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Subject[ 011/184] usermodehelper: implement UMH_KILLABLE
2.6.32-longterm review patch.  If anyone has any objections, please let me know.

------------------

From: Oleg Nesterov <oleg@redhat.com>

commit d0bd587a80960d7ba7e0c8396e154028c9045c54 upstream

Implement UMH_KILLABLE, should be used along with UMH_WAIT_EXEC/PROC.
The caller must ensure that subprocess_info->path/etc can not go away
until call_usermodehelper_freeinfo().

call_usermodehelper_exec(UMH_KILLABLE) does
wait_for_completion_killable. If it fails, it uses
xchg(&sub_info->complete, NULL) to serialize with umh_complete() which
does the same xhcg() to access sub_info->complete.

If call_usermodehelper_exec wins, it can safely return. umh_complete()
should get NULL and call call_usermodehelper_freeinfo().

Otherwise we know that umh_complete() was already called, in this case
call_usermodehelper_exec() falls back to wait_for_completion() which
should succeed "very soon".

Note: UMH_NO_WAIT == -1 but it obviously should not be used with
UMH_KILLABLE. We delay the neccessary cleanup to simplify the back
porting.

Signed-off-by: Oleg Nesterov <oleg@redhat.com>
Cc: Tetsuo Handa <penguin-kernel@I-love.SAKURA.ne.jp>
Cc: Rusty Russell <rusty@rustcorp.com.au>
Cc: Tejun Heo <tj@kernel.org>
Cc: David Rientjes <rientjes@google.com>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>
[dannf: backported to Debian's 2.6.32]
Signed-off-by: Willy Tarreau <w@1wt.eu>
---
include/linux/kmod.h | 2 ++
kernel/kmod.c | 27 +++++++++++++++++++++++++--
2 files changed, 27 insertions(+), 2 deletions(-)

diff --git a/include/linux/kmod.h b/include/linux/kmod.h
index 0546fe7..93e732e 100644
--- a/include/linux/kmod.h
+++ b/include/linux/kmod.h
@@ -64,6 +64,8 @@ enum umh_wait {
UMH_WAIT_PROC = 1, /* wait for the process to complete */
};

+#define UMH_KILLABLE 4 /* wait for EXEC/PROC killable */
+
/* Actually execute the sub-process */
int call_usermodehelper_exec(struct subprocess_info *info, enum umh_wait wait);

diff --git a/kernel/kmod.c b/kernel/kmod.c
index 2a27d17..2c2a020 100644
--- a/kernel/kmod.c
+++ b/kernel/kmod.c
@@ -208,7 +208,15 @@ EXPORT_SYMBOL(call_usermodehelper_freeinfo);

static void umh_complete(struct subprocess_info *sub_info)
{
- complete(sub_info->complete);
+ struct completion *comp = xchg(&sub_info->complete, NULL);
+ /*
+ * See call_usermodehelper_exec(). If xchg() returns NULL
+ * we own sub_info, the UMH_KILLABLE caller has gone away.
+ */
+ if (comp)
+ complete(comp);
+ else
+ call_usermodehelper_freeinfo(sub_info);
}

/* Keventd can't block, but this (a child) can. */
@@ -264,6 +272,9 @@ static void __call_usermodehelper(struct work_struct *work)

BUG_ON(atomic_read(&sub_info->cred->usage) != 1);

+ if (wait != UMH_NO_WAIT)
+ wait &= ~UMH_KILLABLE;
+
/* CLONE_VFORK: wait until the usermode helper has execve'd
* successfully We need the data structures to stay around
* until that is done. */
@@ -525,9 +536,21 @@ int call_usermodehelper_exec(struct subprocess_info *sub_info,
queue_work(khelper_wq, &sub_info->work);
if (wait == UMH_NO_WAIT) /* task has freed sub_info */
goto unlock;
+
+ if (wait & UMH_KILLABLE) {
+ retval = wait_for_completion_killable(&done);
+ if (!retval)
+ goto wait_done;
+
+ /* umh_complete() will see NULL and free sub_info */
+ if (xchg(&sub_info->complete, NULL))
+ goto unlock;
+ /* fallthrough, umh_complete() was already called */
+ }
+
wait_for_completion(&done);
+wait_done:
retval = sub_info->retval;
-
out:
call_usermodehelper_freeinfo(sub_info);
unlock:
--
1.7.12.2.21.g234cd45.dirty




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