netdev - Re: [PATCH bpf-next v11 1/2] bpf: add new helper get_file_path for mapping a file descriptor to a pathname

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Message-ID: <20191205071858.entnj2c27n44kwit@ast-mbp.dhcp.thefacebook.com>
Date:   Wed, 4 Dec 2019 23:19:00 -0800
From:   Alexei Starovoitov <alexei.starovoitov@...il.com>
To:     Wenbo Zhang <ethercflow@...il.com>
Cc:     bpf@...r.kernel.org, ast@...nel.org, daniel@...earbox.net,
        yhs@...com, andrii.nakryiko@...il.com, netdev@...r.kernel.org
Subject: Re: [PATCH bpf-next v11 1/2] bpf: add new helper get_file_path for
 mapping a file descriptor to a pathname

On Wed, Dec 04, 2019 at 11:20:35PM -0500, Wenbo Zhang wrote:
>  
> +BPF_CALL_3(bpf_get_file_path, char *, dst, u32, size, int, fd)
> +{
> +	struct file *f;
> +	char *p;
> +	int ret = -EBADF;
> +
> +	/* Ensure we're in user context which is safe for the helper to
> +	 * run. This helper has no business in a kthread.
> +	 */
> +	if (unlikely(in_interrupt() ||
> +		     current->flags & (PF_KTHREAD | PF_EXITING))) {
> +		ret = -EPERM;
> +		goto error;
> +	}
> +
> +	/* Use fget_raw instead of fget to support O_PATH, and it doesn't
> +	 * have any sleepable code, so it's ok to be here.
> +	 */
> +	f = fget_raw(fd);
> +	if (!f)
> +		goto error;
> +
> +	/* For unmountable pseudo filesystem, it seems to have no meaning
> +	 * to get their fake paths as they don't have path, and to be no
> +	 * way to validate this function pointer can be always safe to call
> +	 * in the current context.
> +	 */
> +	if (f->f_path.dentry->d_op && f->f_path.dentry->d_op->d_dname) {
> +		ret = -EINVAL;
> +		fput(f);
> +		goto error;
> +	}
> +
> +	/* After filter unmountable pseudo filesytem, d_path won't call
> +	 * dentry->d_op->d_name(), the normally path doesn't have any
> +	 * sleepable code, and despite it uses the current macro to get
> +	 * fs_struct (current->fs), we've already ensured we're in user
> +	 * context, so it's ok to be here.
> +	 */
> +	p = d_path(&f->f_path, dst, size);

Above 'if's are not enough to make sure that it won't dead lock.
Allowing it in tracing_func_proto() means that it's available to kprobe too.
Hence deadlock is possible. Please see previous email thread.
This helper is safe in tracepoint+bpf only.