[Xcb] [PATCH] Force XCB event structures with 64-bit extended fields to be packed.

Kenneth Graunke kenneth at whitecape.org
Fri Jan 3 15:08:33 PST 2014

With the advent of the Present extension, some events (such as
PresentCompleteNotify) now use native 64-bit types on the wire.

For XGE events, we insert an extra "uint32_t full_sequence" field
immediately after the first 32 bytes of data.  Normally, this causes
the subsequent fields to be shifted over by 4 bytes, and the structure
to grow in size by 4 bytes.  Everything works fine.

However, if event contains 64-bit extended fields, this may result in
the compiler adding an extra 4 bytes of padding so that those fields
remain aligned on 64-bit boundaries.  This causes the structure to grow
by 8 bytes, not 4.  Unfortunately, XCB doesn't realize this, and
always believes that the length only increased by 4.  read_packet()
then fails to malloc enough memory to hold the event, and the event
processing code uses the wrong offsets.

To fix this, mark any event structures containing 64-bit extended
fields with __attribute__((__packed__)).

v2: Use any(...) instead of True in (...), as suggested by
    Daniel Martin.

v3 (Alan Coopersmith): Fix build with Solaris Studio 12.3 by moving the
attribute to after the structure definition.

Signed-off-by: Kenneth Graunke <kenneth at whitecape.org>
Reviewed-by: Keith Packard <keithp at keithp.com> [v1]
Reviewed-by: Josh Triplett <josh at joshtriplett.org> [v1]
Reviewed-by: Daniel Martin <consume.noise at gmail.com>
 src/c_client.py | 12 +++++++++---
 src/xcb.h       |  2 ++
 2 files changed, 11 insertions(+), 3 deletions(-)

Thanks Alan!  Feel free to add your Signed-off-by if you like - I wasn't
sure whether to include it or not.

diff --git a/src/c_client.py b/src/c_client.py
index 99fd307..45de544 100644
--- a/src/c_client.py
+++ b/src/c_client.py
@@ -1762,7 +1762,7 @@ def c_simple(self, name):
         # Iterator
         _c_iterator(self, name)
-def _c_complex(self):
+def _c_complex(self, force_packed = False):
     Helper function for handling all structure types.
     Called for all structs, requests, replies, events, errors.
@@ -1817,7 +1817,7 @@ def _c_complex(self):
             if b.type.has_name:
                 _h('    } %s;', b.c_field_name)
-    _h('} %s;', self.c_type)
+    _h('} %s%s;', 'XCB_PACKED ' if force_packed else '', self.c_type)
 def c_struct(self, name):
@@ -2902,6 +2902,7 @@ def c_event(self, name):
     # events while generating the structure for them. Otherwise we would read
     # garbage (the internal full_sequence) when accessing normal event fields
     # there.
+    force_packed = False
     if hasattr(self, 'is_ge_event') and self.is_ge_event and self.name == name:
         event_size = 0
         for field in self.fields:
@@ -2911,6 +2912,11 @@ def c_event(self, name):
                 full_sequence = Field(tcard32, tcard32.name, 'full_sequence', False, True, True)
                 idx = self.fields.index(field)
                 self.fields.insert(idx + 1, full_sequence)
+                # If the event contains any 64-bit extended fields, they need
+                # to remain aligned on a 64-bit boundary.  Adding full_sequence
+                # would normally break that; force the struct to be packed.
+                force_packed = any(f.type.size == 8 and f.type.is_simple for f in self.fields[(idx+1):])
     _c_type_setup(self, name, ('event',))
@@ -2920,7 +2926,7 @@ def c_event(self, name):
     if self.name == name:
         # Structure definition
-        _c_complex(self)
+        _c_complex(self, force_packed)
         # Typedef
diff --git a/src/xcb.h b/src/xcb.h
index e62c985..73c77a3 100644
--- a/src/xcb.h
+++ b/src/xcb.h
@@ -51,6 +51,8 @@ extern "C" {
  * @file xcb.h
+#define XCB_PACKED __attribute__((__packed__))
  * @defgroup XCB_Core_API XCB Core API
  * @brief Core API of the XCB library.

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