Re: [PATCH v2 1/3] tracing: detect the string termination character when parsing user input string

[Steven Rostedt]

On Mon, 15 Jan 2018 19:41:12 +0800
changbin.du@xxxxxxxxx wrote:

> From: Changbin Du <changbin.du@xxxxxxxxx>
> 
> The usersapce can give a '\0' terminated C string in the input buffer.
> Before this change, trace_get_user() will return a parsed string "\0" in
> below case which is not expected (expects it skip all inputs) and cause the
> caller failed.

The above totally does not parse (no pun intended).

Are you trying to say:

"User space can pass in a C nul character '\0' along with its input.
The function trace_get_user() will try to process it as a normal
character, and that will fail to parse.

> 
> open("/sys/kernel/debug/tracing//set_ftrace_pid", O_WRONLY|O_TRUNC) = 3
> write(3, " \0", 2)                      = -1 EINVAL (Invalid argument)
> 
> while parse can handle spaces, so below works.
> 
> $ echo "" > set_ftrace_pid
> $ echo " " > set_ftrace_pid
> $ echo -n " " > set_ftrace_pid
> 
> This patch try to make the parser '\0' aware to fix such issue. When parser
> sees a '\0' it stops further parsing. With this change, write(3, " \0", 2)
> will work.

The above should be something like:

"Have the parser stop on '\0' and cease any further parsing. Only
process the characters up to the nul '\0' character and do not process
it."

-- Steve


> 
> Signed-off-by: Changbin Du <changbin.du@xxxxxxxxx>
> 
> ---
>   v2: Stop parsing when '\0' found.
> ---
>  kernel/trace/trace.c | 6 +++---
>  1 file changed, 3 insertions(+), 3 deletions(-)
> 
> diff --git a/kernel/trace/trace.c b/kernel/trace/trace.c
> index 2a8d8a2..144d08e 100644
> --- a/kernel/trace/trace.c
> +++ b/kernel/trace/trace.c
> @@ -1237,7 +1237,7 @@ int trace_get_user(struct trace_parser *parser, const char __user *ubuf,
>  		}
>  
>  		/* only spaces were written */
> -		if (isspace(ch)) {
> +		if (isspace(ch) || !ch) {
>  			*ppos += read;
>  			ret = read;
>  			goto out;
> @@ -1247,7 +1247,7 @@ int trace_get_user(struct trace_parser *parser, const char __user *ubuf,
>  	}
>  
>  	/* read the non-space input */
> -	while (cnt && !isspace(ch)) {
> +	while (cnt && !isspace(ch) && ch) {
>  		if (parser->idx < parser->size - 1)
>  			parser->buffer[parser->idx++] = ch;
>  		else {
> @@ -1262,7 +1262,7 @@ int trace_get_user(struct trace_parser *parser, const char __user *ubuf,
>  	}
>  
>  	/* We either got finished input or we have to wait for another call. */
> -	if (isspace(ch)) {
> +	if (isspace(ch) || !ch) {
>  		parser->buffer[parser->idx] = 0;
>  		parser->cont = false;
>  	} else if (parser->idx < parser->size - 1) {