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

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Date:   Wed, 10 Jan 2018 11:01:20 +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 1/3] tracing: detect the string termination character
 when parsing user input string

hi Rostedt,

On Tue, Jan 09, 2018 at 05:54:34PM -0500, Steven Rostedt wrote:
> On Tue,  9 Jan 2018 17:55:46 +0800
> changbin.du@...el.com wrote:
> 
> > From: Changbin Du <changbin.du@...el.com>
> > 
> > The usersapce can give a '\0' terminated C string or even has '\0' at the
> > middle of input buffer. We need handle both these two cases correctly.
> 
> What do you define as correctly. Because I'm not seeing it.
>
Soory I don't fully understand your question. What I meant is want to get clear that
how will tracing parser below strings.
  "", "  ",  "\0", " \0 ", "aa\0bb"
The parser may only recognize certain formats, but whatever the behaviour should
be clear and coherent for all tracing interfaces.

> > 
> > Before this change, trace_get_user() will return a parsed string "\0" in
> > below case. It is not expected (expects it skip all inputs) and cause the
> > caller failed.
> > 
> > open("/sys/kernel/debug/tracing//set_ftrace_pid", O_WRONLY|O_TRUNC) = 3
> > write(3, " \0", 2)                      = -1 EINVAL (Invalid argument)
> 
> That looks more like a feature and not a bug.
> 
I point this out because I think the parser should take this as an emptry string
per the comments of trace_get_user().
/*
 * trace_get_user - reads the user input string separated by  space
 * (matched by isspace(ch))
 *
 * For each string found the 'struct trace_parser' is updated,
 * and the function returns.
 *
 * Returns number of bytes read.
 *
 * See kernel/trace/trace.h for 'struct trace_parser' details.
 */

> > 
> > This patch try to make the parser '\0' aware to fix such issue.
> 
> Why?
> 

> > 
> > Since the caller expects trace_get_user() to parse whole input buffer, so
> > this patch treat '\0' as a separator as whitespace.
> 
> It looks more like we are trying to fix a userspace bug via the kernel.
> 

> I'm not liking this. So NACK.
> 
> -- Steve
> 
> > 
> > Signed-off-by: Changbin Du <changbin.du@...el.com>
> > ---
> >  kernel/trace/trace.c | 17 +++++++++++------
> >  1 file changed, 11 insertions(+), 6 deletions(-)
> > 
> > diff --git a/kernel/trace/trace.c b/kernel/trace/trace.c
> > index 2a8d8a2..18526a1 100644
> > --- a/kernel/trace/trace.c
> > +++ b/kernel/trace/trace.c
> > @@ -1194,9 +1194,14 @@ void trace_parser_put(struct trace_parser *parser)
> >  	parser->buffer = NULL;
> >  }
> >  
> > +static inline bool is_space_or_zero(char ch)
> > +{
> > +	return isspace(ch) || !ch;
> > +}
> > +
> >  /*
> > - * trace_get_user - reads the user input string separated by  space
> > - * (matched by isspace(ch))
> > + * trace_get_user - reads the user input string separated by space or '\0'
> > + * (matched by is_space_or_zero(ch))
> >   *
> >   * For each string found the 'struct trace_parser' is updated,
> >   * and the function returns.
> > @@ -1228,7 +1233,7 @@ int trace_get_user(struct trace_parser *parser, const char __user *ubuf,
> >  	 */
> >  	if (!parser->cont) {
> >  		/* skip white space */
> > -		while (cnt && isspace(ch)) {
> > +		while (cnt && is_space_or_zero(ch)) {
> >  			ret = get_user(ch, ubuf++);
> >  			if (ret)
> >  				goto out;
> > @@ -1237,7 +1242,7 @@ int trace_get_user(struct trace_parser *parser, const char __user *ubuf,
> >  		}
> >  
> >  		/* only spaces were written */
> > -		if (isspace(ch)) {
> > +		if (is_space_or_zero(ch)) {
> >  			*ppos += read;
> >  			ret = read;
> >  			goto out;
> > @@ -1247,7 +1252,7 @@ int trace_get_user(struct trace_parser *parser, const char __user *ubuf,
> >  	}
> >  
> >  	/* read the non-space input */
> > -	while (cnt && !isspace(ch)) {
> > +	while (cnt && !is_space_or_zero(ch)) {
> >  		if (parser->idx < parser->size - 1)
> >  			parser->buffer[parser->idx++] = ch;
> >  		else {
> > @@ -1262,7 +1267,7 @@ 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 (is_space_or_zero(ch)) {
> >  		parser->buffer[parser->idx] = 0;
> >  		parser->cont = false;
> >  	} else if (parser->idx < parser->size - 1) {
> 

-- 
Thanks,
Changbin Du