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

Eric W. Biederman
2020-08-17T22:10:34Z

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@redhat.com
Suggested-by: Oleg Nesterov <oleg@redhat.com>
Signed-off-by: Eric W. Biederman <ebiederm@xmission.com>
---
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