(release/25.0) miext/sync: Handle SIGBUS from truncated shm fences

[metux]

A client can truncate the shared memory backing a DRI3/xshmfence sync
fence and cause SIGBUS when the server accesses it.

Register the mapping with busfault and wrap xshmfence accesses with
sigsetjmp so truncation unwinds safely, then discard the mapping while
keeping the fence XID.

Signed-off-by: Olivier Fourdan ofourdan@redhat.com
Assisted-by: AI
Closes: https://gitlab.freedesktop.org/xorg/xserver/-/work_items/1911
Part-of: https://gitlab.freedesktop.org/xorg/xserver/-/merge_requests/2270
(cherry picked from commit cd083d9)

[metux]

🤖 Automated review — generated by Starfleet ship Enterprise (model: cruiser) on behalf of @metux. Not a human review.

🤖 Automated review — generated by Starfleet ship Enterprise on behalf of @metux. Not a human review.

Summary: Adds BUSFAULT-guarded SIGBUS handling to miext/sync/misyncshm.c. Sync SHM fences now register with the busfault subsystem (introduced in #4016) and use sigsetjmp/siglongjmp to safely handle truncated shared memory fences instead of crashing. Includes fence discard on SIGBUS and registration on fence creation.

Assessment:

• Backport-worthiness: Yes, crash-safety/DoS fix for the Sync extension, applies to release/25.0.
• Driver-ABI impact: No - changes are in miext/sync/misyncshm.c, an internal server subsystem. No driver-visible structures or _X_EXPORT symbols affected.
• Correctness: Uses the existing busfault_set_jmp()/busfault_unregister() API correctly; jmp buffer is cleared after successful access and after longjmp. Safe.

Label: bot-review-passed