[hackers] [dwm][PATCH] ensure layout symbol is properly nul-terminated

From: NRK <nrk_AT_disroot.org>
Date: Sat, 13 Aug 2022 09:00:06 +0600

the dwm source code somewhat silently assumes that the symbol will fit
into 16 bytes. but this may not be the case since users can define
longer symbols in `config.h` (and there's no comment or note stating
anything about the size requirement).

this patch syncs the sizeof Layout->symbol and Monitor->ltsymbol to be
the same which should emit a warning if the symbol in config.h is longer
than 17 bytes.

however, there's still an edge case where the symbol is exactly 17 bytes
(including the nul character), in which case there won't be any
warnings. so use stpncpy and manually ensure nul termination.

---
 dwm.c | 10 +++++-----
 1 file changed, 5 insertions(+), 5 deletions(-)
diff --git a/dwm.c b/dwm.c
index 967c9e8..86c4985 100644
--- a/dwm.c
+++ b/dwm.c
_AT_@ -107,12 +107,12 @@ typedef struct {
 } Key;
 
 typedef struct {
-	const char *symbol;
+	const char symbol[16];
 	void (*arrange)(Monitor *);
 } Layout;
 
 struct Monitor {
-	char ltsymbol[16];
+	char ltsymbol[sizeof ((Layout *)0)->symbol];
 	float mfact;
 	int nmaster;
 	int num;
_AT_@ -397,7 +397,7 @@ arrange(Monitor *m)
 void
 arrangemon(Monitor *m)
 {
-	strncpy(m->ltsymbol, m->lt[m->sellt]->symbol, sizeof m->ltsymbol);
+	*stpncpy(m->ltsymbol, m->lt[m->sellt]->symbol, sizeof m->ltsymbol - 1) = '\0';
 	if (m->lt[m->sellt]->arrange)
 		m->lt[m->sellt]->arrange(m);
 }
_AT_@ -644,7 +644,7 @@ createmon(void)
 	m->topbar = topbar;
 	m->lt[0] = &layouts[0];
 	m->lt[1] = &layouts[1 % LENGTH(layouts)];
-	strncpy(m->ltsymbol, layouts[0].symbol, sizeof m->ltsymbol);
+	*stpncpy(m->ltsymbol, layouts[0].symbol, sizeof m->ltsymbol - 1) = '\0';
 	return m;
 }
 
_AT_@ -1507,7 +1507,7 @@ setlayout(const Arg *arg)
 		selmon->sellt ^= 1;
 	if (arg && arg->v)
 		selmon->lt[selmon->sellt] = (Layout *)arg->v;
-	strncpy(selmon->ltsymbol, selmon->lt[selmon->sellt]->symbol, sizeof selmon->ltsymbol);
+	*stpncpy(selmon->ltsymbol, selmon->lt[selmon->sellt]->symbol, sizeof selmon->ltsymbol - 1) = '\0';
 	if (selmon->sel)
 		arrange(selmon);
 	else
-- 
2.35.1

Received on Sat Aug 13 2022 - 05:00:06 CEST