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

Sargun Dhillon committed 6 years ago

Commit 5e876fb1 parent c79f46a2 files · +22 −2Browse files

Unified · Split

2 files

fs/file.c +20 −2
include/linux/file.h +2 −0

fs/file.c+20−2

@@ -706,9 +706,9 @@ void do_close_on_exec(struct files_struct *files) · 705 unmodified lines
706	706	spin_unlock(&files->file_lock);
707	707	}
708	708
709		−static struct file *__fget(unsigned int fd, fmode_t mask, unsigned int refs)
	709	+static struct file __fget_files(struct files_struct files, unsigned int fd,
	710	+ fmode_t mask, unsigned int refs)
710	711	{
711		− struct files_struct *files = current->files;
712	712	struct file *file;
713	713
714	714	rcu_read_lock();
@@ -729,6 +729,12 @@ loop: · 14 unmodified lines
729	729	return file;
730	730	}
731	731
	732	+static inline struct file *__fget(unsigned int fd, fmode_t mask,
	733	+ unsigned int refs)
	734	+{
	735	+ return __fget_files(current->files, fd, mask, refs);
	736	+}
	737	+
732	738	struct file *fget_many(unsigned int fd, unsigned int refs)
733	739	{
734	740	return __fget(fd, FMODE_PATH, refs);
@@ -746,6 +752,18 @@ struct file *fget_raw(unsigned int fd) · 11 unmodified lines
746	752	}
747	753	EXPORT_SYMBOL(fget_raw);
748	754
	755	+struct file fget_task(struct task_struct task, unsigned int fd)
	756	+{
	757	+ struct file *file = NULL;
	758	+
	759	+ task_lock(task);
	760	+ if (task->files)
	761	+ file = __fget_files(task->files, fd, 0, 1);
	762	+ task_unlock(task);
	763	+
	764	+ return file;
	765	+}
	766	+
749	767	/*
750	768	* Lightweight file lookup - no refcnt increment if fd table isn't shared.
751	769	*

include/linux/file.h+2−0

@@ -16,6 +16,7 @@ extern void fput(struct file *); · 15 unmodified lines
16	16	extern void fput_many(struct file *, unsigned int);
17	17
18	18	struct file_operations;
	19	+struct task_struct;
19	20	struct vfsmount;
20	21	struct dentry;
21	22	struct inode;
@@ -47,6 +48,7 @@ static inline void fdput(struct fd fd) · 25 unmodified lines
47	48	extern struct file *fget(unsigned int fd);
48	49	extern struct file *fget_many(unsigned int fd, unsigned int refs);
49	50	extern struct file *fget_raw(unsigned int fd);
	51	+extern struct file fget_task(struct task_struct task, unsigned int fd);
50	52	extern unsigned long __fdget(unsigned int fd);
51	53	extern unsigned long __fdget_raw(unsigned int fd);
52	54	extern unsigned long __fdget_pos(unsigned int fd);