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

On 2/4/26 10:03 PM, Waiman Long wrote:
> 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? 

Well, a complete change log is as follows.

Cheers,
Longman

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

diff --git a/fs/fs_struct.c b/fs/fs_struct.c
index b8c46c5a38a0..67e08d8db058 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 count;

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

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

  static inline int replace_path(struct path *p, const struct path *old, 
const s>
@@ -70,6 +74,8 @@ void chroot_fs_refs(const struct path *old_root, const 
struct>
                                 count++;
                                 path_get(new_root);
                         }
+                       count += fs->pwd_xrefs;
+                       fs->pwd_xrefs = 0;
                         write_sequnlock(&fs->seq);
                 }
                 task_unlock(p);
@@ -81,8 +87,11 @@ void chroot_fs_refs(const struct path *old_root, 
const struc>

  void free_fs_struct(struct fs_struct *fs)
  {
+       int count = fs->pwd_xrefs + 1;
+
         path_put(&fs->root);
-       path_put(&fs->pwd);
+       while (count--)
+               path_put(&fs->pwd);
         kmem_cache_free(fs_cachep, fs);
  }

@@ -110,6 +119,7 @@ struct fs_struct *copy_fs_struct(struct fs_struct *old)
         if (fs) {
                 fs->users = 1;
                 fs->in_exec = 0;
+               fs->pwd_xrefs = 0;
                 seqlock_init(&fs->seq);
                 fs->umask = old->umask;

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)