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Sat, 14 Jun 2025 14:36:15 -0400 (EDT) Message-ID: <75ea3f6b-cf5b-4e97-9214-cbd3f299008c@maowtm.org> Date: Sat, 14 Jun 2025 19:36:14 +0100 Precedence: bulk X-Mailing-List: linux-kernel@vger.kernel.org List-Id: List-Subscribe: List-Unsubscribe: MIME-Version: 1.0 User-Agent: Mozilla Thunderbird Subject: Re: [PATCH v4 bpf-next 1/5] namei: Introduce new helper function path_walk_parent() To: Song Liu Cc: bpf@vger.kernel.org, linux-fsdevel@vger.kernel.org, linux-kernel@vger.kernel.org, linux-security-module@vger.kernel.org, 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, amir73il@gmail.com, repnop@google.com, jlayton@kernel.org, josef@toxicpanda.com, mic@digikod.net, gnoack@google.com, neil@brown.name References: <20250611220220.3681382-1-song@kernel.org> <20250611220220.3681382-2-song@kernel.org> Content-Language: en-US From: Tingmao Wang In-Reply-To: <20250611220220.3681382-2-song@kernel.org> Content-Type: text/plain; charset=UTF-8 Content-Transfer-Encoding: 7bit On 6/11/25 23:02, 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 | 99 +++++++++++++++++++++++++++++++++++++------ > include/linux/namei.h | 2 + > 2 files changed, 87 insertions(+), 14 deletions(-) > > diff --git a/fs/namei.c b/fs/namei.c > index 4bb889fc980b..bc65361c5d13 100644 > --- a/fs/namei.c > +++ b/fs/namei.c > @@ -2048,36 +2048,107 @@ 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); > - if (unlikely(!path_connected(nd->path.mnt, parent))) { > + parent = dget_parent(path->dentry); > + if (unlikely(!path_connected(path->mnt, parent))) { This is checking path_connected here but also in follow_dotdot, path_connected is checked again. Is this check meant to be here? It will also change the landlock behaviour right? (For some reason patch 2 rejects when I tried to apply it on v6.16-rc1, so I haven't actually tested this patch to see if this is really an issue) > dput(parent); > return ERR_PTR(-ENOENT); > } > return parent; > > in_root: > - if (unlikely(nd->flags & LOOKUP_BENEATH)) > + if (unlikely(flags & LOOKUP_BENEATH)) > return ERR_PTR(-EXDEV); > - return dget(nd->path.dentry); > + 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 > + * released and zero'ed. > + * > + * Returns: > + * true - if @path is updated to its parent. > + * false - if @path is already the root (real root or @root). > + */ > +bool 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)) > + goto false_out; > + > + if (parent == path->dentry) { > + dput(parent); > + goto false_out; > + } > + dput(path->dentry); > + path->dentry = parent; > + return true; > + > +false_out: > + path_put(path); > + memset(path, 0, sizeof(*path)); > + return false; > +} > + > +static struct dentry *follow_dotdot(struct nameidata *nd) > +{ > + struct dentry *parent = __path_walk_parent(&nd->path, &nd->root, nd->flags); > + > + if (IS_ERR(parent)) > + return parent; > + if (unlikely(!path_connected(nd->path.mnt, parent))) { > + dput(parent); > + return ERR_PTR(-ENOENT); > + } > + nd->inode = nd->path.dentry->d_inode; > + return parent; > } > > static const char *handle_dots(struct nameidata *nd, int type) > diff --git a/include/linux/namei.h b/include/linux/namei.h > index 5d085428e471..cba5373ecf86 100644 > --- a/include/linux/namei.h > +++ b/include/linux/namei.h > @@ -85,6 +85,8 @@ extern int follow_down_one(struct path *); > extern int follow_down(struct path *path, unsigned int flags); > extern int follow_up(struct path *); > > +bool path_walk_parent(struct path *path, const struct path *root); > + > extern struct dentry *lock_rename(struct dentry *, struct dentry *); > extern struct dentry *lock_rename_child(struct dentry *, struct dentry *); > extern void unlock_rename(struct dentry *, struct dentry *);