0700
committer Linus Torvalds <[EMAIL PROTECTED]> Wed, 24 Aug 2005 01:44:29 -0700

[PATCH] preempt race in getppid

With CONFIG_PREEMPT && !CONFIG_SMP, it's possible for sys_getppid to
return a bogus value if the parent's task_struct gets reallocated after
current->group_leader->real_parent is read:

        asmlinkage long sys_getppid(void)
                int pid;
                struct task_struct *me = current;
                struct task_struct *parent;

                parent = me->group_leader->real_parent;
RACE HERE =>    for (;;) {
                        pid = parent->tgid;
        #ifdef CONFIG_SMP
                        struct task_struct *old = parent;

                         * Make sure we read the pid before re-reading the
                         * parent pointer:
                        parent = me->group_leader->real_parent;
                        if (old != parent)
                return pid;

If the process gets preempted at the indicated point, the parent process
can go ahead and call exit() and then get wait()'d on to reap its
task_struct. When the preempted process gets resumed, it will not do any
further checks of the parent pointer on !CONFIG_SMP: it will read the
bad pid and return.

So, the same algorithm used when SMP is enabled should be used when
preempt is enabled, which will recheck ->real_parent in this case.

Signed-off-by: David Meybohm <[EMAIL PROTECTED]>
Signed-off-by: Andrew Morton <[EMAIL PROTECTED]>
Signed-off-by: Linus Torvalds <[EMAIL PROTECTED]>

 kernel/timer.c |    2 +-
 1 files changed, 1 insertion(+), 1 deletion(-)

diff --git a/kernel/timer.c b/kernel/timer.c
--- a/kernel/timer.c
+++ b/kernel/timer.c
@@ -1023,7 +1023,7 @@ asmlinkage long sys_getppid(void)
        parent = me->group_leader->real_parent;
        for (;;) {
                pid = parent->tgid;
-#ifdef CONFIG_SMP
+#if defined(CONFIG_SMP) || defined(CONFIG_PREEMPT)
                struct task_struct *old = parent;
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