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=-6.6 required=3.0 tests=DKIM_SIGNED,DKIM_VALID, DKIM_VALID_AU,FREEMAIL_FORGED_FROMDOMAIN,FREEMAIL_FROM, HEADER_FROM_DIFFERENT_DOMAINS,INCLUDES_PATCH,MAILING_LIST_MULTI,SIGNED_OFF_BY, SPF_HELO_NONE,SPF_PASS,URIBL_BLOCKED autolearn=unavailable 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 C378BC55189 for ; Wed, 22 Apr 2020 06:06:16 +0000 (UTC) Received: from vger.kernel.org (vger.kernel.org [23.128.96.18]) by mail.kernel.org (Postfix) with ESMTP id 9E08C206D6 for ; Wed, 22 Apr 2020 06:06:16 +0000 (UTC) Authentication-Results: mail.kernel.org; dkim=pass (2048-bit key) header.d=gmail.com header.i=@gmail.com header.b="aN+izJ3/" Received: (majordomo@vger.kernel.org) by vger.kernel.org via listexpand id S1726444AbgDVGGN (ORCPT ); Wed, 22 Apr 2020 02:06:13 -0400 Received: from lindbergh.monkeyblade.net ([23.128.96.19]:52204 "EHLO lindbergh.monkeyblade.net" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP id S1725912AbgDVGGM (ORCPT ); Wed, 22 Apr 2020 02:06:12 -0400 Received: from mail-ed1-x541.google.com (mail-ed1-x541.google.com [IPv6:2a00:1450:4864:20::541]) by lindbergh.monkeyblade.net (Postfix) with ESMTPS id 86031C03C1A6; Tue, 21 Apr 2020 23:06:12 -0700 (PDT) Received: by mail-ed1-x541.google.com with SMTP id t12so642136edw.3; Tue, 21 Apr 2020 23:06:12 -0700 (PDT) DKIM-Signature: v=1; a=rsa-sha256; c=relaxed/relaxed; d=gmail.com; s=20161025; h=mime-version:references:in-reply-to:reply-to:from:date:message-id :subject:to:cc; bh=Hf6xP/vargsYyqhriKqrcqcKu8IaknYAwCr2SAXzhXI=; b=aN+izJ3/KqQp3SAdWwIWSP+jcBw5otAN/1GD5K+BlWRji7Zd3bMHJzSqn7qsh7vAdK QXJV1CGsw102VJx6a1lvvX1cW0L3ps5LZh30zWvS1SkFwSb+XBig/NUk/pWP8GJtmBH7 5IfjGFW1eUjF4/GQWX+9fMIJalMR0VrFF9vzKYNZurzjs9IqaeC4x5bf/Hdd+XCCHYsi albDIhy/E2gI2yvuF3cSVGxeV9A+wNbYK/GJNtYW1Mqrl8bcaOu4+ExKEmnFkp9bL5tP CxG3QuWgX0HPdmCvbUkia9CFzgUieDzExNpJSmvh8Uh1nRWWUafYiDal2wXaj6MB+0tF G7Yg== X-Google-DKIM-Signature: v=1; a=rsa-sha256; c=relaxed/relaxed; d=1e100.net; s=20161025; h=x-gm-message-state:mime-version:references:in-reply-to:reply-to :from:date:message-id:subject:to:cc; bh=Hf6xP/vargsYyqhriKqrcqcKu8IaknYAwCr2SAXzhXI=; b=d/dRm1a8KOF4ETZ40hOlUe23dVmp2/4tPWS0QqMmQmAhSc6FAWZX4xgZLorh2v+hC5 cP5rPmO8hcIoX7juiNz+kU8o69v9n6REf6Fffjh93YlochOsXp0up3unPAPewPS8TJX3 3nzJRMmON7vbBON+mTInsfh6dm2i9as1R/8RIKiCZG3d6n+6sCQCNpNYj8C9+UGtKKc4 cptS6IeybL6e3ltiu0lxsIct9L7ENH+qJPPvrKRIAhYADlxzZDJbbsiudtAgHDf6q4aa BjOT3SG+zf8xWXPNxCDgJbKl/Hq6YH1/cthJbANc46hKb712RmXsiK3iVeeDXhfgIHuu +ajw== X-Gm-Message-State: AGi0PubrmzEL9MSxdVPmp5LTNK1LCgtcmjS8sZqk9nWYi965NyXYm3Jh dU6Pn/R4dsYFb4RrEO4JQHbC+2ucVdt+5moDblgFAFPl X-Google-Smtp-Source: APiQypKp3kYkG8TVoAcF4xcC377lvWoZyGD7nQURS2s/XU3TjDbIFNmVwIkeXXgllN7sKYVf37cWeyi+0nFEOU18pTk= X-Received: by 2002:aa7:d513:: with SMTP id y19mr22472621edq.367.1587535571151; Tue, 21 Apr 2020 23:06:11 -0700 (PDT) MIME-Version: 1.0 References: <05c9a6725490c5a5c4ee71be73326c2fedf35ba5.1587531463.git.josh@joshtriplett.org> In-Reply-To: <05c9a6725490c5a5c4ee71be73326c2fedf35ba5.1587531463.git.josh@joshtriplett.org> Reply-To: mtk.manpages@gmail.com From: "Michael Kerrisk (man-pages)" Date: Wed, 22 Apr 2020 08:06:00 +0200 Message-ID: Subject: Re: [PATCH v5 1/3] fs: Support setting a minimum fd for "lowest available fd" allocation To: Josh Triplett Cc: io-uring@vger.kernel.org, "linux-fsdevel@vger.kernel.org" , lkml , Alexander Viro , Arnd Bergmann , Jens Axboe , Aleksa Sarai , linux-man , Linux API Content-Type: text/plain; charset="UTF-8" Sender: io-uring-owner@vger.kernel.org Precedence: bulk List-ID: X-Mailing-List: io-uring@vger.kernel.org [CC += linux-api] On Wed, 22 Apr 2020 at 07:19, Josh Triplett wrote: > > 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; > -- > 2.26.2 > -- Michael Kerrisk Linux man-pages maintainer; http://www.kernel.org/doc/man-pages/ Linux/UNIX System Programming Training: http://man7.org/training/