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[163.53.146.70]) by smtp.gmail.com with ESMTPSA id d2e1a72fcca58-8a0894b49dcsm730783b3a.16.2026.10.21.03.58.29 (version=TLS1_3 cipher=TLS_AES_256_GCM_SHA384 bits=256/256); Wed, 21 Oct 2026 03:58:35 -0700 (PDT) Date: Wed, 21 Oct 2026 21:58:09 +1100 From: Matt Bobrowski To: Alexei Starovoitov Cc: bpf@vger.kernel.org, Daniel Borkmann , Andrii Nakryiko , Eduard Zingerman , Kumar Kartikeya Dwivedi , Martin KaFai Lau , Song Liu , Yonghong Song , Jiri Olsa , Emil Tsalapatis , Ihor Solodrai , Christian Brauner , Alexander Viro , Jan Kara , linux-fsdevel@vger.kernel.org, stable@vger.kernel.org Subject: Re: [PATCH bpf-next 1/2] bpf: fix crash in bpf_path_d_path() for exiting tasks Message-ID: References: <20261020105014.97985-1-matt@bobrowski.net> <20261020105014.97985-2-matt@bobrowski.net> X-Mailing-List: bpf@vger.kernel.org List-Id: List-Post: List-Help: List-Subscribe: List-Unsubscribe: List-Owner: Precedence: list MIME-Version: 1.0 Content-Type: text/plain; charset=us-ascii Content-Disposition: inline In-Reply-To: On Tue, Oct 20, 2026 at 06:24:42PM +0000, Alexei Starovoitov wrote: > On Tue, Oct 20, 2026 at 03:50 AM Matt Bobrowski wrote: > > Fix this by returning -EINVAL from bpf_path_d_path() when current->fs > > is NULL. Only the task itself clears its fs, in exit_fs(), so the check > > cannot race with that. > > The crash is gone, but so is the path. file_release progs will get > -EINVAL for every file that a task still has open when it exits. > > It also breaks what works today. The d_dname branch in d_path() returns > before it looks at current->fs, so at task exit a pipe or a socket > resolves to "pipe:[123]" now and to -EINVAL with this patch. > > And it's not specific to bpf. The comment in fs/cachefiles/namei.c says > that current->fs == NULL in do_exit() breaks d_path() called from ext4 > open. > > can we fix it in d_path() instead ? > With an empty root prepend_path() goes up to the global root, the same > way d_absolute_path() does. Then the check in bpf_path_d_path() is not > needed and the selftest can expect the path. A chrooted task gets the > path from the global root, but it doesn't have a root anymore. > > Something like this (compile tested only): Prior to sending out the fix, I also considered addressing this directly within d_path(), much like you've proposed here, but decided against it. d_path()'s contract is to build a pathname relative to the current root. It does fall back to a root-independent name when the path isn't reachable from that root, but that fallback still presupposes the caller has a root to measure against. An exiting task past exit_fs() has no root, so d_path() has no correct answer to give in that state, and I'm not convinced we should invent one for it. That's what d_absolute_path() is for. You're right that my patch regresses the d_dname case though, since pipes and sockets resolve today without ever looking at current->fs. Rather than replicating the d_dname check in bpf_path_d_path(), which would mean duplicating VFS logic such as the mounted pseudo-inode special case, I think the cleaner fix is to move the "no root, no answer" check into d_path() itself, after the d_dname branch: if (unlikely(!current->fs)) return ERR_PTR(-ENOENT); That keeps d_path()'s semantics intact, keeps pipe:[N] and socket:[N] working at exit, and turns the oops into an error for every caller, including the ext4 open path you mentioned. Nothing that works today changes behaviour; only calls that currently oops are affected. Callers that want a name in that state then have to choose one explicitly, which I take to be Al's point. For BPF, that choice could be a new BPF kfunc wrapping d_absolute_path(), so programs can pick whichever semantics they actually want, and file_release programs that need a name at exit get one whose format doesn't depend on how far the task got through do_exit(). I'd prefer that over silently falling back to d_absolute_path() inside bpf_path_d_path(), which would return chroot-relative names before exit_fs() and absolute ones after, and still wouldn't help pipes and sockets since d_absolute_path() rejects internal mounts. Al, would a check like the above in d_path() be acceptable to you? > diff --git a/fs/d_path.c b/fs/d_path.c > index c253090..ea49d33 100644 > --- a/fs/d_path.c > +++ b/fs/d_path.c > @@ -265,7 +265,7 @@ static void get_fs_root_rcu(struct fs_struct *fs, struct path *root) > char *d_path(const struct path *path, char *buf, int buflen) > { > DECLARE_BUFFER(b, buf, buflen); > - struct path root; > + struct path root = {}; > > /* > * We have various synthetic filesystems that never get mounted. On > @@ -284,7 +284,9 @@ char *d_path(const struct path *path, char *buf, int buflen) > return path->dentry->d_op->d_dname(path->dentry, buf, buflen); > > rcu_read_lock(); > - get_fs_root_rcu(current->fs, &root); > + /* Past exit_fs() there is no root to stop at, go up to the global one. */ > + if (current->fs) > + get_fs_root_rcu(current->fs, &root); > if (unlikely(d_unlinked(path->dentry))) > prepend(&b, " (deleted)", 11); > else > > pw-bot: cr