Commit 5b17b618 authored by Eric W. Biederman's avatar Eric W. Biederman
Browse files

proc/fd: In proc_readfd_common use task_lookup_next_fd_rcu 

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.

Using task_lookup_next_fd_rcu simplifies proc_readfd_common, by moving
the checking for the maximum file descritor into the generic code, and
by remvoing the need for capturing and releasing a reference on
files_struct.

As task_lookup_fd_rcu may update the fd ctx->pos has been changed
to be the fd +2 after task_lookup_fd_rcu returns.

[1] https://lkml.kernel.org/r/20180915160423.GA31461@redhat.com


Suggested-by: default avatarOleg Nesterov <oleg@redhat.com>
Tested-by: default avatarAndy Lavr <andy.lavr@gmail.com>
v1: https://lkml.kernel.org/r/20200817220425.9389-10-ebiederm@xmission.com
Link: https://lkml.kernel.org/r/20201120231441.29911-15-ebiederm@xmission.com


Signed-off-by: default avatarEric W. Biederman <ebiederm@xmission.com>
parent e9a53aeb

fs/proc/fd.c

+5 −12
Original line number Diff line number Diff line
@@ -217,7 +217,6 @@ static int proc_readfd_common(struct file *file, struct dir_context *ctx, 
			      instantiate_t instantiate)

{

	struct task_struct *p = get_proc_task(file_inode(file));

	struct files_struct *files;

	unsigned int fd;



	if (!p)
@@ -225,22 +224,18 @@ static int proc_readfd_common(struct file *file, struct dir_context *ctx, 


	if (!dir_emit_dots(file, ctx))

		goto out;

	files = get_files_struct(p);

	if (!files)

		goto out;



	rcu_read_lock();

	for (fd = ctx->pos - 2;

	     fd < files_fdtable(files)->max_fds;

	     fd++, ctx->pos++) {

	for (fd = ctx->pos - 2;; fd++) {

		struct file *f;

		struct fd_data data;

		char name[10 + 1];

		unsigned int len;



		f = files_lookup_fd_rcu(files, fd);

		f = task_lookup_next_fd_rcu(p, &fd);

		ctx->pos = fd + 2LL;

		if (!f)

			continue;

			break;

		data.mode = f->f_mode;

		rcu_read_unlock();

		data.fd = fd;
@@ -249,13 +244,11 @@ static int proc_readfd_common(struct file *file, struct dir_context *ctx, 
		if (!proc_fill_cache(file, ctx,

				     name, len, instantiate, p,

				     &data))

			goto out_fd_loop;

			goto out;

		cond_resched();

		rcu_read_lock();

	}

	rcu_read_unlock();

out_fd_loop:

	put_files_struct(files);

out:

	put_task_struct(p);

	return 0;