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

[Masami Hiramatsu (Google) <mhiramat@kernel.org>]

On Tue, 16 Apr 2024 22:41:01 +0000
Beau Belgrave <beaub@linux.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

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@linux.microsoft.com>
> ---
>  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.

> +{
> +	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++ = ' ';
}

> +
> +	/*
> +	 * 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);

	...

Thank you,

OT: BTW, can this also simplify synthetic events?

> +		/* 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@kernel.org>