On Tue, Feb 11, 2020 at 10:45:53PM +0000, Al Viro wrote:

On Mon, Feb 10, 2020 at 04:05:15PM +0100, Alexey Gladkov wrote:

quoted

This allows to flush dcache entries of a task on multiple procfs mounts
per pid namespace.

The RCU lock is used because the number of reads at the task exit time
is much larger than the number of procfs mounts.

Cc: Kees Cook <redacted>
Cc: Andy Lutomirski <luto@kernel.org>
Signed-off-by: Djalal Harouni <redacted>
Suggested-by: Linus Torvalds <torvalds@linux-foundation.org>
Signed-off-by: Alexey Gladkov <redacted>
---
 fs/proc/base.c                | 20 +++++++++++++++-----
 fs/proc/root.c                | 27 ++++++++++++++++++++++++++-
 include/linux/pid_namespace.h |  2 ++
 include/linux/proc_fs.h       |  2 ++
 4 files changed, 45 insertions(+), 6 deletions(-)

diff --git a/fs/proc/base.c b/fs/proc/base.c
index 4ccb280a3e79..24b7c620ded3 100644
--- a/fs/proc/base.c
+++ b/fs/proc/base.c

@@ -3133,7 +3133,7 @@ static const struct inode_operations proc_tgid_base_inode_operations = {
 	.permission	= proc_pid_permission,
 };
 
-static void proc_flush_task_mnt(struct vfsmount *mnt, pid_t pid, pid_t tgid)
+static void proc_flush_task_mnt_root(struct dentry *mnt_root, pid_t pid, pid_t tgid)
 {
 	struct dentry *dentry, *leader, *dir;
 	char buf[10 + 1];

@@ -3142,7 +3142,7 @@ static void proc_flush_task_mnt(struct vfsmount *mnt, pid_t pid, pid_t tgid)
 	name.name = buf;
 	name.len = snprintf(buf, sizeof(buf), "%u", pid);
 	/* no ->d_hash() rejects on procfs */
-	dentry = d_hash_and_lookup(mnt->mnt_root, &name);
+	dentry = d_hash_and_lookup(mnt_root, &name);
 	if (dentry) {
 		d_invalidate(dentry);

... which can block

quoted

 		dput(dentry);

... and so can this

quoted

+		rcu_read_lock();
+		list_for_each_entry_rcu(fs_info, &upid->ns->proc_mounts, pidns_entry) {
+			mnt_root = fs_info->m_super->s_root;
+			proc_flush_task_mnt_root(mnt_root, upid->nr, tgid->numbers[i].nr);

... making that more than slightly unsafe.

I see. So, I can't use rcu locks here as well as spinlocks.
Thanks!

-- 
Rgrds, legion