Commit 5b17b618 authored Nov 20, 2020 by Eric W. Biederman

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: Oleg Nesterov <oleg@redhat.com>
Tested-by: Andy 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: Eric 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;

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