Commit 9c225f26 authored by Linus Torvalds's avatar Linus Torvalds Committed by Al Viro
Browse files

vfs: atomic f_pos accesses as per POSIX 



Our write() system call has always been atomic in the sense that you get
the expected thread-safe contiguous write, but we haven't actually
guaranteed that concurrent writes are serialized wrt f_pos accesses, so
threads (or processes) that share a file descriptor and use "write()"
concurrently would quite likely overwrite each others data.

This violates POSIX.1-2008/SUSv4 Section XSI 2.9.7 that says:

 "2.9.7 Thread Interactions with Regular File Operations

  All of the following functions shall be atomic with respect to each
  other in the effects specified in POSIX.1-2008 when they operate on
  regular files or symbolic links: [...]"

and one of the effects is the file position update.

This unprotected file position behavior is not new behavior, and nobody
has ever cared.  Until now.  Yongzhi Pan reported unexpected behavior to
Michael Kerrisk that was due to this.

This resolves the issue with a f_pos-specific lock that is taken by
read/write/lseek on file descriptors that may be shared across threads
or processes.

Reported-by: default avatarYongzhi Pan <panyongzhi@gmail.com>
Reported-by: default avatarMichael Kerrisk <mtk.manpages@gmail.com>
Cc: Al Viro <viro@zeniv.linux.org.uk>
Signed-off-by: default avatarLinus Torvalds <torvalds@linux-foundation.org>
Signed-off-by: default avatarAl Viro <viro@zeniv.linux.org.uk>
parent 1b56e989

fs/file_table.c

+1 −0
Original line number Diff line number Diff line
@@ -135,6 +135,7 @@ struct file *get_empty_filp(void) 
	atomic_long_set(&f->f_count, 1);

	rwlock_init(&f->f_owner.lock);

	spin_lock_init(&f->f_lock);

	mutex_init(&f->f_pos_lock);

	eventpoll_init_file(f);

	/* f->f_version: 0 */

	return f;

fs/namei.c

+1 −1
Original line number Diff line number Diff line
@@ -1884,7 +1884,7 @@ static int path_init(int dfd, const char *name, unsigned int flags, 


		nd->path = f.file->f_path;

		if (flags & LOOKUP_RCU) {

			if (f.need_put)

			if (f.flags & FDPUT_FPUT)

				*fp = f.file;

			nd->seq = __read_seqcount_begin(&nd->path.dentry->d_seq);

			rcu_read_lock();

fs/open.c

+4 −0
Original line number Diff line number Diff line
@@ -705,6 +705,10 @@ static int do_dentry_open(struct file *f, 
		return 0;

	}



	/* POSIX.1-2008/SUSv4 Section XSI 2.9.7 */

	if (S_ISREG(inode->i_mode))

		f->f_mode |= FMODE_ATOMIC_POS;



	f->f_op = fops_get(inode->i_fop);

	if (unlikely(WARN_ON(!f->f_op))) {

		error = -ENODEV;

fs/read_write.c

+40 −14
Original line number Diff line number Diff line
@@ -264,10 +264,36 @@ loff_t vfs_llseek(struct file *file, loff_t offset, int whence) 
}

EXPORT_SYMBOL(vfs_llseek);



/*

 * We only lock f_pos if we have threads or if the file might be

 * shared with another process. In both cases we'll have an elevated

 * file count (done either by fdget() or by fork()).

 */

static inline struct fd fdget_pos(int fd)

{

	struct fd f = fdget(fd);

	struct file *file = f.file;



	if (file && (file->f_mode & FMODE_ATOMIC_POS)) {

		if (file_count(file) > 1) {

			f.flags |= FDPUT_POS_UNLOCK;

			mutex_lock(&file->f_pos_lock);

		}

	}

	return f;

}



static inline void fdput_pos(struct fd f)

{

	if (f.flags & FDPUT_POS_UNLOCK)

		mutex_unlock(&f.file->f_pos_lock);

	fdput(f);

}



SYSCALL_DEFINE3(lseek, unsigned int, fd, off_t, offset, unsigned int, whence)

{

	off_t retval;

	struct fd f = fdget(fd);

	struct fd f = fdget_pos(fd);

	if (!f.file)

		return -EBADF;
@@ -278,7 +304,7 @@ SYSCALL_DEFINE3(lseek, unsigned int, fd, off_t, offset, unsigned int, whence) 
		if (res != (loff_t)retval)

			retval = -EOVERFLOW;	/* LFS: should only happen on 32 bit platforms */

	}

	fdput(f);

	fdput_pos(f);

	return retval;

}
@@ -498,7 +524,7 @@ static inline void file_pos_write(struct file *file, loff_t pos) 


SYSCALL_DEFINE3(read, unsigned int, fd, char __user *, buf, size_t, count)

{

	struct fd f = fdget(fd);

	struct fd f = fdget_pos(fd);

	ssize_t ret = -EBADF;



	if (f.file) {
@@ -506,7 +532,7 @@ SYSCALL_DEFINE3(read, unsigned int, fd, char __user *, buf, size_t, count) 
		ret = vfs_read(f.file, buf, count, &pos);

		if (ret >= 0)

			file_pos_write(f.file, pos);

		fdput(f);

		fdput_pos(f);

	}

	return ret;

}
@@ -514,7 +540,7 @@ SYSCALL_DEFINE3(read, unsigned int, fd, char __user *, buf, size_t, count) 
SYSCALL_DEFINE3(write, unsigned int, fd, const char __user *, buf,

		size_t, count)

{

	struct fd f = fdget(fd);

	struct fd f = fdget_pos(fd);

	ssize_t ret = -EBADF;



	if (f.file) {
@@ -522,7 +548,7 @@ SYSCALL_DEFINE3(write, unsigned int, fd, const char __user *, buf, 
		ret = vfs_write(f.file, buf, count, &pos);

		if (ret >= 0)

			file_pos_write(f.file, pos);

		fdput(f);

		fdput_pos(f);

	}



	return ret;
@@ -797,7 +823,7 @@ EXPORT_SYMBOL(vfs_writev); 
SYSCALL_DEFINE3(readv, unsigned long, fd, const struct iovec __user *, vec,

		unsigned long, vlen)

{

	struct fd f = fdget(fd);

	struct fd f = fdget_pos(fd);

	ssize_t ret = -EBADF;



	if (f.file) {
@@ -805,7 +831,7 @@ SYSCALL_DEFINE3(readv, unsigned long, fd, const struct iovec __user *, vec, 
		ret = vfs_readv(f.file, vec, vlen, &pos);

		if (ret >= 0)

			file_pos_write(f.file, pos);

		fdput(f);

		fdput_pos(f);

	}



	if (ret > 0)
@@ -817,7 +843,7 @@ SYSCALL_DEFINE3(readv, unsigned long, fd, const struct iovec __user *, vec, 
SYSCALL_DEFINE3(writev, unsigned long, fd, const struct iovec __user *, vec,

		unsigned long, vlen)

{

	struct fd f = fdget(fd);

	struct fd f = fdget_pos(fd);

	ssize_t ret = -EBADF;



	if (f.file) {
@@ -825,7 +851,7 @@ SYSCALL_DEFINE3(writev, unsigned long, fd, const struct iovec __user *, vec, 
		ret = vfs_writev(f.file, vec, vlen, &pos);

		if (ret >= 0)

			file_pos_write(f.file, pos);

		fdput(f);

		fdput_pos(f);

	}



	if (ret > 0)
@@ -968,7 +994,7 @@ COMPAT_SYSCALL_DEFINE3(readv, compat_ulong_t, fd, 
		const struct compat_iovec __user *,vec,

		compat_ulong_t, vlen)

{

	struct fd f = fdget(fd);

	struct fd f = fdget_pos(fd);

	ssize_t ret;

	loff_t pos;
@@ -978,7 +1004,7 @@ COMPAT_SYSCALL_DEFINE3(readv, compat_ulong_t, fd, 
	ret = compat_readv(f.file, vec, vlen, &pos);

	if (ret >= 0)

		f.file->f_pos = pos;

	fdput(f);

	fdput_pos(f);

	return ret;

}
@@ -1035,7 +1061,7 @@ COMPAT_SYSCALL_DEFINE3(writev, compat_ulong_t, fd, 
		const struct compat_iovec __user *, vec,

		compat_ulong_t, vlen)

{

	struct fd f = fdget(fd);

	struct fd f = fdget_pos(fd);

	ssize_t ret;

	loff_t pos;
@@ -1045,7 +1071,7 @@ COMPAT_SYSCALL_DEFINE3(writev, compat_ulong_t, fd, 
	ret = compat_writev(f.file, vec, vlen, &pos);

	if (ret >= 0)

		f.file->f_pos = pos;

	fdput(f);

	fdput_pos(f);

	return ret;

}

include/linux/file.h

+4 −2
Original line number Diff line number Diff line
@@ -28,12 +28,14 @@ static inline void fput_light(struct file *file, int fput_needed) 


struct fd {

	struct file *file;

	int need_put;

	unsigned int flags;

};

#define FDPUT_FPUT       1

#define FDPUT_POS_UNLOCK 2



static inline void fdput(struct fd fd)

{

	if (fd.need_put)

	if (fd.flags & FDPUT_FPUT)

		fput(fd.file);

}

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