[v2,1/4] vfs, fdtable: Add get_task_file helper
diff mbox series

Message ID 20191209070609.GA32438@ircssh-2.c.rugged-nimbus-611.internal
State New
Headers show
Series
  • Add ptrace get_fd request
Related show

Commit Message

Sargun Dhillon Dec. 9, 2019, 7:06 a.m. UTC
This introduces a function which can be used to fetch a file, given an
arbitrary task. As long as the user holds a reference (refcnt) to the
task_struct it is safe to call, and will either return NULL on failure,
or a pointer to the file, with a refcnt.

Signed-off-by: Sargun Dhillon <sargun@sargun.me>
---
 fs/file.c               | 19 +++++++++++++++++++
 include/linux/fdtable.h | 10 ++++++++++
 2 files changed, 29 insertions(+)

Comments

Oleg Nesterov Dec. 9, 2019, 3:52 p.m. UTC | #1
On 12/09, Sargun Dhillon wrote:
>
> +struct file *get_task_file(struct task_struct *task, unsigned int fd)
> +{
> +	struct file *file = NULL;
> +
> +	task_lock(task);
> +	rcu_read_lock();
> +
> +	if (task->files) {
> +		file = fcheck_files(task->files, fd);
> +		if (file && !get_file_rcu(file))
> +			file = NULL;
> +	}

On second thought this is not exactly right, get_file_rcu() can fail if
get_task_file() races with dup2(), in this case we need to do fcheck_files()
again. And this is what __fget() already does, so may be the patch below
makes more sense?

I will leave this to other reviewers, but suddenly I recall that I have
already sent the patch which adds a similar helper a while ago.

See https://lore.kernel.org/lkml/20180915160423.GA31461@redhat.com/

In short, get_files_struct() should be avoided because it can race with
exec() and break POSIX locks which use ->fl_owner = files_struct.

Oleg.

--- x/fs/file.c
+++ x/fs/file.c
@@ -706,9 +706,9 @@ void do_close_on_exec(struct files_struc
 	spin_unlock(&files->file_lock);
 }
 
-static struct file *__fget(unsigned int fd, fmode_t mask, unsigned int refs)
+static struct file *__fget_files(struct files_struct *files, unsigned int fd,
+					fmode_t mask, unsigned int refs)
 {
-	struct files_struct *files = current->files;
 	struct file *file;
 
 	rcu_read_lock();
@@ -729,6 +729,23 @@ loop:
 	return file;
 }
 
+struct file *fget_task(struct task_struct *task, unsigned int fd)
+{
+	struct file *file;
+
+	task_lock(task);
+	if (task->files)
+		file = __fget_files(task->files, fd, 0, 1);
+	task_unlock(task);
+
+	return file;
+}
+
+static struct file *__fget(unsigned int fd, fmode_t mask, unsigned int refs)
+{
+	return __fget_files(current->files, fd, mask, refs);
+}
+
 struct file *fget_many(unsigned int fd, unsigned int refs)
 {
 	return __fget(fd, FMODE_PATH, refs);

Patch
diff mbox series

diff --git a/fs/file.c b/fs/file.c
index 3da91a112bab..98601a503a0f 100644
--- a/fs/file.c
+++ b/fs/file.c
@@ -1015,3 +1015,22 @@  int iterate_fd(struct files_struct *files, unsigned n,
 	return res;
 }
 EXPORT_SYMBOL(iterate_fd);
+
+struct file *get_task_file(struct task_struct *task, unsigned int fd)
+{
+	struct file *file = NULL;
+
+	task_lock(task);
+	rcu_read_lock();
+
+	if (task->files) {
+		file = fcheck_files(task->files, fd);
+		if (file && !get_file_rcu(file))
+			file = NULL;
+	}
+
+	rcu_read_unlock();
+	task_unlock(task);
+
+	return file;
+}
diff --git a/include/linux/fdtable.h b/include/linux/fdtable.h
index f07c55ea0c22..eacb1a56df44 100644
--- a/include/linux/fdtable.h
+++ b/include/linux/fdtable.h
@@ -115,6 +115,16 @@  int iterate_fd(struct files_struct *, unsigned,
 		int (*)(const void *, struct file *, unsigned),
 		const void *);
 
+/*
+ * get_task_file - get a reference to a file from another task
+ * @task: the task to get the file descriptor from
+ * @fd: the file descriptor number to fetch
+ *
+ * returns NULL on failure, or pointer to the file on success, with a reference
+ * It requires that the task is pinned prior to calling it.
+ */
+struct file *get_task_file(struct task_struct *task, unsigned int fd);
+
 extern int __alloc_fd(struct files_struct *files,
 		      unsigned start, unsigned end, unsigned flags);
 extern void __fd_install(struct files_struct *files,