Let's define two helpers strdupa_safe() + strndupa_safe() which do the
same as their non-safe counterparts, except that they abort if called
with allocations larger than ALLOCA_MAX.
This should ensure that all our alloca() based allocations are subject
to this limit.
afaics glibc offers three alloca() based APIs: alloca() itself,
strndupa() + strdupa(). With this we have now replacements for all of
them, that take the limit into account.
procfs_memory_get_current is renamed to procfs_memory_get_used, because
"current" can mean anything, including total memory, used memory, and free
memory, as long as the value is up to date.
No functional change.
This is preparation for emulating the "usage_usec" keyed attribute of
the "cpu.stat" property of the root cgroup from data in /proc. Similar,
for emulating the "memory.current" attribute.
