A mmap cache belonging to a tcb was updated when a system call which
changed the memory mapping was called. This implementation was assumed
the mapping was changed only by the tcb. However, this assumption is
incorrect if the target application is multi-threaded; more than two
tcbs can shared the same memory mapping and a tcb can modify it without
being noticed by the others.
This change introduces a global integer variable mmap_cache_generation,
and mmap_cache_generation field to struct tcb. The variable
is incremented each time a process enters a syscall that can modify its
memory mapping. Each tcb records the value of this variable at the
moment if building its mmap cache. Every mmap cache associated with
the given tcb can be validated by comparing its mmap_cache_generation
field with the variable mmap_cache_generation.
This implementation is inefficient. If strace attaches two processes
which don't share the memory mapping, rebuilding mmap cache of a tcb
triggered by another tcb's mmap system call is not necessary.