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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
  1. fs/file.c +20 −2
  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 spin_unlock(&files->file_lock);706 spin_unlock(&files->file_lock);
707}707}
708 708
709static struct file *__fget(unsigned int fd, fmode_t mask, unsigned int refs)709static 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 struct file *file;712 struct file *file;
713 713
714 rcu_read_lock();714 rcu_read_lock();
@@ -729,6 +729,12 @@ loop: · 14 unmodified lines
729 return file;729 return file;
730}730}
731 731
732static 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
732struct file *fget_many(unsigned int fd, unsigned int refs)738struct file *fget_many(unsigned int fd, unsigned int refs)
733{739{
734 return __fget(fd, FMODE_PATH, refs);740 return __fget(fd, FMODE_PATH, refs);
@@ -746,6 +752,18 @@ struct file *fget_raw(unsigned int fd) · 11 unmodified lines
746}752}
747EXPORT_SYMBOL(fget_raw);753EXPORT_SYMBOL(fget_raw);
748 754
755struct 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 * Lightweight file lookup - no refcnt increment if fd table isn't shared.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
16extern void fput_many(struct file *, unsigned int);16extern void fput_many(struct file *, unsigned int);
17 17
18struct file_operations;18struct file_operations;
19struct task_struct;
19struct vfsmount;20struct vfsmount;
20struct dentry;21struct dentry;
21struct inode;22struct inode;
@@ -47,6 +48,7 @@ static inline void fdput(struct fd fd) · 25 unmodified lines
47extern struct file *fget(unsigned int fd);48extern struct file *fget(unsigned int fd);
48extern struct file *fget_many(unsigned int fd, unsigned int refs);49extern struct file *fget_many(unsigned int fd, unsigned int refs);
49extern struct file *fget_raw(unsigned int fd);50extern struct file *fget_raw(unsigned int fd);
51extern struct file *fget_task(struct task_struct *task, unsigned int fd);
50extern unsigned long __fdget(unsigned int fd);52extern unsigned long __fdget(unsigned int fd);
51extern unsigned long __fdget_raw(unsigned int fd);53extern unsigned long __fdget_raw(unsigned int fd);
52extern unsigned long __fdget_pos(unsigned int fd);54extern unsigned long __fdget_pos(unsigned int fd);