eventfs: Test for ei->is_freed when accessing ei->dentry

The eventfs_inode (ei) is protected by SRCU, but the ei->dentry is not. It
is protected by the eventfs_mutex. Anytime the eventfs_mutex is released,
and access to the ei->dentry needs to be done, it should first check if
ei->is_freed is set under the eventfs_mutex. If it is, then the ei->dentry
is invalid and must not be used. The ei->dentry must only be accessed
under the eventfs_mutex and after checking if ei->is_freed is set.

When the ei is being freed, it will (under the eventfs_mutex) set is_freed
and at the same time move the dentry to a free list to be cleared after
the eventfs_mutex is released. This means that any access to the
ei->dentry must check first if ei->is_freed is set, because if it is, then
the dentry is on its way to be freed.

Also add comments to describe this better.

Link: https://lore.kernel.org/all/CA+G9fYt6pY+tMZEOg=SoEywQOe19fGP3uR15SGowkdK+_X85Cg@mail.gmail.com/
Link: https://lore.kernel.org/all/CA+G9fYuDP3hVQ3t7FfrBAjd_WFVSurMgCepTxunSJf=MTe=6aA@mail.gmail.com/
Link: https://lkml.kernel.org/r/20231101172649.477608228@goodmis.org

Cc: Ajay Kaher <akaher@vmware.com>
Cc: Mark Rutland <mark.rutland@arm.com>
Cc: Andrew Morton <akpm@linux-foundation.org>
Fixes: 5790b1fb3d672 ("eventfs: Remove eventfs_file and just use eventfs_inode")
Reported-by: Linux Kernel Functional Testing <lkft@linaro.org>
Reported-by: Naresh Kamboju <naresh.kamboju@linaro.org>
Reported-by: Beau Belgrave <beaub@linux.microsoft.com>
Reviewed-by: Masami Hiramatsu (Google) <mhiramat@kernel.org>
Tested-by: Linux Kernel Functional Testing <lkft@linaro.org>
Tested-by: Naresh Kamboju <naresh.kamboju@linaro.org>
Tested-by: Beau Belgrave <beaub@linux.microsoft.com>
Signed-off-by: Steven Rostedt (Google) <rostedt@goodmis.org>

1 files changed, 2 insertions, 1 deletions

diff --git a/fs/tracefs/internal.h b/fs/tracefs/internal.h
index c7d88aaa949f..5a98e87dd3d1 100644
--- a/fs/tracefs/internal.h
+++ b/fs/tracefs/internal.h
@@ -24,6 +24,7 @@ struct tracefs_inode {
  * @d_children: The array of dentries to represent the files when created
  * @data:	The private data to pass to the callbacks
  * @is_freed:	Flag set if the eventfs is on its way to be freed
+ *                Note if is_freed is set, then dentry is corrupted.
  * @nr_entries: The number of items in @entries
  */
 struct eventfs_inode {
@@ -31,7 +32,7 @@ struct eventfs_inode {
 	const struct eventfs_entry	*entries;
 	const char			*name;
 	struct list_head		children;
-	struct dentry			*dentry;
+	struct dentry			*dentry; /* Check is_freed to access */
 	struct dentry			*d_parent;
 	struct dentry			**d_children;
 	void				*data;