[v2,06/24] proc/fd: In proc_fd_link use fget_task

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


Message ID 20201120231441.29911-6-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.

Simplifying proc_fd_link is a little bit tricky.  It is necessary to
know that there is a reference to fd_f	 ile while path_get is running.
This reference can either be guaranteed to exist either by locking the
fdtable as the code currently does or by taking a reference on the
file in question.

Use fget_task to remove the need for get_files_struct and
to take a reference to file in question.

[1] https://lkml.kernel.org/r/20180915160423.GA31461@redhat.com
Suggested-by: Oleg Nesterov <oleg@redhat.com>
v1: https://lkml.kernel.org/r/20200817220425.9389-8-ebiederm@xmission.com
Signed-off-by: "Eric W. Biederman" <ebiederm@xmission.com>
 fs/proc/fd.c | 13 +++----------
 1 file changed, 3 insertions(+), 10 deletions(-)

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diff --git a/fs/proc/fd.c b/fs/proc/fd.c
index 81882a13212d..d58960f6ee52 100644
--- a/fs/proc/fd.c
+++ b/fs/proc/fd.c
@@ -146,29 +146,22 @@  static const struct dentry_operations tid_fd_dentry_operations = {
 static int proc_fd_link(struct dentry *dentry, struct path *path)
-	struct files_struct *files = NULL;
 	struct task_struct *task;
 	int ret = -ENOENT;
 	task = get_proc_task(d_inode(dentry));
 	if (task) {
-		files = get_files_struct(task);
-		put_task_struct(task);
-	}
-	if (files) {
 		unsigned int fd = proc_fd(d_inode(dentry));
 		struct file *fd_file;
-		spin_lock(&files->file_lock);
-		fd_file = fcheck_files(files, fd);
+		fd_file = fget_task(task, fd);
 		if (fd_file) {
 			*path = fd_file->f_path;
 			ret = 0;
+			fput(fd_file);
-		spin_unlock(&files->file_lock);
-		put_files_struct(files);
+		put_task_struct(task);
 	return ret;