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Subject[PATCH] sched: check tgid in is_global_init
Date
Global init can have sub-threads, so ->pid check is not reliable enough
for is_global_init(), we need to check tgid instead. This has been spotted
by Oleg and a fix was proposed by Richard a long time ago (see the link
below).

Oleg wrote:
:Because is_global_init() is only true for the main thread of /sbin/init.
:
:Just look at oom_unkillable_task(). It tries to not kill init. But, say,
:select_bad_process() can happily find a sub-thread of is_global_init()
:and still kill it.

I recently hit the problem in question; re-sending the patch (to the
best of my knowledge it has never been submitted) with updated function
comment. Credit goes to Oleg and Richard.

Link: https://www.redhat.com/archives/linux-audit/2013-December/msg00086.html
Reported-by: Oleg Nesterov <oleg@redhat.com>
Suggested-by: Richard Guy Briggs <rgb@redhat.com>
Acked-by: Serge Hallyn <serge.hallyn@canonical.com>
Cc: <stable@vger.kernel.org>
Signed-off-by: Sergey Senozhatsky <sergey.senozhatsky@gmail.com>
---
include/linux/sched.h | 5 +++--
1 file changed, 3 insertions(+), 2 deletions(-)

diff --git a/include/linux/sched.h b/include/linux/sched.h
index daf600e..d33de52 100644
--- a/include/linux/sched.h
+++ b/include/linux/sched.h
@@ -2013,7 +2013,8 @@ static inline int pid_alive(const struct task_struct *p)
}

/**
- * is_global_init - check if a task structure is init
+ * is_global_init - check if a task structure is init. Since init
+ * is free to have sub-threads we need to check tgid.
* @tsk: Task structure to be checked.
*
* Check if a task structure is the first user space task the kernel created.
@@ -2022,7 +2023,7 @@ static inline int pid_alive(const struct task_struct *p)
*/
static inline int is_global_init(struct task_struct *tsk)
{
- return tsk->pid == 1;
+ return task_tgid_nr(tsk) == 1;
}

extern struct pid *cad_pid;
--
2.7.0.rc0.20.g4b9ab0e


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