[vlc-devel] [PATCH] control/globalhotkeys: xcb: fix p_sys->p_map population

[Filip Roséen]

As described in #17747, certain configurations end up causing a
double-free in xcb.c:Close due to the same value returned from
xcb_key_symbols_get_keycode appearing multiple times in p_sys->p_map.

The code responsible for clean-up assumes that every value refers to a
separate allocated resource, whereas Mapping potentially populates
p_sys->p_map with the same value several times.

These changes make sure that initialization vs clean-up is in harmony.

fixes: #18136

--

A side-effect of these changes is that the we are allocating more
resources than before. There are two alternative fixes (each suffering
from added code complexity):

 1. store all return-values xcb_key_symbols_get_keycode in a separate
    array (with unique entries), so that clean-up can use this array
    instead of looping through p_sys->p_map.

 2. As duplicate entries in p_sys->p_map in terms of ->p_keys are
    guaranteed to be seqential, one could also update the clean-up loop
    so that it does not free the about-to-be-freed value if it compares
    equal to the previous item.

It is important to note that xcb_keysym_t is (at least currently)
typedef'd to "uint8_t", meaning that even though we in theory allocate
3 times more memory at the affected code-path - the real impact on
memory usage is very small.

Best Regards,\
Filip Roséen
---
 modules/control/globalhotkeys/xcb.c | 11 ++++++-----
 1 file changed, 6 insertions(+), 5 deletions(-)

diff --git a/modules/control/globalhotkeys/xcb.c b/modules/control/globalhotkeys/xcb.c
index 42fb9f4854..f083b81336 100644
--- a/modules/control/globalhotkeys/xcb.c
+++ b/modules/control/globalhotkeys/xcb.c
@@ -308,11 +308,6 @@ static bool Mapping( intf_thread_t *p_intf )
         if( i_vlc_key == KEY_UNSET )
             continue;
 
-        xcb_keycode_t *p_keys = xcb_key_symbols_get_keycode(
-                p_sys->p_symbols, GetX11Key( i_vlc_key & ~KEY_MODIFIER ) );
-        if( !p_keys )
-            continue;
-
         const unsigned i_modifier = GetX11Modifier( p_sys->p_connection,
                 p_sys->p_symbols, i_vlc_key & KEY_MODIFIER );
 
@@ -325,6 +320,12 @@ static bool Mapping( intf_thread_t *p_intf )
             if( i != 0 && i_ignored == 0)
                 continue;
 
+            xcb_keycode_t *p_keys = xcb_key_symbols_get_keycode(
+                p_sys->p_symbols, GetX11Key( i_vlc_key & ~KEY_MODIFIER ) );
+
+            if( !p_keys )
+                break;
+
             hotkey_mapping_t *p_map = realloc( p_sys->p_map,
                               sizeof(*p_sys->p_map) * (p_sys->i_map+1) );
             if( !p_map )
-- 
2.12.0