[PATCH 12/17] proc/fd: In fdinfo seq_show don't use get_files_struct

Eric W. Biederman
2020-08-17T22:12:26Z

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>
Signed-off-by: "Eric W. Biederman" <ebiederm@xmission.com>
---
fs/proc/fd.c | 9 +++++----
1 file changed, 5 insertions(+), 4 deletions(-)

diff --git a/fs/proc/fd.c b/fs/proc/fd.c
index d9fee5390fd7..0b46eea154b7 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;
--
2.25.0