From: Christoph Hellwig <hch@xxxxxx> Subject: kernel: set USER_DS in kthread_use_mm Some architectures like arm64 and s390 require USER_DS to be set for kernel threads to access user address space, which is the whole purpose of kthread_use_mm, but other like x86 don't. That has lead to a huge mess where some callers are fixed up once they are tested on said architectures, while others linger around and yet other like io_uring try to do "clever" optimizations for what usually is just a trivial asignment to a member in the thread_struct for most architectures. Make kthread_use_mm set USER_DS, and kthread_unuse_mm restore to the previous value instead. Link: http://lkml.kernel.org/r/20200404094101.672954-7-hch@xxxxxx Signed-off-by: Christoph Hellwig <hch@xxxxxx> Acked-by: Michael S. Tsirkin <mst@xxxxxxxxxx> Reviewed-by: Jens Axboe <axboe@xxxxxxxxx> Tested-by: Jens Axboe <axboe@xxxxxxxxx> Cc: Alex Deucher <alexander.deucher@xxxxxxx> Cc: Al Viro <viro@xxxxxxxxxxxxxxxxxx> Cc: Felipe Balbi <balbi@xxxxxxxxxx> Cc: Felix Kuehling <Felix.Kuehling@xxxxxxx> Cc: Jason Wang <jasowang@xxxxxxxxxx> Cc: Zhenyu Wang <zhenyuw@xxxxxxxxxxxxxxx> Cc: Zhi Wang <zhi.a.wang@xxxxxxxxx> Cc: Greg Kroah-Hartman <gregkh@xxxxxxxxxxxxxxxxxxx> Signed-off-by: Andrew Morton <akpm@xxxxxxxxxxxxxxxxxxxx> --- drivers/usb/gadget/function/f_fs.c | 4 ---- drivers/vhost/vhost.c | 3 --- fs/io-wq.c | 8 ++------ fs/io_uring.c | 4 ---- kernel/kthread.c | 6 ++++++ 5 files changed, 8 insertions(+), 17 deletions(-) --- a/drivers/usb/gadget/function/f_fs.c~kernel-set-user_ds-in-kthread_use_mm +++ a/drivers/usb/gadget/function/f_fs.c @@ -824,13 +824,9 @@ static void ffs_user_copy_worker(struct bool kiocb_has_eventfd = io_data->kiocb->ki_flags & IOCB_EVENTFD; if (io_data->read && ret > 0) { - mm_segment_t oldfs = get_fs(); - - set_fs(USER_DS); kthread_use_mm(io_data->mm); ret = ffs_copy_to_iter(io_data->buf, ret, &io_data->data); kthread_unuse_mm(io_data->mm); - set_fs(oldfs); } io_data->kiocb->ki_complete(io_data->kiocb, ret, ret); --- a/drivers/vhost/vhost.c~kernel-set-user_ds-in-kthread_use_mm +++ a/drivers/vhost/vhost.c @@ -329,9 +329,7 @@ static int vhost_worker(void *data) struct vhost_dev *dev = data; struct vhost_work *work, *work_next; struct llist_node *node; - mm_segment_t oldfs = get_fs(); - set_fs(USER_DS); kthread_use_mm(dev->mm); for (;;) { @@ -361,7 +359,6 @@ static int vhost_worker(void *data) } } kthread_unuse_mm(dev->mm); - set_fs(oldfs); return 0; } --- a/fs/io_uring.c~kernel-set-user_ds-in-kthread_use_mm +++ a/fs/io_uring.c @@ -5989,15 +5989,12 @@ static int io_sq_thread(void *data) { struct io_ring_ctx *ctx = data; const struct cred *old_cred; - mm_segment_t old_fs; DEFINE_WAIT(wait); unsigned long timeout; int ret = 0; complete(&ctx->sq_thread_comp); - old_fs = get_fs(); - set_fs(USER_DS); old_cred = override_creds(ctx->creds); timeout = jiffies + ctx->sq_thread_idle; @@ -6102,7 +6099,6 @@ static int io_sq_thread(void *data) if (current->task_works) task_work_run(); - set_fs(old_fs); io_sq_thread_drop_mm(ctx); revert_creds(old_cred); --- a/fs/io-wq.c~kernel-set-user_ds-in-kthread_use_mm +++ a/fs/io-wq.c @@ -169,7 +169,6 @@ static bool __io_worker_unuse(struct io_ dropped_lock = true; } __set_current_state(TASK_RUNNING); - set_fs(KERNEL_DS); kthread_unuse_mm(worker->mm); mmput(worker->mm); worker->mm = NULL; @@ -421,14 +420,11 @@ static void io_wq_switch_mm(struct io_wo mmput(worker->mm); worker->mm = NULL; } - if (!work->mm) { - set_fs(KERNEL_DS); + if (!work->mm) return; - } + if (mmget_not_zero(work->mm)) { kthread_use_mm(work->mm); - if (!worker->mm) - set_fs(USER_DS); worker->mm = work->mm; /* hang on to this mm */ work->mm = NULL; --- a/kernel/kthread.c~kernel-set-user_ds-in-kthread_use_mm +++ a/kernel/kthread.c @@ -52,6 +52,7 @@ struct kthread { unsigned long flags; unsigned int cpu; void *data; + mm_segment_t oldfs; struct completion parked; struct completion exited; #ifdef CONFIG_BLK_CGROUP @@ -1235,6 +1236,9 @@ void kthread_use_mm(struct mm_struct *mm if (active_mm != mm) mmdrop(active_mm); + + to_kthread(tsk)->oldfs = get_fs(); + set_fs(USER_DS); } EXPORT_SYMBOL_GPL(kthread_use_mm); @@ -1249,6 +1253,8 @@ void kthread_unuse_mm(struct mm_struct * WARN_ON_ONCE(!(tsk->flags & PF_KTHREAD)); WARN_ON_ONCE(!tsk->mm); + set_fs(to_kthread(tsk)->oldfs); + task_lock(tsk); sync_mm_rss(mm); tsk->mm = NULL; _
