Re: [PATCH] eventfs: Stop using dcache_readdir() for getdents()

[Linus Torvalds <torvalds@linux-foundation.org>]

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On Wed, 3 Jan 2024 at 10:12, Linus Torvalds
<torvalds@linux-foundation.org> wrote:
>
> Much better. Now eventfs looks more like a real filesystem, and less
> like an eldritch horror monster that is parts of dcache tackled onto a
> pseudo-filesystem.

Oh, except I think you still need to just remove the 'set_gid()' mess.

It's disgusting and it's wrong, and it's not even what the 'uid'
option does (it only sets the root inode uid).

If you remount the filesystem with different gid values, you get to
keep both broken pieces. And if it isn't a remount, then setting the
root uid is sufficient.

I think the whole thing was triggered by commit 49d67e445742, and
maybe the fix is to just revert that commit.

That commit makes no sense in general, since the default mounting
position for tracefs that the kernel sets up is only accessible to
root anyway.

Alternatively, just do the ->permissions() thing, and allow access to
the group in the mount options.

Getting rid of set_gid() would be this attached lovely patch:

 fs/tracefs/inode.c | 83 ++----------------------------------------------------
 1 file changed, 2 insertions(+), 81 deletions(-)

and would get rid of the final (?) piece of disgusting dcache hackery
that tracefs most definitely should not have.

             Linus

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 fs/tracefs/inode.c | 83 ++----------------------------------------------------
 1 file changed, 2 insertions(+), 81 deletions(-)

diff --git a/fs/tracefs/inode.c b/fs/tracefs/inode.c
index 62524b20964e..a22253037e3e 100644
--- a/fs/tracefs/inode.c
+++ b/fs/tracefs/inode.c
@@ -183,83 +183,6 @@ struct tracefs_fs_info {
 	struct tracefs_mount_opts mount_opts;
 };
 
-static void change_gid(struct dentry *dentry, kgid_t gid)
-{
-	if (!dentry->d_inode)
-		return;
-	dentry->d_inode->i_gid = gid;
-}
-
-/*
- * Taken from d_walk, but without he need for handling renames.
- * Nothing can be renamed while walking the list, as tracefs
- * does not support renames. This is only called when mounting
- * or remounting the file system, to set all the files to
- * the given gid.
- */
-static void set_gid(struct dentry *parent, kgid_t gid)
-{
-	struct dentry *this_parent;
-	struct list_head *next;
-
-	this_parent = parent;
-	spin_lock(&this_parent->d_lock);
-
-	change_gid(this_parent, gid);
-repeat:
-	next = this_parent->d_subdirs.next;
-resume:
-	while (next != &this_parent->d_subdirs) {
-		struct tracefs_inode *ti;
-		struct list_head *tmp = next;
-		struct dentry *dentry = list_entry(tmp, struct dentry, d_child);
-		next = tmp->next;
-
-		spin_lock_nested(&dentry->d_lock, DENTRY_D_LOCK_NESTED);
-
-		change_gid(dentry, gid);
-
-		/* If this is the events directory, update that too */
-		ti = get_tracefs(dentry->d_inode);
-		if (ti && (ti->flags & TRACEFS_EVENT_INODE))
-			eventfs_update_gid(dentry, gid);
-
-		if (!list_empty(&dentry->d_subdirs)) {
-			spin_unlock(&this_parent->d_lock);
-			spin_release(&dentry->d_lock.dep_map, _RET_IP_);
-			this_parent = dentry;
-			spin_acquire(&this_parent->d_lock.dep_map, 0, 1, _RET_IP_);
-			goto repeat;
-		}
-		spin_unlock(&dentry->d_lock);
-	}
-	/*
-	 * All done at this level ... ascend and resume the search.
-	 */
-	rcu_read_lock();
-ascend:
-	if (this_parent != parent) {
-		struct dentry *child = this_parent;
-		this_parent = child->d_parent;
-
-		spin_unlock(&child->d_lock);
-		spin_lock(&this_parent->d_lock);
-
-		/* go into the first sibling still alive */
-		do {
-			next = child->d_child.next;
-			if (next == &this_parent->d_subdirs)
-				goto ascend;
-			child = list_entry(next, struct dentry, d_child);
-		} while (unlikely(child->d_flags & DCACHE_DENTRY_KILLED));
-		rcu_read_unlock();
-		goto resume;
-	}
-	rcu_read_unlock();
-	spin_unlock(&this_parent->d_lock);
-	return;
-}
-
 static int tracefs_parse_options(char *data, struct tracefs_mount_opts *opts)
 {
 	substring_t args[MAX_OPT_ARGS];
@@ -332,10 +255,8 @@ static int tracefs_apply_options(struct super_block *sb, bool remount)
 	if (!remount || opts->opts & BIT(Opt_uid))
 		inode->i_uid = opts->uid;
 
-	if (!remount || opts->opts & BIT(Opt_gid)) {
-		/* Set all the group ids to the mount option */
-		set_gid(sb->s_root, opts->gid);
-	}
+	if (!remount || opts->opts & BIT(Opt_gid))
+		inode->i_gid = opts->gid;
 
 	return 0;
 }