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

[Steven Rostedt]

On Tue, 23 Apr 2024 16:23:37 +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

I'm curious, what happens if you have: "u8 field1; u8 field2;" ?

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.


> 
> 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 | 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.

-- 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;
> +}
> +