linux-kernel - Re: [PATCH v2 1/2] tracing/user_events: Fix non-spaced field matching

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Date: Thu, 2 May 2024 15:58:53 -0700
From: Beau Belgrave <beaub@...ux.microsoft.com>
To: Steven Rostedt <rostedt@...dmis.org>
Cc: mhiramat@...nel.org, mathieu.desnoyers@...icios.com,
	linux-kernel@...r.kernel.org, linux-trace-kernel@...r.kernel.org,
	dcook@...ux.microsoft.com
Subject: Re: [PATCH v2 1/2] tracing/user_events: Fix non-spaced field matching

On Thu, May 02, 2024 at 05:16:34PM -0400, Steven Rostedt wrote:
> On Tue, 23 Apr 2024 16:23:37 +0000
> Beau Belgrave <beaub@...ux.microsoft.com> 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
> 
> I'm curious, what happens if you have: "u8 field1; u8 field2;" ?
> 

You'll get an extra whitespace during the copy, assuming it was really:
"u8 field1;u8 field2"

If it had spaces, this code wouldn't run.

> Do you care? As you will then create "u8 field1; u8 field2; "
> 
> but I'm guessing the extra whitespace at the end doesn't affect anything.
> 

Right, you get an extra byte allocated, but the argv_split() with ignore
it. The compare will work correctly (I've verified this just now to
double check).

IE these all match:
"Test u8 a; u8 b; "
"Test u8 a; u8 b;"
"Test u8 a; u8 b"

> 
> > 
> > Fixes: ba470eebc2f6 ("tracing/user_events: Prevent same name but different args event")
> > Signed-off-by: Beau Belgrave <beaub@...ux.microsoft.com>
> > ---
> >  kernel/trace/trace_events_user.c | 76 +++++++++++++++++++++++++++++++-
> >  1 file changed, 75 insertions(+), 1 deletion(-)
> > 
> > diff --git a/kernel/trace/trace_events_user.c b/kernel/trace/trace_events_user.c
> > index 70d428c394b6..82b191f33a28 100644
> > --- a/kernel/trace/trace_events_user.c
> > +++ b/kernel/trace/trace_events_user.c
> > @@ -1989,6 +1989,80 @@ 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++;
> 
> This will count that "..;" 
> 
> This is most likely not an issue, but since I didn't see this case
> anywhere, I figured I bring it up just to confirm that it's not an issue.
> 

It's not an issue on the matching/logic. However, you do get an extra
byte alloc (which doesn't bother me in this edge case).

Thanks,
-Beau

> -- Steve
> 
> 
> > +	}
> > +
> > +	return count;
> > +}
> > +
> > +/*
> > + * Copies the arguments while ensuring all ';' have a trailing space.
> > + */
> > +static char *insert_space_after_semis(char *args, int count)
> > +{
> > +	char *fixed, *pos;
> > +	int len;
> > +
> > +	len = strlen(args) + count;
> > +	fixed = kmalloc(len + 1, GFP_KERNEL);
> > +
> > +	if (!fixed)
> > +		return NULL;
> > +
> > +	pos = fixed;
> > +
> > +	/* Insert a space after ';' if there is no trailing space. */
> > +	while (*args) {
> > +		*pos = *args++;
> > +
> > +		if (*pos++ == ';' && !isspace(*args))
> > +			*pos++ = ' ';
> > +	}
> > +
> > +	*pos = '\0';
> > +
> > +	return fixed;
> > +}
> > +