Re: [PATCH v7 13/13] user_events: Use __get_rel_str for relative string fields

[Beau Belgrave <beaub@linux.microsoft.com>]

On Fri, Dec 10, 2021 at 10:23:27AM +0900, Masami Hiramatsu wrote:
> Hi Beau,
> 
> On Thu,  9 Dec 2021 14:32:10 -0800
> Beau Belgrave <beaub@linux.microsoft.com> wrote:
> 
> > Switch between __get_str and __get_rel_str within the print_fmt of
> > user_events. Add unit test to ensure print_fmt is correct on known
> > types.
> > 
> > Signed-off-by: Beau Belgrave <beaub@linux.microsoft.com>
> > ---
> >  kernel/trace/trace_events_user.c              |  24 ++-
> >  .../selftests/user_events/ftrace_test.c       | 166 ++++++++++++++++++
> >  2 files changed, 182 insertions(+), 8 deletions(-)
> > 
> > diff --git a/kernel/trace/trace_events_user.c b/kernel/trace/trace_events_user.c
> > index 56eb58ddb4cf..3779fa2ca14a 100644
> > --- a/kernel/trace/trace_events_user.c
> > +++ b/kernel/trace/trace_events_user.c
> > @@ -257,7 +257,7 @@ static int user_event_add_field(struct user_event *user, const char *type,
> >  	goto add_field;
> >  
> >  add_validator:
> > -	if (strstr(type, "char[") != 0)
> > +	if (strstr(type, "char") != 0)
> >  		validator_flags |= VALIDATOR_ENSURE_NULL;
> 
> What is this change for? This seems not related to the other changes.
> (Also, what happen if it is a single char type?)
> 

I'm glad you asked, it appears like __data_loc / __rel_loc can take char
as it's type (It doesn't appear to be limited to char[] cases). I wanted
to ensure something malicious couldn't sneak past by using this corner
case.

IE: __data_loc char test

In trace_events_filter.c:
int filter_assign_type(const char *type)
{
        if (strstr(type, "__data_loc") && strstr(type, "char"))
                return FILTER_DYN_STRING;

        if (strchr(type, '[') && strstr(type, "char"))
                return FILTER_STATIC_STRING;

        if (strcmp(type, "char *") == 0 || strcmp(type, "const char *") == 0)
                return FILTER_PTR_STRING;

        return FILTER_OTHER;
}

char[ is only checked if __data_loc is not specified.

> >  
> >  	validator = kmalloc(sizeof(*validator), GFP_KERNEL);
> > @@ -456,14 +456,21 @@ static const char *user_field_format(const char *type)
> >  	return "%llu";
> >  }
> >  
> > -static bool user_field_is_dyn_string(const char *type)
> > +static bool user_field_is_dyn_string(const char *type, const char **str_func)
> >  {
> > -	if (str_has_prefix(type, "__data_loc ") ||
> > -	    str_has_prefix(type, "__rel_loc "))
> > -		if (strstr(type, "char[") != 0)
> > -			return true;
> > +	if (str_has_prefix(type, "__data_loc ")) {
> > +		*str_func = "__get_str";
> > +		goto check;
> > +	}
> > +
> > +	if (str_has_prefix(type, "__rel_loc ")) {
> > +		*str_func = "__get_rel_str";
> > +		goto check;
> > +	}
> >  
> >  	return false;
> > +check:
> > +	return strstr(type, "char") != 0;
> >  }
> >  
> >  #define LEN_OR_ZERO (len ? len - pos : 0)
> > @@ -472,6 +479,7 @@ static int user_event_set_print_fmt(struct user_event *user, char *buf, int len)
> >  	struct ftrace_event_field *field, *next;
> >  	struct list_head *head = &user->fields;
> >  	int pos = 0, depth = 0;
> > +	const char *str_func;
> >  
> >  	pos += snprintf(buf + pos, LEN_OR_ZERO, "\"");
> >  
> > @@ -488,9 +496,9 @@ static int user_event_set_print_fmt(struct user_event *user, char *buf, int len)
> >  	pos += snprintf(buf + pos, LEN_OR_ZERO, "\"");
> >  
> >  	list_for_each_entry_safe_reverse(field, next, head, link) {
> > -		if (user_field_is_dyn_string(field->type))
> > +		if (user_field_is_dyn_string(field->type, &str_func))
> >  			pos += snprintf(buf + pos, LEN_OR_ZERO,
> > -					", __get_str(%s)", field->name);
> > +					", %s(%s)", str_func, field->name);
> >  		else
> >  			pos += snprintf(buf + pos, LEN_OR_ZERO,
> >  					", REC->%s", field->name);
> > diff --git a/tools/testing/selftests/user_events/ftrace_test.c b/tools/testing/selftests/user_events/ftrace_test.c
> 
> Just a nitpick, if possible, please split this part from the kernel update.
> 

I will try to do so, could you help me understand why I would split this
out? (For future patches)

I thought the intention of each would be to contain it's logical grouping:
I wanted to show, yes the code changed, and yes we have a unit test for
that new condition.

> > index 16aff1fb295a..b2e5c0765a68 100644
> > --- a/tools/testing/selftests/user_events/ftrace_test.c
> > +++ b/tools/testing/selftests/user_events/ftrace_test.c
> > @@ -20,6 +20,7 @@ const char *data_file = "/sys/kernel/debug/tracing/user_events_data";
> >  const char *status_file = "/sys/kernel/debug/tracing/user_events_status";
> >  const char *enable_file = "/sys/kernel/debug/tracing/events/user_events/__test_event/enable";
> >  const char *trace_file = "/sys/kernel/debug/tracing/trace";
> > +const char *fmt_file = "/sys/kernel/debug/tracing/events/user_events/__test_event/format";
> >  
> >  static int trace_bytes(void)
> >  {
> > @@ -47,6 +48,61 @@ static int trace_bytes(void)
> >  	return bytes;
> >  }
> >  
> > +static int get_print_fmt(char *buffer, int len)
> > +{
> > +	FILE *fp = fopen(fmt_file, "r");
> > +	int c, index = 0, last = 0;
> > +
> > +	if (!fp)
> > +		return -1;
> > +
> > +	/* Read until empty line (Skip Common) */
> > +	while (true) {
> > +		c = getc(fp);
> > +
> > +		if (c == EOF)
> > +			break;
> > +
> > +		if (last == '\n' && c == '\n')
> > +			break;
> > +
> > +		last = c;
> > +	}
> 
> Another nitpick, maybe you need a function like skip_until_empty_line(fp)
> and repeat it like this.
> 
> 	if (skip_until_empty_line(fp) < 0)
> 		goto out;
> 	if (skip_until_empty_line(fp) < 0)
> 		goto out;
> 

Sure thing.

> > +
> > +	last = 0;
> > +
> > +	/* Read until empty line (Skip Properties) */
> > +	while (true) {
> > +		c = getc(fp);
> > +
> > +		if (c == EOF)
> > +			break;
> > +
> > +		if (last == '\n' && c == '\n')
> > +			break;
> > +
> > +		last = c;
> > +	}
> > +
> > +	/* Read in print_fmt: */
> > +	while (len > 1) {
> > +		c = getc(fp);
> > +
> > +		if (c == EOF || c == '\n')
> > +			break;
> > +
> > +		buffer[index++] = c;
> > +
> > +		len--;
> > +	}
> 
> And here you can use fgets(buffer, len, fp).
> 

Makes sense.

> 
> Thank you,
> 
> 

[..]

> 
> -- 
> Masami Hiramatsu <mhiramat@kernel.org>

Thanks,
-Beau