Re: [PATCH v2] audit: Avoid excessive dput/dget in audit_context setup and reset paths

[Waiman Long <llong@redhat.com>]

On 2/4/26 3:18 PM, Al Viro wrote:
> On Wed, Feb 04, 2026 at 01:16:15PM -0500, Waiman Long wrote:
>
>
>> Thanks for the detailed explanation. I am thinking about something like
>> the code diff below. Of course, there are other corner cases like unshare(2)
>> that still needs to be handled. Do you think something like this is viable?
> Deadlocks aside, the immediate problem here is that consensus number is too
> low.  Take three threads sharing the same fs_struct instance.  The first one
> calls your get_fs_pwd_share(); then the remaining two threads call set_fs_pwd()
> (e.g. by calling chdir(2) in userland code).  The reference stored into
> fs->pwd_waiter by the first of those two gets overwritten by that stored
> by the second.  When the caller of get_fs_pwd_share() gets to put_fs_pwd_share(),
> only one of the sleepers gets woken up...
>
> And it's very easy to end up with something as simple as chdir("foo") deadlocking -
> we start with resolving the relative pathname we'd been given, audit wants to
> record the current directory, on the theory that relative pathname is none too
> useful in logs without knowing what had it been relative to.  Then, in the
> same thread, you call set_fs_pwd() - after all, that's the main effect of chdir(2).
> Deadlock...
>
> IOW, it's not just unshare(2) that needs to be taken care of - chdir(2) would need
> to be treated differently.

Now I realize that there is indeed a deadlock problem. Scrap that. Now I 
have a simpler idea that shouldn't have this type of deadlock problem. 
So what do you think about the sample code below?

Thanks,
Longman

=======================[ Cut here ]================================

diff --git a/fs/fs_struct.c b/fs/fs_struct.c
index b8c46c5a38a0..daeeb80cf088 100644
--- a/fs/fs_struct.c
+++ b/fs/fs_struct.c
@@ -32,15 +32,19 @@ void set_fs_root(struct fs_struct *fs, const struct 
path *p>
  void set_fs_pwd(struct fs_struct *fs, const struct path *path)
  {
         struct path old_pwd;
+       int xrefs;

         path_get(path);
         write_seqlock(&fs->seq);
         old_pwd = fs->pwd;
         fs->pwd = *path;
+       xrefs = fs->pwd_xrefs + 1;
+       fs->pwd_xrefs = 0;
         write_sequnlock(&fs->seq);

         if (old_pwd.dentry)
-               path_put(&old_pwd);
+               while (xrefs--)
+                       path_put(&old_pwd);
  }

  static inline int replace_path(struct path *p, const struct path *old, 
const s>
diff --git a/include/linux/fs_struct.h b/include/linux/fs_struct.h
index 0070764b790a..0d79d51de240 100644
--- a/include/linux/fs_struct.h
+++ b/include/linux/fs_struct.h
@@ -8,10 +8,11 @@
  #include <linux/seqlock.h>

  struct fs_struct {
-       int users;
         seqlock_t seq;
+       int users;
         int umask;
         int in_exec;
+       int pwd_xrefs;  /* Extra references of pwd */
         struct path root, pwd;
  } __randomize_layout;

@@ -40,6 +41,31 @@ static inline void get_fs_pwd(struct fs_struct *fs, 
struct p>
         read_sequnlock_excl(&fs->seq);
  }

+static inline void get_fs_pwd_share(struct fs_struct *fs, struct path *pwd)
+{
+       read_seqlock_excl(&fs->seq);
+       *pwd = fs->pwd;
+       if (fs->pwd_xrefs)
+               fs->pwd_xrefs--;
+       else
+               path_get(pwd);
+       read_sequnlock_excl(&fs->seq);
+}
+
+static inline void put_fs_pwd_share(struct fs_struct *fs, struct path *pwd)
+{
+       bool put = false;
+
+       read_seqlock_excl(&fs->seq);
+       if ((fs->pwd.dentry == pwd->dentry) && (fs->pwd.mnt == pwd->mnt))
+               fs->pwd_xrefs++;
+       else
+               put = true;
+       read_sequnlock_excl(&fs->seq);
+       if (put)
+               path_put(pwd);
+}
+
  extern bool current_chrooted(void);

  static inline int current_umask(void)