Let get_items_log_attrs take the start-of-text offset into account

...when interpreting item->offset values.

I ran into this when executing tests of recent LibreOffice master with ASan on
Fedora 32 (with pango-1.44.7-2.fc32.x86_64), where one of the tests renders
various dialogs with a Tamil localization and failed with

> ==97247==ERROR: AddressSanitizer: SEGV on unknown address 0x60b000210006 (pc 0x7fd6c5b22b54 bp 0x61d0004b4150 sp 0x7fff107a0d18 T0)
> ==97247==The signal is caused by a READ memory access.
>  #0 in g_utf8_get_char at ../glib/gutf8.c:319:37 (/lib64/libglib-2.0.so.0 +0x85b54)
>  #1 in break_indic at ../pango/break-indic.c:119:17 (/lib64/libpango-1.0.so.0 +0x1076d)
>  #2 in break_script at ../pango/break.c:1896:7 (/lib64/libpango-1.0.so.0 +0x1076d)
>  #3 in tailor_break at ../pango/break.c:1606:9 (/lib64/libpango-1.0.so.0 +0x147db)
>  #4 in pango_tailor_break at ../pango/break.c:1774:7 (/lib64/libpango-1.0.so.0 +0x147db)
>  #5 in get_items_log_attrs at ../pango/pango-layout.c:4032:7 (/lib64/libpango-1.0.so.0 +0x2729c)
>  #6 in pango_layout_check_lines at ../pango/pango-layout.c:4289:7 (/lib64/libpango-1.0.so.0 +0x2729c)
>  #7 in pango_layout_get_extents_internal at ../pango/pango-layout.c:2623:3 (/lib64/libpango-1.0.so.0 +0x29068)
>  #8 in gtk_label_get_measuring_layout at /usr/src/debug/gtk3-3.24.22-1.fc32.x86_64/gtk/gtklabel.c:3376:3 (/lib64/libgtk-3.so.0 +0x2454d0)
[...]

From some debugging, it smells like `pango_layout_check_lines` calls
`pango_itemize_with_base_dir` to compute `state.items` that are relative to the
beginning of `layout->text`, but then passes `state.items` together with the
offset'ed `start` into `get_items_log_attrs`, so that the latter misinterpreted
the items' locations relative to the offset'ed start.

Just adding

      g_assert (item->offset <= length);
      g_assert (item->length <= length - item->offset);

to the original `get_items_log_attrs` would make various tests in the `meson
test` suite fail, but which pass again with the complete fix, matching the above
speculation.

1 files changed, 6 insertions, 2 deletions

diff --git a/pango/pango-layout.c b/pango/pango-layout.c
index 7d5b9b10..68ffd190 100644
--- a/pango/pango-layout.c
+++ b/pango/pango-layout.c
@@ -4088,6 +4088,7 @@ process_line (PangoLayout    *layout,
 
 static void
 get_items_log_attrs (const char   *text,
+                     int           start,
                      int           length,
 		     GList        *items,
 		     PangoLogAttr *log_attrs,
@@ -4096,11 +4097,13 @@ get_items_log_attrs (const char   *text,
   int offset = 0;
   GList *l;
 
-  pango_default_break (text, length, NULL, log_attrs, log_attrs_len);
+  pango_default_break (text + start, length, NULL, log_attrs, log_attrs_len);
 
   for (l = items; l; l = l->next)
     {
       PangoItem *item = l->data;
+      g_assert (item->offset <= start + length);
+      g_assert (item->length <= (start + length) - item->offset);
 
       pango_tailor_break (text + item->offset,
                           item->length,
@@ -4371,7 +4374,8 @@ pango_layout_check_lines (PangoLayout *layout)
 
       apply_attributes_to_items (state.items, shape_attrs);
 
-      get_items_log_attrs (start,
+      get_items_log_attrs (layout->text,
+                           start - layout->text,
                            delimiter_index + delim_len,
                            state.items,
 			   layout->log_attrs + start_offset,