From mboxrd@z Thu Jan 1 00:00:00 1970 Received: from smtp.kernel.org (aws-us-west-2-korg-mail-alma10-1.taild15c8.ts.net [100.103.45.18]) (using TLSv1.2 with cipher ECDHE-RSA-AES256-GCM-SHA384 (256/256 bits)) (No client certificate requested) by smtp.subspace.kernel.org (Postfix) with ESMTPS id B4F453C4155; Mon, 1 Jun 2026 13:57:39 +0000 (UTC) Authentication-Results: smtp.subspace.kernel.org; arc=none smtp.client-ip=100.103.45.18 ARC-Seal:i=1; a=rsa-sha256; d=subspace.kernel.org; s=arc-20240116; t=1780322260; cv=none; b=O9PuSipvMTWGJkO4ZY0wShVx2azdsNpKTlQ4KBJ3sOCpibnw4HBbC+x81PbfGsQQp4udtvj54fFEN+sNlhjTAfluKRCux57vkd6Al9s/13sztCN60a5FCybA/08MqZeE230jFc4LzmZyIpjg4pP4E9iYq0+dLsp9/GkcYF9+2u8= ARC-Message-Signature:i=1; a=rsa-sha256; d=subspace.kernel.org; s=arc-20240116; t=1780322260; c=relaxed/simple; bh=Um5qFBTonfqtToK8ivjVhOxkc2Bf9eG0Su0nAvqlLJY=; h=From:Date:Subject:MIME-Version:Content-Type:Message-Id:References: In-Reply-To:To:Cc; b=I5yN6w5NWTmwBkdrVD/V/DXm1G+s7f033Ick7w/9+TZQuze5Dj+Rq7agimbiOHO2GGbO8dqoID5YL5Mz1cmtCxRlYO8c+TLvEXG15QF6bfNrYbMhtCulGVYvkcEk63tHEfpqHFCocCCpWBLIj/crO3QgQB4NESglBiWZhiZ6PMQ= ARC-Authentication-Results:i=1; smtp.subspace.kernel.org; dkim=pass (2048-bit key) header.d=kernel.org header.i=@kernel.org header.b=Sbrrku9x; arc=none smtp.client-ip=100.103.45.18 Authentication-Results: smtp.subspace.kernel.org; dkim=pass (2048-bit key) header.d=kernel.org header.i=@kernel.org header.b="Sbrrku9x" Received: by smtp.kernel.org (Postfix) with ESMTPSA id C70541F00893; Mon, 1 Jun 2026 13:57:37 +0000 (UTC) DKIM-Signature: v=1; a=rsa-sha256; c=relaxed/relaxed; d=kernel.org; s=k20260515; t=1780322259; bh=ezJrg3c6vUTHy3zJ0IWV8cRNjVA2L6WwpesMAQucO4M=; h=From:Date:Subject:References:In-Reply-To:To:Cc; b=Sbrrku9xOIJMny16jvq6oAeRXrV402IT67789XF9sn2Fe7jThJBQGBFfAWIRmw0+1 TPL7q11gD2/aZxRrLIIRpTmMfyWCUdNwoiOqEIKH6tI9LIrAxooT25DSHi7PYniYFw fwOMlSkLwCiOhWf7fd++NSLF/rkh00F0PN3VlxYLpx4b6s+8dKUGX1usa4ucREKgve TUUE29eGY4muzGl5ljjusNCXMMQuywqMUEGaqSs7gHIJ6b8bExxezG6Ne5+/XcYRjG pcaZdf6ZGIM83L9VEE8lAAFqcbGtGoPALG2/REtMULGen6wLv+qBqpYyFG5TLju3UV OFTC+ZAmThj4Q== From: Christian Brauner Date: Mon, 01 Jun 2026 15:56:53 +0200 Subject: [PATCH RFC v4 20/25] fs: add umh argument to struct kernel_clone_args Precedence: bulk X-Mailing-List: linux-kernel@vger.kernel.org List-Id: List-Subscribe: List-Unsubscribe: MIME-Version: 1.0 Content-Type: text/plain; charset="utf-8" Content-Transfer-Encoding: 7bit Message-Id: <20260601-work-kthread-nullfs-v4-20-77ee053060e0@kernel.org> References: <20260601-work-kthread-nullfs-v4-0-77ee053060e0@kernel.org> In-Reply-To: <20260601-work-kthread-nullfs-v4-0-77ee053060e0@kernel.org> To: linux-fsdevel@vger.kernel.org Cc: Linus Torvalds , linux-kernel@vger.kernel.org, Alexander Viro , Jan Kara , Jann Horn , "Christian Brauner (Amutable)" X-Mailer: b4 0.16-dev-fffa9 X-Developer-Signature: v=1; a=openpgp-sha256; l=3653; i=brauner@kernel.org; h=from:subject:message-id; bh=Um5qFBTonfqtToK8ivjVhOxkc2Bf9eG0Su0nAvqlLJY=; b=owGbwMvMwCU28Zj0gdSKO4sYT6slMWTJ9i/PWM9TGKy87P0Kr5MTG2Jql+ZukFwTp6xvtX/rB 8XzNn2JHaUsDGJcDLJiiiwO7Sbhcst5KjYbZWrAzGFlAhnCwMUpABP5toyRYVdBoeNUJbNvS7Qv zfwau/z88p6r1+7v3TznvW7EAesD3WoMPxnlsuL+eDpPrvCa8UbE8Ye0sf66srSbm59/WOdY/6i ymw0A X-Developer-Key: i=brauner@kernel.org; a=openpgp; fpr=4880B8C9BD0E5106FC070F4F7B3C391EFEA93624 Add a umh field to struct kernel_clone_args. When set, copy_fs() copies from pid 1's fs_struct instead of the kthread's fs_struct. This ensures usermodehelper threads always get init's filesystem state regardless of their parent's (kthreadd's) fs. Usermodehelper threads are not allowed to create mount namespaces (CLONE_NEWNS), share filesystem state (CLONE_FS), or be started from a non-initial mount namespace. No usermodehelper currently does this so we don't need to worry about this restriction. Set .umh = 1 in user_mode_thread(). At this stage pid 1's fs points to rootfs which is the same as kthreadd's fs, so this is functionally equivalent. Signed-off-by: Christian Brauner (Amutable) --- include/linux/sched/task.h | 1 + kernel/fork.c | 25 +++++++++++++++++++++---- kernel/umh.c | 6 ++---- 3 files changed, 24 insertions(+), 8 deletions(-) diff --git a/include/linux/sched/task.h b/include/linux/sched/task.h index 41ed884cffc9..e0c1ca8c6a18 100644 --- a/include/linux/sched/task.h +++ b/include/linux/sched/task.h @@ -31,6 +31,7 @@ struct kernel_clone_args { u32 io_thread:1; u32 user_worker:1; u32 no_files:1; + u32 umh:1; unsigned long stack; unsigned long stack_size; unsigned long tls; diff --git a/kernel/fork.c b/kernel/fork.c index bf153b2929d4..f1accd7a2455 100644 --- a/kernel/fork.c +++ b/kernel/fork.c @@ -1593,11 +1593,27 @@ static int copy_mm(u64 clone_flags, struct task_struct *tsk) return 0; } -static int copy_fs(u64 clone_flags, struct task_struct *tsk) +static int copy_fs(u64 clone_flags, struct task_struct *tsk, bool umh) { - struct fs_struct *fs = current->fs; + struct fs_struct *fs; + + /* + * Usermodehelper may copy userspace_init_fs filesystem state but + * they don't get to create mount namespaces, share the + * filesystem state, or be started from a non-initial mount + * namespace. + */ + if (umh) { + if (clone_flags & (CLONE_NEWNS | CLONE_FS)) + return -EINVAL; + if (current->nsproxy->mnt_ns != &init_mnt_ns) + return -EINVAL; + fs = userspace_init_fs; + } else { + fs = current->fs; + VFS_WARN_ON_ONCE(current->fs != current->real_fs); + } - VFS_WARN_ON_ONCE(current->fs != current->real_fs); if (clone_flags & CLONE_FS) { /* tsk->fs is already what we want */ read_seqlock_excl(&fs->seq); @@ -2252,7 +2268,7 @@ __latent_entropy struct task_struct *copy_process( retval = copy_files(clone_flags, p, args->no_files); if (retval) goto bad_fork_cleanup_semundo; - retval = copy_fs(clone_flags, p); + retval = copy_fs(clone_flags, p, args->umh); if (retval) goto bad_fork_cleanup_files; retval = copy_sighand(clone_flags, p); @@ -2796,6 +2812,7 @@ pid_t user_mode_thread(int (*fn)(void *), void *arg, unsigned long flags) .exit_signal = (flags & CSIGNAL), .fn = fn, .fn_arg = arg, + .umh = 1, }; return kernel_clone(&args); diff --git a/kernel/umh.c b/kernel/umh.c index cffda97d961c..d3f4b308b85d 100644 --- a/kernel/umh.c +++ b/kernel/umh.c @@ -71,10 +71,8 @@ static int call_usermodehelper_exec_async(void *data) spin_unlock_irq(¤t->sighand->siglock); /* - * Initial kernel threads share ther FS with init, in order to - * get the init root directory. But we've now created a new - * thread that is going to execve a user process and has its own - * 'struct fs_struct'. Reset umask to the default. + * Usermodehelper threads get a copy of userspace init's + * fs_struct. Reset umask to the default. */ current->fs->umask = 0022; -- 2.47.3