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From: Alexei Starovoitov <alexei.starovoitov@gmail.com>
To: Wenbo Zhang <ethercflow@gmail.com>
Cc: bpf@vger.kernel.org, ast@kernel.org, daniel@iogearbox.net,
	yhs@fb.com, andrii.nakryiko@gmail.com, netdev@vger.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
Date: Wed, 4 Dec 2019 23:19:00 -0800
Message-ID: <20191205071858.entnj2c27n44kwit@ast-mbp.dhcp.thefacebook.com> (raw)
In-Reply-To: <afe4deb020b781c76e9df8403a744f88a8725cd2.1575517685.git.ethercflow@gmail.com>

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.


  reply index

Thread overview: 48+ messages / expand[flat|nested]  mbox.gz  Atom feed  top
2019-11-19 13:27 [PATCH bpf-next v10 0/2] bpf: adding get_file_path helper Wenbo Zhang
2019-11-19 13:27 ` [PATCH bpf-next v10 1/2] bpf: add new helper get_file_path for mapping a file descriptor to a pathname Wenbo Zhang
2019-11-23  3:18   ` Alexei Starovoitov
2019-11-23  4:43     ` Al Viro
2019-11-23  4:51     ` Al Viro
2019-11-23  5:19       ` Alexei Starovoitov
2019-11-23  5:35         ` Al Viro
2019-11-23  6:04           ` Alexei Starovoitov
2019-12-13 19:51             ` Brendan Gregg
2019-12-05  4:20   ` [PATCH bpf-next v11 0/2] bpf: adding get_file_path helper Wenbo Zhang
2019-12-05  4:20     ` [PATCH bpf-next v11 1/2] bpf: add new helper get_file_path for mapping a file descriptor to a pathname Wenbo Zhang
2019-12-05  7:19       ` Alexei Starovoitov [this message]
2019-12-05  9:47         ` Wenbo Zhang
2019-12-15  4:01       ` [PATCH bpf-next v12 0/2] bpf: adding get_file_path helper Wenbo Zhang
2019-12-15  4:01         ` [PATCH bpf-next v12 1/2] bpf: add new helper get_file_path for mapping a file descriptor to a pathname Wenbo Zhang
2019-12-15 16:05           ` Yonghong Song
2019-12-17  6:26             ` Wenbo Zhang
2019-12-17  6:33               ` Yonghong Song
2019-12-15 16:10           ` Yonghong Song
2019-12-17  6:27             ` Wenbo Zhang
2019-12-16 22:09           ` Brendan Gregg
2019-12-17  4:05             ` Wenbo Zhang
2019-12-17  9:47           ` [PATCH bpf-next v13 0/2] bpf: adding get_fd_path helper Wenbo Zhang
2019-12-17  9:47             ` [PATCH bpf-next v13 1/2] bpf: add new helper get_fd_path for mapping a file descriptor to a pathname Wenbo Zhang
2019-12-17 16:29               ` Yonghong Song
2019-12-17 19:39                 ` Daniel Borkmann
2019-12-18  0:11                   ` Wenbo Zhang
2019-12-18  0:06                 ` Wenbo Zhang
2019-12-18  0:56               ` [PATCH bpf-next v14 0/2] bpf: adding get_fd_path helper Wenbo Zhang
2019-12-18  0:56                 ` [PATCH bpf-next v14 1/2] bpf: add new helper get_fd_path for mapping a file descriptor to a pathname Wenbo Zhang
2019-12-18  3:27                   ` Yonghong Song
2019-12-19 16:14                   ` Daniel Borkmann
2019-12-20  3:35                     ` Wenbo Zhang
2020-01-16  8:59                       ` Jiri Olsa
2020-02-10  4:43                         ` Brendan Gregg
2020-02-11  0:01                           ` Daniel Borkmann
2020-02-12 15:21                             ` Jiri Olsa
2019-12-18  0:56                 ` [PATCH bpf-next v14 2/2] selftests/bpf: test for bpf_get_fd_path() from tracepoint Wenbo Zhang
2019-12-18  3:27                   ` Yonghong Song
2019-12-17  9:47             ` [PATCH bpf-next v13 " Wenbo Zhang
2019-12-17 16:32               ` Yonghong Song
2019-12-15  4:01         ` [PATCH bpf-next v12 2/2] selftests/bpf: test for bpf_get_file_path() " Wenbo Zhang
2019-12-15 16:24           ` Yonghong Song
2019-12-17  4:01             ` Wenbo Zhang
2019-12-17  4:13               ` Yonghong Song
2019-12-17  9:44                 ` Wenbo Zhang
2019-12-05  4:20     ` [PATCH bpf-next v11 " Wenbo Zhang
2019-11-19 13:27 ` [PATCH bpf-next v10 " Wenbo Zhang

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