[PATCH RFC 18/38] proc: don't WARN about a name that is already registered

[Christian Brauner <brauner@kernel.org>]

proc_register() WARNs when the directory already has an entry of the
same name and fails the registration. Callers that pick their names
themselves should never get there but some take names from userspace.
pktgen creates a file for each device a writer of "add_device" adds,
named after the device. A device can have any name, including "pgctrl"
or "kpktgend_0", the names of pktgen's own files:

  ip link set lo name pgctrl
  ip link set pgctrl up
  echo "add_device pgctrl" > /proc/net/pktgen/kpktgend_0

  proc_dir_entry 'pktgen/pgctrl' already registered
  WARNING: fs/proc/generic.c:404 at proc_register+0x555/0x8b0, CPU#0: bash/241
  Call Trace:
   proc_create_data+0x9f/0xf0
   pktgen_thread_write+0xd9b/0x1780
   proc_reg_write+0x1c2/0x3c0
   vfs_write+0x1cd/0xfa0

The files of a network namespace belong to the root of its user
namespace and none of this requires a capability. So once pktgen is
loaded an unprivileged user can taint the kernel, or crash it with
panic_on_warn, from namespaces of its own. pktgen itself copes with
the failed registration just fine.

So go back to what proc_register() did before commit 6c2f91e077f1
("proc: use WARN() rather than printk+backtrace") and what
sysfs_warn_dup() does for sysfs: report the name with pr_warn() and
dump the stack. While at it, do that after dropping proc_subdir_lock
instead of printing a backtrace while holding the lock that every
lookup and readdir in procfs needs.

Reported-by: Chris Mason <mason@kernel.org>
Fixes: 6c2f91e077f1 ("proc: use WARN() rather than printk+backtrace")
Cc: stable@vger.kernel.org
Signed-off-by: Christian Brauner (Amutable) <brauner@kernel.org>
---
 fs/proc/generic.c | 5 +++--
 1 file changed, 3 insertions(+), 2 deletions(-)

diff --git a/fs/proc/generic.c b/fs/proc/generic.c
index c560c96a64eb..ef804cd3ec45 100644
--- a/fs/proc/generic.c
+++ b/fs/proc/generic.c
@@ -401,9 +401,10 @@ struct proc_dir_entry *proc_register(struct proc_dir_entry *dir,
 	write_lock(&proc_subdir_lock);
 	dp->parent = dir;
 	if (pde_subdir_insert(dir, dp) == false) {
-		WARN(1, "proc_dir_entry '%s/%s' already registered\n",
-		     dir->name, dp->name);
 		write_unlock(&proc_subdir_lock);
+		pr_warn("proc_dir_entry '%s/%s' already registered\n",
+			dir->name, dp->name);
+		dump_stack();
 		goto out_free_inum;
 	}
 	write_unlock(&proc_subdir_lock);

-- 
2.53.0