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

[Josh Triplett]

On Fri, Jan 03, 2014 at 03:08:33PM -0800, Kenneth Graunke wrote:
> 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]

v3 as well.

> 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):])
>                  break
>  
>      _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)
>      else:
>          # Typedef
>          _h('')
> 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.
> -- 
> 1.8.5.2
> 
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