From mboxrd@z Thu Jan 1 00:00:00 1970 Received: from out-183.mta1.migadu.com (out-183.mta1.migadu.com [95.215.58.183]) (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 1FF952E36EB for ; Fri, 4 Jul 2025 17:40:43 +0000 (UTC) Authentication-Results: smtp.subspace.kernel.org; arc=none smtp.client-ip=95.215.58.183 ARC-Seal:i=1; a=rsa-sha256; d=subspace.kernel.org; s=arc-20240116; t=1751650846; cv=none; b=ssRNQ1TZFpIAHZze2Tcaw+PU4JpimSQ6nCRTXits/V/d5b3nsYynqA0ytcPx2ew2SQPCntT9d3UnDYjf6hr+Il1v6axOrpRsPWL1pA23RSaOSi84CrC9ZG73Qeb3nbCq/+B3csE1B1+4rk+spWYEpntvaLqYLYQ87N1/xOLkAt0= ARC-Message-Signature:i=1; a=rsa-sha256; d=subspace.kernel.org; s=arc-20240116; t=1751650846; c=relaxed/simple; bh=j1y5fK2pSIFjeIzluaoTZB7LiubY9SK90IFdNX+JcVI=; h=Message-ID:Date:MIME-Version:Subject:To:Cc:References:From: In-Reply-To:Content-Type; b=fYQEaw+p3vDMrkFOw8dc/Xc8qlaBKpn+lynLTATcM3Tvv0iONShzjh9A1fCS5nVC7rLAlSOGOZqA4iqS7zxHtVg7Y+FE0ECqeej+6TZt5p8uz8Rd2b5HJeQyLsP5NRPmabkB7IziR2iECR729JTq9aMf5m2GLRcnHYPubxNhO0w= ARC-Authentication-Results:i=1; smtp.subspace.kernel.org; dmarc=pass (p=none dis=none) header.from=linux.dev; spf=pass smtp.mailfrom=linux.dev; dkim=pass (1024-bit key) header.d=linux.dev header.i=@linux.dev header.b=pKspoYDT; arc=none smtp.client-ip=95.215.58.183 Authentication-Results: smtp.subspace.kernel.org; dmarc=pass (p=none dis=none) header.from=linux.dev Authentication-Results: smtp.subspace.kernel.org; spf=pass smtp.mailfrom=linux.dev Authentication-Results: smtp.subspace.kernel.org; dkim=pass (1024-bit key) header.d=linux.dev header.i=@linux.dev header.b="pKspoYDT" Message-ID: <2459c10e-d74c-4118-9b6d-c37d05ecec02@linux.dev> DKIM-Signature: v=1; a=rsa-sha256; c=relaxed/relaxed; d=linux.dev; s=key1; t=1751650841; h=from:from:reply-to:subject:subject:date:date:message-id:message-id: to:to:cc:cc:mime-version:mime-version:content-type:content-type: content-transfer-encoding:content-transfer-encoding: in-reply-to:in-reply-to:references:references; bh=xIRcNmmy2VbVp7gJQIkaTFfPIXvag7FLLr7FrIV0lQo=; b=pKspoYDT8P7/dYSLFT3Wcf6V3S1xlnly+2wS8ALTKH+MecAV9raqgDtRFHb0+N2lny17mW Urcm4546PBB3ywrgOhyT2lNXU6S1ODfdsqWVsxtfy8Ky7lG6s44qnb6dsS029GqH9Obmh5 BPBmV3Y+sZ0PNlQN6O1PuI38XaEGhZU= Date: Fri, 4 Jul 2025 10:40:18 -0700 Precedence: bulk X-Mailing-List: linux-kernel@vger.kernel.org List-Id: List-Subscribe: List-Unsubscribe: MIME-Version: 1.0 Subject: Re: [PATCH v5 bpf-next 1/5] namei: Introduce new helper function path_walk_parent() Content-Language: en-GB To: Song Liu , bpf@vger.kernel.org, linux-fsdevel@vger.kernel.org, linux-kernel@vger.kernel.org, linux-security-module@vger.kernel.org Cc: kernel-team@meta.com, andrii@kernel.org, eddyz87@gmail.com, ast@kernel.org, daniel@iogearbox.net, martin.lau@linux.dev, viro@zeniv.linux.org.uk, brauner@kernel.org, jack@suse.cz, kpsingh@kernel.org, mattbobrowski@google.com, m@maowtm.org, neil@brown.name References: <20250617061116.3681325-1-song@kernel.org> <20250617061116.3681325-2-song@kernel.org> X-Report-Abuse: Please report any abuse attempt to abuse@migadu.com and include these headers. From: Yonghong Song In-Reply-To: <20250617061116.3681325-2-song@kernel.org> Content-Type: text/plain; charset=UTF-8; format=flowed Content-Transfer-Encoding: 7bit X-Migadu-Flow: FLOW_OUT On 6/16/25 11:11 PM, Song Liu wrote: > This helper walks an input path to its parent. Logic are added to handle > walking across mount tree. > > This will be used by landlock, and BPF LSM. > > Suggested-by: Neil Brown > Signed-off-by: Song Liu > --- > fs/namei.c | 95 +++++++++++++++++++++++++++++++++++-------- > include/linux/namei.h | 2 + > 2 files changed, 79 insertions(+), 18 deletions(-) > > diff --git a/fs/namei.c b/fs/namei.c > index 4bb889fc980b..d0557c0b5cc8 100644 > --- a/fs/namei.c > +++ b/fs/namei.c > @@ -2048,36 +2048,95 @@ static struct dentry *follow_dotdot_rcu(struct nameidata *nd) > return nd->path.dentry; > } > > -static struct dentry *follow_dotdot(struct nameidata *nd) > +/** > + * __path_walk_parent - Find the parent of the given struct path > + * @path - The struct path to start from > + * @root - A struct path which serves as a boundary not to be crosses. > + * - If @root is zero'ed, walk all the way to global root. > + * @flags - Some LOOKUP_ flags. > + * > + * Find and return the dentry for the parent of the given path > + * (mount/dentry). If the given path is the root of a mounted tree, it > + * is first updated to the mount point on which that tree is mounted. > + * > + * If %LOOKUP_NO_XDEV is given, then *after* the path is updated to a new > + * mount, the error EXDEV is returned. > + * > + * If no parent can be found, either because the tree is not mounted or > + * because the @path matches the @root, then @path->dentry is returned > + * unless @flags contains %LOOKUP_BENEATH, in which case -EXDEV is returned. > + * > + * Returns: either an ERR_PTR() or the chosen parent which will have had > + * the refcount incremented. > + */ > +static struct dentry *__path_walk_parent(struct path *path, const struct path *root, int flags) > { > - struct dentry *parent; > - > - if (path_equal(&nd->path, &nd->root)) > + if (path_equal(path, root)) > goto in_root; > - if (unlikely(nd->path.dentry == nd->path.mnt->mnt_root)) { > - struct path path; > + if (unlikely(path->dentry == path->mnt->mnt_root)) { > + struct path new_path; > > - if (!choose_mountpoint(real_mount(nd->path.mnt), > - &nd->root, &path)) > + if (!choose_mountpoint(real_mount(path->mnt), > + root, &new_path)) > goto in_root; > - path_put(&nd->path); > - nd->path = path; > - nd->inode = path.dentry->d_inode; > - if (unlikely(nd->flags & LOOKUP_NO_XDEV)) > + path_put(path); > + *path = new_path; > + if (unlikely(flags & LOOKUP_NO_XDEV)) > return ERR_PTR(-EXDEV); > } > /* rare case of legitimate dget_parent()... */ > - parent = dget_parent(nd->path.dentry); > + return dget_parent(path->dentry); I have some confusion with this patch when crossing mount boundary. In d_path.c, we have static int __prepend_path(const struct dentry *dentry, const struct mount *mnt, const struct path *root, struct prepend_buffer *p) { while (dentry != root->dentry || &mnt->mnt != root->mnt) { const struct dentry *parent = READ_ONCE(dentry->d_parent); if (dentry == mnt->mnt.mnt_root) { struct mount *m = READ_ONCE(mnt->mnt_parent); struct mnt_namespace *mnt_ns; if (likely(mnt != m)) { dentry = READ_ONCE(mnt->mnt_mountpoint); mnt = m; continue; } /* Global root */ mnt_ns = READ_ONCE(mnt->mnt_ns); /* open-coded is_mounted() to use local mnt_ns */ if (!IS_ERR_OR_NULL(mnt_ns) && !is_anon_ns(mnt_ns)) return 1; // absolute root else return 2; // detached or not attached yet } if (unlikely(dentry == parent)) /* Escaped? */ return 3; prefetch(parent); if (!prepend_name(p, &dentry->d_name)) break; dentry = parent; } return 0; } At the mount boundary and not at root mount, the code has dentry = READ_ONCE(mnt->mnt_mountpoint); mnt = m; /* 'mnt' will be parent mount */ continue; After that, we have const struct dentry *parent = READ_ONCE(dentry->d_parent); if (dentry == mnt->mnt.mnt_root) { /* assume this is false */ } ... prefetch(parent); if (!prepend_name(p, &dentry->d_name)) break; dentry = parent; So the prepend_name(p, &dentry->d_name) is actually from mnt->mnt_mountpoint. In your above code, maybe we should return path->dentry in the below if statement? if (unlikely(path->dentry == path->mnt->mnt_root)) { struct path new_path; if (!choose_mountpoint(real_mount(path->mnt), root, &new_path)) goto in_root; path_put(path); *path = new_path; if (unlikely(flags & LOOKUP_NO_XDEV)) return ERR_PTR(-EXDEV); + return path->dentry; } /* rare case of legitimate dget_parent()... */ return dget_parent(path->dentry); Also, could you add some selftests cross mount points? This will have more coverages with __path_walk_parent(). > + > +in_root: > + if (unlikely(flags & LOOKUP_BENEATH)) > + return ERR_PTR(-EXDEV); > + return dget(path->dentry); > +} > + > +/** > + * path_walk_parent - Walk to the parent of path > + * @path: input and output path. > + * @root: root of the path walk, do not go beyond this root. If @root is > + * zero'ed, walk all the way to real root. > + * > + * Given a path, find the parent path. Replace @path with the parent path. > + * If we were already at the real root or a disconnected root, @path is > + * not changed. > + * > + * Returns: > + * 0 - if @path is updated to its parent. > + * <0 - if @path is already the root (real root or @root). > + */ > +int path_walk_parent(struct path *path, const struct path *root) > +{ > + struct dentry *parent; > + > + parent = __path_walk_parent(path, root, LOOKUP_BENEATH); > + > + if (IS_ERR(parent)) > + return PTR_ERR(parent); > + > + if (parent == path->dentry) { > + dput(parent); > + return -ENOENT; > + } > + dput(path->dentry); > + path->dentry = parent; > + return 0; > +} > + [...]