linux-kernel - Re: [PATCH v3 bpf-next 1/5] namei: Introduce new helper function path_walk

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Message-ID: <CAPhsuW75h3efyXmCcYE_UEmZGXR5KMSEw8h-_vrZH82BYU=WVw@mail.gmail.com>
Date: Tue, 10 Jun 2025 16:08:08 -0700
From: Song Liu <song@...nel.org>
To: Tingmao Wang <m@...wtm.org>
Cc: Mickaël Salaün <mic@...ikod.net>, 
	bpf@...r.kernel.org, linux-fsdevel@...r.kernel.org, 
	linux-kernel@...r.kernel.org, linux-security-module@...r.kernel.org, 
	kernel-team@...a.com, andrii@...nel.org, eddyz87@...il.com, ast@...nel.org, 
	daniel@...earbox.net, martin.lau@...ux.dev, viro@...iv.linux.org.uk, 
	brauner@...nel.org, jack@...e.cz, kpsingh@...nel.org, 
	mattbobrowski@...gle.com, amir73il@...il.com, repnop@...gle.com, 
	jlayton@...nel.org, josef@...icpanda.com, gnoack@...gle.com
Subject: Re: [PATCH v3 bpf-next 1/5] namei: Introduce new helper function path_walk_parent()

On Tue, Jun 10, 2025 at 3:26 PM Tingmao Wang <m@...wtm.org> wrote:
[..]
> >
> >                 if (!choose_mountpoint(real_mount(path->mnt), root, &p))
> >                         return false;
> >                 path_put(path);
> >                 *path = p;
> >                 ret = true;
> >         }
> >
> >         if (unlikely(IS_ROOT(path->dentry)))
> >                 return ret;
>
> Returning true here would be the wrong semantic right?  This whole thing
> is only possible when some mount shadows "/".  Say if you have a landlock
> rule on the old "/", but then we mount a new "/" and chroot into it (via
> "/.."), the landlock rule on the old "/" should not apply, but if we
> change *path and return true here then this will "expose" that old "/" to
> landlock.

Could you please provide more specific information about this case?

Thanks,
Song

> A quick suggestion although I haven't tested anything - maybe we should do
> a special case check for IS_ROOT inside the
>     if (unlikely(path->dentry == path->mnt->mnt_root))
> ? Before "path_put(path);", if IS_ROOT(p.dentry) then we just path_get(p)
> and return false.
>
> >
> >         parent = dget_parent(path->dentry);
> >         dput(path->dentry);
> >         path->dentry = parent;
> >         return true;
> > }
> >
> > Thanks,
> > Song
>