[PATCH 06/11] fork: add kernel_clone_args flag to not dup/clone files

Mike Christie
2023-03-10T22:06:16Z

Each vhost device gets a thread that is used to perform IO and management
operations. Instead of a thread that is accessing a device, the thread is
part of the device, so when it creates a thread using a helper based on
copy_process we can't dup or clone the parent's files/FDS because it
would do an extra increment on ourself.

Later, when we do:

Qemu process exits:
do_exit -> exit_files -> put_files_struct -> close_files

we would leak the device's resources because of that extra refcount
on the fd or file_struct.

This patch adds a no_files option so these worker threads can prevent
taking an extra refcount on themselves.

Signed-off-by: Mike Christie <michael.christie@oracle.com>
Acked-by: Christian Brauner <brauner@kernel.org>
Reviewed-by: Christoph Hellwig <hch@lst.de>
Acked-by: Michael S. Tsirkin <mst@redhat.com>
---
include/linux/sched/task.h | 1 +
kernel/fork.c | 10 ++++++++--
2 files changed, 9 insertions(+), 2 deletions(-)

diff --git a/include/linux/sched/task.h b/include/linux/sched/task.h
index 2950e83d5382..4f816048794f 100644
--- a/include/linux/sched/task.h
+++ b/include/linux/sched/task.h
@@ -28,6 +28,7 @@ struct kernel_clone_args {
u32 kthread:1;
u32 io_thread:1;
u32 user_worker:1;
+ u32 no_files:1;
unsigned long stack;
unsigned long stack_size;
unsigned long tls;
diff --git a/kernel/fork.c b/kernel/fork.c
index 0dec38276363..258163ea5cd2 100644
--- a/kernel/fork.c
+++ b/kernel/fork.c
@@ -1624,7 +1624,8 @@ static int copy_fs(unsigned long clone_flags, struct task_struct *tsk)
return 0;
}

-static int copy_files(unsigned long clone_flags, struct task_struct *tsk)
+static int copy_files(unsigned long clone_flags, struct task_struct *tsk,
+ int no_files)
{
struct files_struct *oldf, *newf;
int error = 0;
@@ -1636,6 +1637,11 @@ static int copy_files(unsigned long clone_flags, struct task_struct *tsk)
if (!oldf)
goto out;

+ if (no_files) {
+ tsk->files = NULL;
+ goto out;
+ }
+
if (clone_flags & CLONE_FILES) {
atomic_inc(&oldf->count);
goto out;
@@ -2256,7 +2262,7 @@ static __latent_entropy struct task_struct *copy_process(
retval = copy_semundo(clone_flags, p);
if (retval)
goto bad_fork_cleanup_security;
- retval = copy_files(clone_flags, p);
+ retval = copy_files(clone_flags, p, args->no_files);
if (retval)
goto bad_fork_cleanup_semundo;
retval = copy_fs(clone_flags, p);
--
2.25.1