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

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Date:   Tue, 16 Jan 2018 10:41:31 +0800
From:   "Du, Changbin" <changbin.du@...el.com>
To:     Steven Rostedt <rostedt@...dmis.org>
Cc:     changbin.du@...el.com, jolsa@...hat.com, peterz@...radead.org,
        mingo@...hat.com, alexander.shishkin@...ux.intel.com,
        linux-kernel@...r.kernel.org, linux-perf-users@...r.kernel.org
Subject: Re: [PATCH v2 1/3] tracing: detect the string termination character
 when parsing user input string

Hi Rostedt,
Thanks for your polish, let me update commit msg with your words.

On Mon, Jan 15, 2018 at 06:20:00PM -0500, Steven Rostedt wrote:
> On Mon, 15 Jan 2018 19:41:12 +0800
> changbin.du@...el.com wrote:
> 
> > From: Changbin Du <changbin.du@...el.com>
> > 
> > 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@...el.com>
> > 
> > ---
> >   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) {
> 

-- 
Thanks,
Changbin Du