From 9c225f2655e36a470c4f58dbbc99244c5fc7f2d4 Mon Sep 17 00:00:00 2001 From: Linus Torvalds Date: Mon, 3 Mar 2014 09:36:58 -0800 Subject: 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: Yongzhi Pan Reported-by: Michael Kerrisk Cc: Al Viro Signed-off-by: Linus Torvalds Signed-off-by: Al Viro --- include/linux/file.h | 6 ++++-- 1 file changed, 4 insertions(+), 2 deletions(-) (limited to 'include/linux/file.h') diff --git a/include/linux/file.h b/include/linux/file.h index cbacf4faf447..f2517fa2d610 100644 --- a/include/linux/file.h +++ b/include/linux/file.h @@ -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); } -- cgit v1.2.3-58-ga151