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vfs, fdtable: Add fget_task helper

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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: Sargun Dhillon <sargun@sargun.me>
Suggested-by: Oleg Nesterov <oleg@redhat.com>
Acked-by: Christian Brauner <christian.brauner@ubuntu.com>
Reviewed-by: Arnd Bergmann <arnd@arndb.de>
Link: https://lore.kernel.org/r/20200107175927.4558-2-sargun@sargun.me
Signed-off-by: Christian Brauner <christian.brauner@ubuntu.com>
This commit is contained in:
Sargun Dhillon 2020-01-07 09:59:24 -08:00 committed by Christian Brauner
parent c79f46a282
commit 5e876fb43d
No known key found for this signature in database
GPG Key ID: 91C61BC06578DCA2
2 changed files with 22 additions and 2 deletions
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22
fs/file.c
View File

@ -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.
*

2
include/linux/file.h
View File

@ -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);