Re: [PATCH 1/2] tracing/user_events: Fix non-spaced field matching

[Google]

On Fri, 19 Apr 2024 14:13:34 -0700
Beau Belgrave <beaub@xxxxxxxxxxxxxxxxxxx> wrote:

> On Fri, Apr 19, 2024 at 11:33:05AM +0900, Masami Hiramatsu wrote:
> > On Tue, 16 Apr 2024 22:41:01 +0000
> > Beau Belgrave <beaub@xxxxxxxxxxxxxxxxxxx> wrote:
> > 
> > > When the ABI was updated to prevent same name w/different args, it
> > > missed an important corner case when fields don't end with a space.
> > > Typically, space is used for fields to help separate them, like
> > > "u8 field1; u8 field2". If no spaces are used, like
> > > "u8 field1;u8 field2", then the parsing works for the first time.
> > > However, the match check fails on a subsequent register, leading to
> > > confusion.
> > > 
> > > This is because the match check uses argv_split() and assumes that all
> > > fields will be split upon the space. When spaces are used, we get back
> > > { "u8", "field1;" }, without spaces we get back { "u8", "field1;u8" }.
> > > This causes a mismatch, and the user program gets back -EADDRINUSE.
> > > 
> > > Add a method to detect this case before calling argv_split(). If found
> > > force a space after the field separator character ';'. This ensures all
> > > cases work properly for matching.
> > > 
> > > With this fix, the following are all treated as matching:
> > > u8 field1;u8 field2
> > > u8 field1; u8 field2
> > > u8 field1;\tu8 field2
> > > u8 field1;\nu8 field2
> > 
> > Sounds good to me. I just have some nits.
> > 
> > > 
> > > Fixes: ba470eebc2f6 ("tracing/user_events: Prevent same name but different args event")
> > > Signed-off-by: Beau Belgrave <beaub@xxxxxxxxxxxxxxxxxxx>
> > > ---
> > >  kernel/trace/trace_events_user.c | 88 +++++++++++++++++++++++++++++++-
> > >  1 file changed, 87 insertions(+), 1 deletion(-)
> > > 
> > > diff --git a/kernel/trace/trace_events_user.c b/kernel/trace/trace_events_user.c
> > > index 70d428c394b6..9184d3962b2a 100644
> > > --- a/kernel/trace/trace_events_user.c
> > > +++ b/kernel/trace/trace_events_user.c
> > > @@ -1989,6 +1989,92 @@ static int user_event_set_tp_name(struct user_event *user)
> > >  	return 0;
> > >  }
> > >  
> > > +/*
> > > + * Counts how many ';' without a trailing space are in the args.
> > > + */
> > > +static int count_semis_no_space(char *args)
> > > +{
> > > +	int count = 0;
> > > +
> > > +	while ((args = strchr(args, ';'))) {
> > > +		args++;
> > > +
> > > +		if (!isspace(*args))
> > > +			count++;
> > > +	}
> > > +
> > > +	return count;
> > > +}
> > > +
> > > +/*
> > > + * Copies the arguments while ensuring all ';' have a trailing space.
> > > + */
> > > +static char *fix_semis_no_space(char *args, int count)
> > 
> > nit: This name does not represent what it does. 'insert_space_after_semis()'
> > is more self-described.
> > 
> 
> Sure, will fix in a v2.
> 
> > > +{
> > > +	char *fixed, *pos;
> > > +	char c, last;
> > > +	int len;
> > > +
> > > +	len = strlen(args) + count;
> > > +	fixed = kmalloc(len + 1, GFP_KERNEL);
> > > +
> > > +	if (!fixed)
> > > +		return NULL;
> > > +
> > > +	pos = fixed;
> > > +	last = '\0';
> > > +
> > > +	while (len > 0) {
> > > +		c = *args++;
> > > +
> > > +		if (last == ';' && !isspace(c)) {
> > > +			*pos++ = ' ';
> > > +			len--;
> > > +		}
> > > +
> > > +		if (len > 0) {
> > > +			*pos++ = c;
> > > +			len--;
> > > +		}
> > > +
> > > +		last = c;
> > > +	}
> > 
> > nit: This loop can be simpler, because we are sure fixed has enough length;
> > 
> > /* insert a space after ';' if there is no space. */
> > while(*args) {
> > 	*pos = *args++;
> > 	if (*pos++ == ';' && !isspace(*args))
> > 		*pos++ = ' ';
> > }
> > 
> 
> I was worried that if count_semis_no_space() ever had different logic
> (maybe after this commit) that it could cause an overflow if the count
> was wrong, etc.
> 
> I don't have an issue making it shorter, but I was trying to be more on
> the safe side, since this isn't a fast path (event register).

OK, anyway current code looks correct. But note that I don't think
"pos++; len--;" is safer, since it is not atomic. This pattern
easily loose "len--;" in my experience. So please carefully use it ;)

> 
> > > +
> > > +	/*
> > > +	 * len is the length of the copy excluding the null.
> > > +	 * This ensures we always have room for a null.
> > > +	 */
> > > +	*pos = '\0';
> > > +
> > > +	return fixed;
> > > +}
> > > +
> > > +static char **user_event_argv_split(char *args, int *argc)
> > > +{
> > > +	/* Count how many ';' without a trailing space */
> > > +	int count = count_semis_no_space(args);
> > > +
> > > +	if (count) {
> > 
> > nit: it is better to exit fast, so 
> > 
> > 	if (!count)
> > 		return argv_split(GFP_KERNEL, args, argc);
> > 
> > 	...
> 
> Sure, will fix in a v2.
> 
> > 
> > Thank you,
> > 
> > OT: BTW, can this also simplify synthetic events?
> > 
> 
> I'm not sure, I'll check when I have some time. I want to get this fix
> in sooner rather than later.

Ah, nevermind. Synthetic event parses the field by strsep(';') first
and argv_split(). So it does not have this issue.

Thank you,

> 
> Thanks,
> -Beau
> 
> > > +		/* We must fixup 'field;field' to 'field; field' */
> > > +		char *fixed = fix_semis_no_space(args, count);
> > > +		char **split;
> > > +
> > > +		if (!fixed)
> > > +			return NULL;
> > > +
> > > +		/* We do a normal split afterwards */
> > > +		split = argv_split(GFP_KERNEL, fixed, argc);
> > > +
> > > +		/* We can free since argv_split makes a copy */
> > > +		kfree(fixed);
> > > +
> > > +		return split;
> > > +	}
> > > +
> > > +	/* No fixup is required */
> > > +	return argv_split(GFP_KERNEL, args, argc);
> > > +}
> > > +
> > >  /*
> > >   * Parses the event name, arguments and flags then registers if successful.
> > >   * The name buffer lifetime is owned by this method for success cases only.
> > > @@ -2012,7 +2098,7 @@ static int user_event_parse(struct user_event_group *group, char *name,
> > >  		return -EPERM;
> > >  
> > >  	if (args) {
> > > -		argv = argv_split(GFP_KERNEL, args, &argc);
> > > +		argv = user_event_argv_split(args, &argc);
> > >  
> > >  		if (!argv)
> > >  			return -ENOMEM;
> > > -- 
> > > 2.34.1
> > > 
> > 
> > 
> > -- 
> > Masami Hiramatsu (Google) <mhiramat@xxxxxxxxxx>


-- 
Masami Hiramatsu (Google) <mhiramat@xxxxxxxxxx>