Unverified Commit 5e876fb4 authored by Sargun Dhillon's avatar Sargun Dhillon Committed by Christian Brauner
Browse files

vfs, fdtable: Add fget_task helper 

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.

This patch is based on Oleg Nesterov's (cf. [1]) patch from September
2018.

[1]: Link: https://lore.kernel.org/r/20180915160423.GA31461@redhat.com



Signed-off-by: default avatarSargun Dhillon <sargun@sargun.me>
Suggested-by: default avatarOleg Nesterov <oleg@redhat.com>
Acked-by: default avatarChristian Brauner <christian.brauner@ubuntu.com>
Reviewed-by: default avatarArnd Bergmann <arnd@arndb.de>
Link: https://lore.kernel.org/r/20200107175927.4558-2-sargun@sargun.me


Signed-off-by: default avatarChristian Brauner <christian.brauner@ubuntu.com>
parent c79f46a2

fs/file.c

+20 −2
Original line number Diff line number Diff line
@@ -706,9 +706,9 @@ void do_close_on_exec(struct files_struct *files) 
	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 +729,12 @@ loop: 
	return file;

}



static inline 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);
@@ -746,6 +752,18 @@ struct file *fget_raw(unsigned int fd) 
}

EXPORT_SYMBOL(fget_raw);



struct file *fget_task(struct task_struct *task, unsigned int fd)

{

	struct file *file = NULL;



	task_lock(task);

	if (task->files)

		file = __fget_files(task->files, fd, 0, 1);

	task_unlock(task);



	return file;

}



/*

 * Lightweight file lookup - no refcnt increment if fd table isn't shared.

 *

include/linux/file.h

+2 −0
Original line number Diff line number Diff line
@@ -16,6 +16,7 @@ extern void fput(struct file *); 
extern void fput_many(struct file *, unsigned int);



struct file_operations;

struct task_struct;

struct vfsmount;

struct dentry;

struct inode;
@@ -47,6 +48,7 @@ static inline void fdput(struct fd fd) 
extern struct file *fget(unsigned int fd);

extern struct file *fget_many(unsigned int fd, unsigned int refs);

extern struct file *fget_raw(unsigned int fd);

extern struct file *fget_task(struct task_struct *task, unsigned int fd);

extern unsigned long __fdget(unsigned int fd);

extern unsigned long __fdget_raw(unsigned int fd);

extern unsigned long __fdget_pos(unsigned int fd);

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