charts: clamp %n length to buffer size

It appears I overlooked that %n returns the length that would have
been printed regardless of the destination buffer size, not what
was actually written.  The man page is misleading here:

 n      The  number  of characters written so far is stored into the
        integer pointed to  by  the  corresponding  argument.   That
        argument  shall  be  an int *, or variant whose size matches
        the (optionally) supplied integer length modifier.  No argu‐
        ment  is converted.  (This specifier is not supported by the
        bionic C library.)  The behavior is undefined if the conver‐
        sion  specification  includes any flags, a field width, or a
        precision.

In testing, it isn't the count of what's actually written.  It's
oblivious of truncated output scenarios where the output buffer has
been exhausted before reaching the %n.  The man page should be
clarified here.

This commit does the simplest thing and simply clamps the length to
the destination buffer - 1 (for the \0).  %n is being used to avoid
needing an strlen() in this somewhat hot path, but it might make
sense to instead use the snprintf return value similarly clamped
instead of %n since %n isn't doing what was expected.

1 files changed, 2 insertions, 0 deletions

diff --git a/src/charts.c b/src/charts.c
index 0c0fa70..628b87a 100644
--- a/src/charts.c
+++ b/src/charts.c
@@ -590,6 +590,7 @@ static void draw_heirarchy_row(vwm_charts_t *charts, vwm_chart_t *chart, vmon_pr
 	} else { /* we're a process having threads, suppress the wchan and state, as they will be displayed for the thread of same pid */
 		snprintf(str, sizeof(str), "  %5i   %n", proc->pid, &str_len);
 	}
+	str_len = MIN(sizeof(str) - 1, str_len);
 	str_width = XTextWidth(charts->chart_font, str, str_len);
 
 	/* the process' comm label indented according to depth, followed with their respective argv's */
@@ -829,6 +830,7 @@ static void draw_chart(vwm_charts_t *charts, vwm_chart_t *chart, vmon_proc_t *pr
 	/* only draw the \/\/\ and HZ if necessary */
 	if (chart->redraw_needed || charts->prev_sampling_interval != charts->sampling_interval) {
 		snprintf(str, sizeof(str), "\\/\\/\\    %2iHz %n", (int)(charts->sampling_interval == INFINITY ? 0 : 1 / charts->sampling_interval), &str_len);
+		str_len = MIN(sizeof(str) - 1, str_len);
 		XRenderFillRectangle(xserver->display, PictOpSrc, chart->text_picture, &chart_trans_color,
 			0, 0,						/* dst x, y */
 			chart->visible_width, CHART_ROW_HEIGHT);	/* dst w, h */