[PATCH 10/17] proc/fd: In proc_readfd_common use fnext_task

[Eric W. Biederman]

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 fnext_task 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.

[1] https://lkml.kernel.org/r/20180915160423.GA31461@xxxxxxxxxx
Suggested-by: Oleg Nesterov <oleg@xxxxxxxxxx>
Signed-off-by: Eric W. Biederman <ebiederm@xxxxxxxxxxxx>
---
 fs/proc/fd.c | 17 +++++------------
 1 file changed, 5 insertions(+), 12 deletions(-)

diff --git a/fs/proc/fd.c b/fs/proc/fd.c
index abfdcb21cc79..d9fee5390fd7 100644
--- a/fs/proc/fd.c
+++ b/fs/proc/fd.c
@@ -218,7 +218,6 @@ 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)
@@ -226,22 +225,18 @@ 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++, ctx->pos++) {
 		struct file *f;
 		struct fd_data data;
 		char name[10 + 1];
 		unsigned int len;
 
-		f = fcheck_files(files, fd);
+		f = fnext_task(p, &fd);
+		ctx->pos = fd;
 		if (!f)
-			continue;
+			break;
 		data.mode = f->f_mode;
 		rcu_read_unlock();
 		data.fd = fd;
@@ -250,13 +245,11 @@ 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;
-- 
2.25.0