[v2,17/24] proc/fd: In fdinfo seq_show don't use get_files_struct

Submitted by Eric W. Biederman on Nov. 20, 2020, 11:14 p.m.

Details

Message ID 20201120231441.29911-17-ebiederm@xmission.com
State New
Series "exec: Move unshare_files and guarantee files_struct.count is correct"
Headers show

Commit Message

Eric W. Biederman Nov. 20, 2020, 11:14 p.m.
When discussing[1] exec and posix file locks it was realized that none
of the callers of get_files_struct fundamentally needed to call
get_files_struct, and that by switching them to helper functions
instead it will both simplify their code and remove unnecessary
increments of files_struct.count.  Those unnecessary increments can
result in exec unnecessarily unsharing files_struct which breaking
posix locks, and it can result in fget_light having to fallback to
fget reducing system performance.

Instead hold task_lock for the duration that task->files needs to be
stable in seq_show.  The task_lock was already taken in
get_files_struct, and so skipping get_files_struct performs less work
overall, and avoids the problems with the files_struct reference
count.

[1] https://lkml.kernel.org/r/20180915160423.GA31461@redhat.com
Suggested-by: Oleg Nesterov <oleg@redhat.com>
Acked-by: Christian Brauner <christian.brauner@ubuntu.com>
v1: https://lkml.kernel.org/r/20200817220425.9389-12-ebiederm@xmission.com
Signed-off-by: "Eric W. Biederman" <ebiederm@xmission.com>
---
 fs/proc/fd.c | 9 +++++----
 1 file changed, 5 insertions(+), 4 deletions(-)

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diff --git a/fs/proc/fd.c b/fs/proc/fd.c
index 72c1525b4b3e..cb51763ed554 100644
--- a/fs/proc/fd.c
+++ b/fs/proc/fd.c
@@ -28,9 +28,8 @@  static int seq_show(struct seq_file *m, void *v)
 	if (!task)
 		return -ENOENT;
 
-	files = get_files_struct(task);
-	put_task_struct(task);
-
+	task_lock(task);
+	files = task->files;
 	if (files) {
 		unsigned int fd = proc_fd(m->private);
 
@@ -47,8 +46,9 @@  static int seq_show(struct seq_file *m, void *v)
 			ret = 0;
 		}
 		spin_unlock(&files->file_lock);
-		put_files_struct(files);
 	}
+	task_unlock(task);
+	put_task_struct(task);
 
 	if (ret)
 		return ret;
@@ -57,6 +57,7 @@  static int seq_show(struct seq_file *m, void *v)
 		   (long long)file->f_pos, f_flags,
 		   real_mount(file->f_path.mnt)->mnt_id);
 
+	/* show_fd_locks() never deferences files so a stale value is safe */
 	show_fd_locks(m, file, files);
 	if (seq_has_overflowed(m))
 		goto out;