From f23513e8d96cf5e6cf8d2ff0cb5dd6bbc33995e4 Mon Sep 17 00:00:00 2001 From: Ulrich Drepper Date: Sun, 15 Jul 2007 23:40:32 -0700 Subject: Introduce O_CLOEXEC The problem is as follows: in multi-threaded code (or more correctly: all code using clone() with CLONE_FILES) we have a race when exec'ing. thread #1 thread #2 fd=open() fork + exec fcntl(fd,F_SETFD,FD_CLOEXEC) In some applications this can happen frequently. Take a web browser. One thread opens a file and another thread starts, say, an external PDF viewer. The result can even be a security issue if that open file descriptor refers to a sensitive file and the external program can somehow be tricked into using that descriptor. Just adding O_CLOEXEC support to open() doesn't solve the whole set of problems. There are other ways to create file descriptors (socket, epoll_create, Unix domain socket transfer, etc). These can and should be addressed separately though. open() is such an easy case that it makes not much sense putting the fix off. The test program: #include #include #include #include #ifndef O_CLOEXEC # define O_CLOEXEC 02000000 #endif int main (int argc, char *argv[]) { int fd; if (argc > 1) { fd = atol (argv[1]); printf ("child: fd = %d\n", fd); if (fcntl (fd, F_GETFD) == 0 || errno != EBADF) { puts ("file descriptor valid in child"); return 1; } return 0; } fd = open ("/proc/self/exe", O_RDONLY | O_CLOEXEC); printf ("in parent: new fd = %d\n", fd); char buf[20]; snprintf (buf, sizeof (buf), "%d", fd); execl ("/proc/self/exe", argv[0], buf, NULL); puts ("execl failed"); return 1; } [kyle@parisc-linux.org: parisc fix] Signed-off-by: Ulrich Drepper Acked-by: Ingo Molnar Cc: Davide Libenzi Cc: Michael Kerrisk Cc: Chris Zankel Signed-off-by: Kyle McMartin Acked-by: David S. Miller Signed-off-by: Andrew Morton Signed-off-by: Linus Torvalds --- fs/open.c | 14 +++++++++++--- 1 file changed, 11 insertions(+), 3 deletions(-) (limited to 'fs') diff --git a/fs/open.c b/fs/open.c index 0d515d161974..e6991c1b5874 100644 --- a/fs/open.c +++ b/fs/open.c @@ -855,7 +855,7 @@ EXPORT_SYMBOL(dentry_open); /* * Find an empty file descriptor entry, and mark it busy. */ -int get_unused_fd(void) +static int get_unused_fd_flags(int flags) { struct files_struct * files = current->files; int fd, error; @@ -891,7 +891,10 @@ repeat: } FD_SET(fd, fdt->open_fds); - FD_CLR(fd, fdt->close_on_exec); + if (flags & O_CLOEXEC) + FD_SET(fd, fdt->close_on_exec); + else + FD_CLR(fd, fdt->close_on_exec); files->next_fd = fd + 1; #if 1 /* Sanity check */ @@ -907,6 +910,11 @@ out: return error; } +int get_unused_fd(void) +{ + return get_unused_fd_flags(0); +} + EXPORT_SYMBOL(get_unused_fd); static void __put_unused_fd(struct files_struct *files, unsigned int fd) @@ -959,7 +967,7 @@ long do_sys_open(int dfd, const char __user *filename, int flags, int mode) int fd = PTR_ERR(tmp); if (!IS_ERR(tmp)) { - fd = get_unused_fd(); + fd = get_unused_fd_flags(flags); if (fd >= 0) { struct file *f = do_filp_open(dfd, tmp, flags, mode); if (IS_ERR(f)) { -- cgit v1.2.3