the issue at hand is that many syscalls require as an argument the
kernel-ABI size of sigset_t, intended to allow the kernel to switch to
a larger sigset_t in the future. previously, each arch was defining
this size in syscall_arch.h, which was redundant with the definition
of _NSIG in bits/signal.h. as it's used in some not-quite-portable
application code as well, _NSIG is much more likely to be recognized
and understood immediately by someone reading the code, and it's also
shorter and less cluttered.
note that _NSIG is actually 65/129, not 64/128, but the division takes
care of throwing away the off-by-one part.
some minor changes to how hard-coded sets for thread-related purposes
are handled were also needed, since the old object sizes were not
necessarily sufficient. things have gotten a bit ugly in this area,
and i think a cleanup is in order at some point, but for now the goal
is just to get the code working on all supported archs including mips,
which was badly broken by linux rejecting syscalls with the wrong
sigset_t size.
the new approach relies on the fact that the only ways to create
sigset_t objects without invoking UB are to use the sig*set()
functions, or from the masks returned by sigprocmask, sigaction, etc.
or in the ucontext_t argument to a signal handler. thus, as long as
sigfillset and sigaddset avoid adding the "protected" signals, there
is no way the application will ever obtain a sigset_t including these
bits, and thus no need to add the overhead of checking/clearing them
when sigprocmask or sigaction is called.
note that the old code actually *failed* to remove the bits from
sa_mask when sigaction was called.
the new implementations are also significantly smaller, simpler, and
faster due to ignoring the useless "GNU HURD signals" 65-1024, which
are not used and, if there's any sanity in the world, never will be
used.
a signal handler could fork after the pid/tid were read, causing the
wrong process to be signalled. i'm not sure if this is supposed to
have UB or not, but raise is async-signal-safe, so it probably is
allowed. the current solution is slightly expensive so this
implementation is likely to be changed in the future.
