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Date:   Sun, 15 Dec 2019 16:10:10 +0000
From:   Yonghong Song <yhs@...com>
To:     Wenbo Zhang <ethercflow@...il.com>,
        "bpf@...r.kernel.org" <bpf@...r.kernel.org>
CC:     "ast@...nel.org" <ast@...nel.org>,
        "daniel@...earbox.net" <daniel@...earbox.net>,
        "andrii.nakryiko@...il.com" <andrii.nakryiko@...il.com>,
        "netdev@...r.kernel.org" <netdev@...r.kernel.org>
Subject: Re: [PATCH bpf-next v12 1/2] bpf: add new helper get_file_path for
 mapping a file descriptor to a pathname



On 12/14/19 8:01 PM, Wenbo Zhang wrote:
> When people want to identify which file system files are being opened,
> read, and written to, they can use this helper with file descriptor as
> input to achieve this goal. Other pseudo filesystems are also supported.
> 
> This requirement is mainly discussed here:
> 
>    https://github.com/iovisor/bcc/issues/237
> 
> v11->v12: addressed Alexei's feedback
> - only allow tracepoints to make sure it won't dead lock
> 
> v10->v11: addressed Al and Alexei's feedback
> - fix missing fput()
> 
> v9->v10: addressed Andrii's feedback
> - send this patch together with the patch selftests as one patch series
> 
> v8->v9:
> - format helper description
> 
> v7->v8: addressed Alexei's feedback
> - use fget_raw instead of fdget_raw, as fdget_raw is only used inside fs/
> - ensure we're in user context which is safe fot the help to run
> - filter unmountable pseudo filesystem, because they don't have real path
> - supplement the description of this helper function
> 
> v6->v7:
> - fix missing signed-off-by line
> 
> v5->v6: addressed Andrii's feedback
> - avoid unnecessary goto end by having two explicit returns
> 
> v4->v5: addressed Andrii and Daniel's feedback
> - rename bpf_fd2path to bpf_get_file_path to be consistent with other
> helper's names
> - when fdget_raw fails, set ret to -EBADF instead of -EINVAL
> - remove fdput from fdget_raw's error path
> - use IS_ERR instead of IS_ERR_OR_NULL as d_path ether returns a pointer
> into the buffer or an error code if the path was too long
> - modify the normal path's return value to return copied string length
> including NUL
> - update this helper description's Return bits.
> 
> v3->v4: addressed Daniel's feedback
> - fix missing fdput()
> - move fd2path from kernel/bpf/trace.c to kernel/trace/bpf_trace.c
> - move fd2path's test code to another patch
> - add comment to explain why use fdget_raw instead of fdget
> 
> v2->v3: addressed Yonghong's feedback
> - remove unnecessary LOCKDOWN_BPF_READ
> - refactor error handling section for enhanced readability
> - provide a test case in tools/testing/selftests/bpf
> 
> v1->v2: addressed Daniel's feedback
> - fix backward compatibility
> - add this helper description
> - fix signed-off name
> 
> Signed-off-by: Wenbo Zhang <ethercflow@...il.com>
> ---
>   include/uapi/linux/bpf.h       | 29 +++++++++++++-
>   kernel/trace/bpf_trace.c       | 70 ++++++++++++++++++++++++++++++++++
>   tools/include/uapi/linux/bpf.h | 29 +++++++++++++-
>   3 files changed, 126 insertions(+), 2 deletions(-)
> 
> diff --git a/include/uapi/linux/bpf.h b/include/uapi/linux/bpf.h
> index dbbcf0b02970..71d9705df120 100644
> --- a/include/uapi/linux/bpf.h
> +++ b/include/uapi/linux/bpf.h
> @@ -2821,6 +2821,32 @@ union bpf_attr {
>    * 	Return
>    * 		On success, the strictly positive length of the string,	including
>    * 		the trailing NUL character. On error, a negative value.
> + *
> + * int bpf_get_file_path(char *path, u32 size, int fd)
> + *	Description
> + *		Get **file** atrribute from the current task by *fd*, then call
> + *		**d_path** to get it's absolute path and copy it as string into
> + *		*path* of *size*. Notice the **path** don't support unmountable
> + *		pseudo filesystems as they don't have path (eg: SOCKFS, PIPEFS).
> + *		The *size* must be strictly positive. On success, the helper
> + *		makes sure that the *path* is NUL-terminated, and the buffer
> + *		could be:
> + *		- a regular full path (include mountable fs eg: /proc, /sys)
> + *		- a regular full path with "(deleted)" at the end.
> + *		On failure, it is filled with zeroes.
> + *	Return
> + *		On success, returns the length of the copied string INCLUDING
> + *		the trailing NUL.
> + *
> + *		On failure, the returned value is one of the following:
> + *
> + *		**-EPERM** if no permission to get the path (eg: in irq ctx).
> + *
> + *		**-EBADF** if *fd* is invalid.
> + *
> + *		**-EINVAL** if *fd* corresponds to a unmountable pseudo fs
> + *
> + *		**-ENAMETOOLONG** if full path is longer than *size*
>    */
>   #define __BPF_FUNC_MAPPER(FN)		\
>   	FN(unspec),			\
> @@ -2938,7 +2964,8 @@ union bpf_attr {
>   	FN(probe_read_user),		\
>   	FN(probe_read_kernel),		\
>   	FN(probe_read_user_str),	\
> -	FN(probe_read_kernel_str),
> +	FN(probe_read_kernel_str),	\
> +	FN(get_file_path),
>   
>   /* integer value in 'imm' field of BPF_CALL instruction selects which helper
>    * function eBPF program intends to call
> diff --git a/kernel/trace/bpf_trace.c b/kernel/trace/bpf_trace.c
> index e5ef4ae9edb5..db9c0ec46a5d 100644
> --- a/kernel/trace/bpf_trace.c
> +++ b/kernel/trace/bpf_trace.c
> @@ -762,6 +762,72 @@ static const struct bpf_func_proto bpf_send_signal_proto = {
>   	.arg1_type	= ARG_ANYTHING,
>   };
>   
> +BPF_CALL_3(bpf_get_file_path, char *, dst, u32, size, int, fd)
> +{
> +	struct file *f;
> +	char *p;
> +	int ret = -EBADF;

please try to use reverse Christmas tree for declarations.

> +
> +	/* 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);
> +	if (IS_ERR(p)) {
> +		ret = PTR_ERR(p);
> +		fput(f);
> +		goto error;
> +	}
> +
> +	ret = strlen(p);
> +	memmove(dst, p, ret);
> +	dst[ret++] = '\0';
> +	fput(f);
> +	return ret;
> +
> +error:
> +	memset(dst, '0', size);
> +	return ret;
> +}
> +
[...]

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