msg: sanitize term title and block C1 controls and DEL

Expose the terminal output sanitizer as mp_msg_sanitize() and use it in
term_osd_update_title() to sanitize the property-expanded term-title
string before it reaches mp_msg_set_term_title(). Without this, a
crafted media tag can break out of the OSC-0 sequence via embedded BEL
or ESC bytes when --term-title includes property expansions like
${media-title}.

Extend the sanitizer to cover two gaps:

- UTF-8 encoded C1 controls (U+0080-U+009F, bytes C2 80..C2 9F). xterm
  in UTF-8 mode interprets these codepoints as C1 control functions
  (U+009B as CSI, U+009D as OSC, U+009C as ST), allowing escape
  sequence injection without any ESC or BEL bytes.

- DEL (0x7F), which was not previously filtered.

common/msg.c

@@ -559,7 +559,7 @@ static void write_term_msg(struct mp_log *log, int lev, bstr text, bstr *out)
     }
 }
 
-static void sanitize(bstr *text)
+void mp_msg_sanitize(bstr *text)
 {
     for (size_t i = 0; i < text->len; i++) {
         unsigned char ch = text->start[i];
@@ -587,9 +587,20 @@ static void sanitize(bstr *text)
                 text->start[i] = '?';
         }
         // Allow only printable > 0x20 and 0x08-0x0D (backspace, tab, newline, ...)
-        else if (ch < 0x08 || (ch > 0x0D && ch < 0x20)) {
+        else if (ch < 0x08 || (ch > 0x0D && ch < 0x20) || ch == 0x7F) {
             text->start[i] = '?';
         }
+        // Block UTF-8 encoded C1 controls (U+0080-U+009F = bytes C2 80..C2 9F).
+        // xterm interprets these as C1 control functions (CSI, OSC, DCS, ST, ...),
+        // which allows bypassing the ESC/BEL filter above.
+        else if (ch == 0xC2 && i + 1 < text->len &&
+                 (unsigned char)text->start[i + 1] >= 0x80 &&
+                 (unsigned char)text->start[i + 1] <= 0x9F)
+        {
+            text->start[i] = '?';
+            text->start[i + 1] = '?';
+            i++;
+        }
     }
 }
 
@@ -613,7 +624,7 @@ void mp_msg_va(struct mp_log *log, int lev, const char *format, va_list va)
         bstr_xappend(root, &root->buffer, bstr0(format));
     }
 
-    sanitize(&root->buffer);
+    mp_msg_sanitize(&root->buffer);
 
     // Remember last status message and restore it to ensure that it is
     // always displayed

common/msg.h

@@ -67,6 +67,10 @@ static inline bool mp_msg_test(struct mp_log *log, int lev)
 
 void mp_msg_set_max_level(struct mp_log *log, int lev);
 
+// Sanitize text for terminal output.
+struct bstr;
+void mp_msg_sanitize(struct bstr *text);
+
 // Convenience macros.
 #define mp_fatal(log, ...)      mp_msg(log, MSGL_FATAL, __VA_ARGS__)
 #define mp_err(log, ...)        mp_msg(log, MSGL_ERR, __VA_ARGS__)

player/osd.c

@@ -108,7 +108,9 @@ static void term_osd_update_title(struct MPContext *mpctx)
         return;
 
     char *s = mp_property_expand_escaped_string(mpctx, mpctx->opts->term_title);
-    if (bstr_equals(bstr0(s), bstr0(mpctx->term_osd_title))) {
+    bstr title = bstr0(s);
+    mp_msg_sanitize(&title);
+    if (bstr_equals(title, bstr0(mpctx->term_osd_title))) {
         talloc_free(s);
         return;
     }