From mboxrd@z Thu Jan 1 00:00:00 1970 Return-Path: X-Spam-Checker-Version: SpamAssassin 3.4.0 (2014-02-07) on aws-us-west-2-korg-lkml-1.web.codeaurora.org X-Spam-Level: X-Spam-Status: No, score=-6.8 required=3.0 tests=HEADER_FROM_DIFFERENT_DOMAINS, INCLUDES_PATCH,MAILING_LIST_MULTI,SIGNED_OFF_BY,SPF_HELO_NONE,SPF_PASS, URIBL_BLOCKED autolearn=ham autolearn_force=no version=3.4.0 Received: from mail.kernel.org (mail.kernel.org [198.145.29.99]) by smtp.lore.kernel.org (Postfix) with ESMTP id 170FFC2BA16 for ; Sat, 4 Apr 2020 05:58:23 +0000 (UTC) Received: from vger.kernel.org (vger.kernel.org [209.132.180.67]) by mail.kernel.org (Postfix) with ESMTP id DB24E206CC for ; Sat, 4 Apr 2020 05:58:22 +0000 (UTC) Received: (majordomo@vger.kernel.org) by vger.kernel.org via listexpand id S1725837AbgDDF6W (ORCPT ); Sat, 4 Apr 2020 01:58:22 -0400 Received: from relay1-d.mail.gandi.net ([217.70.183.193]:20169 "EHLO relay1-d.mail.gandi.net" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP id S1725536AbgDDF6W (ORCPT ); Sat, 4 Apr 2020 01:58:22 -0400 X-Originating-IP: 50.39.173.183 Received: from localhost (50-39-173-183.bvtn.or.frontiernet.net [50.39.173.183]) (Authenticated sender: josh@joshtriplett.org) by relay1-d.mail.gandi.net (Postfix) with ESMTPSA id 1A408240008; Sat, 4 Apr 2020 05:58:18 +0000 (UTC) Date: Fri, 3 Apr 2020 22:58:08 -0700 From: Josh Triplett To: io-uring@vger.kernel.org Cc: Jens Axboe Subject: [PATCH v3 1/3] fs: Support setting a minimum fd for "lowest available fd" allocation Message-ID: <90bf6fd43343ca862e7f61b0834baf2bdbd0e24c.1585978979.git.josh@joshtriplett.org> References: MIME-Version: 1.0 Content-Type: text/plain; charset=us-ascii Content-Disposition: inline In-Reply-To: Sender: io-uring-owner@vger.kernel.org Precedence: bulk List-ID: X-Mailing-List: io-uring@vger.kernel.org Some applications want to prevent the usual "lowest available fd" allocation from allocating certain file descriptors. For instance, they may want to prevent allocation of a closed fd 0, 1, or 2 other than via dup2/dup3, or reserve some low file descriptors for other purposes. Add a prctl to increase the minimum fd and return the previous minimum. System calls that allocate a specific file descriptor, such as dup2/dup3, ignore this minimum. exec resets the minimum fd, to prevent one program from interfering with another program's expectations about fd allocation. Test program: #include #include #include #include int main(int argc, char *argv[]) { if (prctl(PR_INCREASE_MIN_FD, 100, 0, 0, 0) < 0) err(1, "prctl"); int fd = open("/dev/null", O_RDONLY); if (fd < 0) err(1, "open"); printf("%d\n", fd); // prints 100 return 0; } Signed-off-by: Josh Triplett --- fs/file.c | 23 +++++++++++++++++------ include/linux/fdtable.h | 1 + include/linux/file.h | 1 + include/uapi/linux/prctl.h | 3 +++ kernel/sys.c | 5 +++++ 5 files changed, 27 insertions(+), 6 deletions(-) diff --git a/fs/file.c b/fs/file.c index c8a4e4c86e55..ba06140d89af 100644 --- a/fs/file.c +++ b/fs/file.c @@ -286,7 +286,6 @@ struct files_struct *dup_fd(struct files_struct *oldf, int *errorp) spin_lock_init(&newf->file_lock); newf->resize_in_progress = false; init_waitqueue_head(&newf->resize_wait); - newf->next_fd = 0; new_fdt = &newf->fdtab; new_fdt->max_fds = NR_OPEN_DEFAULT; new_fdt->close_on_exec = newf->close_on_exec_init; @@ -295,6 +294,7 @@ struct files_struct *dup_fd(struct files_struct *oldf, int *errorp) new_fdt->fd = &newf->fd_array[0]; spin_lock(&oldf->file_lock); + newf->next_fd = newf->min_fd = oldf->min_fd; old_fdt = files_fdtable(oldf); open_files = count_open_files(old_fdt); @@ -487,9 +487,7 @@ int __alloc_fd(struct files_struct *files, spin_lock(&files->file_lock); repeat: fdt = files_fdtable(files); - fd = start; - if (fd < files->next_fd) - fd = files->next_fd; + fd = max3(start, files->min_fd, files->next_fd); if (fd < fdt->max_fds) fd = find_next_fd(fdt, fd); @@ -514,7 +512,7 @@ int __alloc_fd(struct files_struct *files, goto repeat; if (start <= files->next_fd) - files->next_fd = fd + 1; + files->next_fd = max(fd + 1, files->min_fd); __set_open_fd(fd, fdt); if (flags & O_CLOEXEC) @@ -555,7 +553,7 @@ static void __put_unused_fd(struct files_struct *files, unsigned int fd) { struct fdtable *fdt = files_fdtable(files); __clear_open_fd(fd, fdt); - if (fd < files->next_fd) + if (fd < files->next_fd && fd >= files->min_fd) files->next_fd = fd; } @@ -684,6 +682,7 @@ void do_close_on_exec(struct files_struct *files) /* exec unshares first */ spin_lock(&files->file_lock); + files->min_fd = 0; for (i = 0; ; i++) { unsigned long set; unsigned fd = i * BITS_PER_LONG; @@ -865,6 +864,18 @@ bool get_close_on_exec(unsigned int fd) return res; } +unsigned int increase_min_fd(unsigned int num) +{ + struct files_struct *files = current->files; + unsigned int old_min_fd; + + spin_lock(&files->file_lock); + old_min_fd = files->min_fd; + files->min_fd += num; + spin_unlock(&files->file_lock); + return old_min_fd; +} + static int do_dup2(struct files_struct *files, struct file *file, unsigned fd, unsigned flags) __releases(&files->file_lock) diff --git a/include/linux/fdtable.h b/include/linux/fdtable.h index f07c55ea0c22..d1980443d8b3 100644 --- a/include/linux/fdtable.h +++ b/include/linux/fdtable.h @@ -60,6 +60,7 @@ struct files_struct { */ spinlock_t file_lock ____cacheline_aligned_in_smp; unsigned int next_fd; + unsigned int min_fd; /* min for "lowest available fd" allocation */ unsigned long close_on_exec_init[1]; unsigned long open_fds_init[1]; unsigned long full_fds_bits_init[1]; diff --git a/include/linux/file.h b/include/linux/file.h index 142d102f285e..b67986f818d2 100644 --- a/include/linux/file.h +++ b/include/linux/file.h @@ -88,6 +88,7 @@ extern bool get_close_on_exec(unsigned int fd); extern int __get_unused_fd_flags(unsigned flags, unsigned long nofile); extern int get_unused_fd_flags(unsigned flags); extern void put_unused_fd(unsigned int fd); +extern unsigned int increase_min_fd(unsigned int num); extern void fd_install(unsigned int fd, struct file *file); diff --git a/include/uapi/linux/prctl.h b/include/uapi/linux/prctl.h index 07b4f8131e36..916327272d21 100644 --- a/include/uapi/linux/prctl.h +++ b/include/uapi/linux/prctl.h @@ -238,4 +238,7 @@ struct prctl_mm_map { #define PR_SET_IO_FLUSHER 57 #define PR_GET_IO_FLUSHER 58 +/* Increase minimum file descriptor for "lowest available fd" allocation */ +#define PR_INCREASE_MIN_FD 59 + #endif /* _LINUX_PRCTL_H */ diff --git a/kernel/sys.c b/kernel/sys.c index d325f3ab624a..daa0ce43cecc 100644 --- a/kernel/sys.c +++ b/kernel/sys.c @@ -2514,6 +2514,11 @@ SYSCALL_DEFINE5(prctl, int, option, unsigned long, arg2, unsigned long, arg3, error = (current->flags & PR_IO_FLUSHER) == PR_IO_FLUSHER; break; + case PR_INCREASE_MIN_FD: + if (arg3 || arg4 || arg5) + return -EINVAL; + error = increase_min_fd((unsigned int)arg2); + break; default: error = -EINVAL; break; -- 2.26.0