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proc/fd: In proc_fd_link use fget_task

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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
Link: https://lkml.kernel.org/r/20201120231441.29911-6-ebiederm@xmission.com
Signed-off-by: Eric W. Biederman <ebiederm@xmission.com>
This commit is contained in:
Eric W. Biederman 2020-11-20 17:14:23 -06:00
parent b48845af01
commit 439be32656
1 changed files with 3 additions and 10 deletions
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13
fs/proc/fd.c
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@ -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;
path_get(&fd_file->f_path);
ret = 0;
fput(fd_file);
}
spin_unlock(&files->file_lock);
put_files_struct(files);
put_task_struct(task);
}
return ret;