fix integer underflow bug in ReadConsoleForTermEmulModern() (#804)

mgkuhn · web-flow · commit 855942b0d3a7 · 2026-07-21T15:36:15.000-04:00
From directvt/vtm#819 (comment) > When a window resize event is received (terminal generates WINDOW_BUFFER_SIZE_EVENT event when switching between alternate/normal buffer mode), the variable text_len=0 may accidentally decrease by one and become text_len=-1 potentially fixes PowerShell/Win32-OpenSSH#2275
diff --git a/contrib/win32/win32compat/tncon.c b/contrib/win32/win32compat/tncon.c
@@ -183,11 +183,13 @@ ReadConsoleForTermEmulModern(HANDLE hInput, char *destin, int destinlen)
 		}
 	}
 
-	// Pop any lone lead surrogate from the input for later.
-	const wchar_t last_char = text[text_len - 1];
-	if (IS_HIGH_SURROGATE(last_char)) {
-		s_previous_lead = last_char;
-		text_len--;
+	if (text_len) {
+		// Pop any lone lead surrogate from the input for later.
+		const wchar_t last_char = text[text_len - 1];
+		if (IS_HIGH_SURROGATE(last_char)) {
+			s_previous_lead = last_char;
+			text_len--;
+		}
 	}
 
 	// ...and finally convert everything to UTF-8.

Original file line number	Diff line number	Diff line change
`@@ -183,11 +183,13 @@ ReadConsoleForTermEmulModern(HANDLE hInput, char *destin, int destinlen)`
`183`	`183`	`}`
`184`	`184`	`}`
`185`	`185`
`186`		`- // Pop any lone lead surrogate from the input for later.`
`187`		`- const wchar_t last_char = text[text_len - 1];`
`188`		`- if (IS_HIGH_SURROGATE(last_char)) {`
`189`		`- s_previous_lead = last_char;`
`190`		`- text_len--;`
	`186`	`+ if (text_len) {`
	`187`	`+ // Pop any lone lead surrogate from the input for later.`
	`188`	`+ const wchar_t last_char = text[text_len - 1];`
	`189`	`+ if (IS_HIGH_SURROGATE(last_char)) {`
	`190`	`+ s_previous_lead = last_char;`
	`191`	`+ text_len--;`
	`192`	`+ }`
`191`	`193`	`}`
`192`	`194`
`193`	`195`	`// ...and finally convert everything to UTF-8.`