the x32 syscall interfaces treat timespec's tv_nsec member as 64-bit
despite the API type being long and long being 32-bit in the ABI. this
is no problem for syscalls that store timespecs to userspace as
results, but caused uninitialized padding to be misinterpreted as the
high bits in syscalls that take timespecs as input.

since the beginning of the port, we've dealt with this situation with
hacks in syscall_arch.h, and injected between __syscall_cp_c and
__syscall_cp_asm, to special-case the syscall numbers that involve
timespecs as inputs and copy them to a form suitable to pass to the
kernel.

commit 40aa18d5 set the stage for
removal of these hacks by letting us treat the "normal" x32 syscalls
dealing with timespec as if they're x32's "time64" syscalls,
effectively making x32 ax "time64-only 32-bit arch" like riscv32 will
be when it's added. since then, all users of syscalls that x32's
syscall_arch.h had hacks for have been updated to use time64 syscalls,
so the hacks can be removed.

there are still at least a few other timespec-related syscalls broken
on x32, which were overlooked when the x32 hacks were done or added
later. these include at least recvmmsg, adjtimex/clock_adjtime, and
timerfd_settime, and they will be fixed independently later on.

time64 syscall is used only if it's the only one defined for the arch,
or if either of the requested times does not fit in 32 bits. care is
taken to normalize the inputs to account for UTIME_NOW or UTIME_OMIT
in tv_nsec, in which case tv_sec should be ignored. this is needed not
only to avoid spurious time64 syscalls that might waste time failing
with ENOSYS, but also to accurately decide whether fallback is
possible.

if the requested time cannot be represented, the function fails with
ENOTSUP, defined in general as "The implementation does not support
the requested feature or value". neither the time64 syscall, nor this
error, can happen on current 32-bit archs where time_t is a 32-bit
type, and both are statically unreachable.

on 64-bit archs, there are only superficial changes to the
SYS_futimesat fallback path, which has been modified to pass long[4]
instead of struct timeval[2] to the kernel, making it suitable for use
on 32-bit archs even once time_t is changed to 64-bit. for 32-bit
archs, the call to SYS_utimensat has also been changed to copy the
timespecs through an array of long[4] rather than passing the
timespec[2] in-place.

time64 syscall is used only if it's the only one defined for the arch,
or if the requested time does not fit in 32 bits. on current 32-bit
archs where time_t is a 32-bit type, this makes it statically
unreachable.

if the time64 syscall is needed because the requested time does not
fit in 32 bits, we define this as an error ENOTSUP, for "The
implementation does not support the requested feature or value".

on 64-bit archs, there is no change to the code after preprocessing.
on current 32-bit archs, the time is moved through an intermediate
copy to remove the assumption that time_t is a 32-bit type.

time64 syscall is used only if it's the only one defined for the arch,
if either component of the itimerspec does not fit in 32 bits, or if
time_t is 64-bit and the caller requested the old value, in which case
there's a possibility that the old value might not fit in 32 bits. on
current 32-bit archs where time_t is a 32-bit type, this makes it
statically unreachable.

on 64-bit archs, there is no change to the code after preprocessing.
on current 32-bit archs, the time is moved through an intermediate
copy to remove the assumption that time_t is a 32-bit type.

time64 syscall is used only if it's the only one defined for the arch,
or if the requested timeout length does not fit in 32 bits. on current
32-bit archs where time_t is a 32-bit type, this makes it statically
unreachable.

on 64-bit archs, there are only superficial changes to the code after
preprocessing. both before and after these changes, these functions
copied their timeout arguments to avoid letting the kernel clobber the
caller's copies. now, the copying also serves to change the type from
userspace timespec to a pair of longs, which makes a difference only
in the 32-bit fallback case, not on 64-bit.

thanks to the original factorization using the __timedwait function,
there are no FUTEX_WAIT calls anywhere else, giving us a single point
of change to make nearly all the timed thread primitives time64-ready.
the one exception is the FUTEX_LOCK_PI command for PI mutex timedlock.
I haven't tried to make these two points share code, since they have
different fallbacks (no non-private fallback needed for PI since PI
was added later) and FUTEX_LOCK_PI isn't a cancellation point (thus
allowing the whole code path to inline into pthread_mutex_timedlock).

as for other changes in this series, the time64 syscall is used only
if it's the only one defined for the arch, or if the requested timeout
does not fit in 32 bits. on current 32-bit archs where time_t is a
32-bit type, this makes it statically unreachable.

on 64-bit archs, there are only superficial changes to the code after
preprocessing. on current 32-bit archs, the time is passed via an
intermediate copy to remove the assumption that time_t is a 32-bit
type.

time64 syscall is used only if it's the only one defined for the arch,
or if the requested timeout does not fit in 32 bits. on current 32-bit
archs where time_t is a 32-bit type, this makes it statically
unreachable.

on 64-bit archs, there is no change to the code after preprocessing.
on current 32-bit archs, the time is passed via an intermediate copy
to remove the assumption that time_t is a 32-bit type.

to avoid duplicating SYS_ipc/SYS_semtimedop choice logic, the code for
32-bit archs "falls through" after updating the timeout argument ts to
point to a [compound literal] array of longs. in preparation for
"time64-only" 32-bit archs, an extra case is added for neither SYS_ipc
nor the non-time64 SYS_semtimedop existing; the ENOSYS failure path
here should never be reachable, and is added just in case a compiler
can't see that it's not reachable, to avoid spurious static analysis
complaints.

time64 syscall is used only if it's the only one defined for the arch,
or if the requested timeout length does not fit in 32 bits. on current
32-bit archs where time_t is a 32-bit type, this makes it statically
unreachable.

on 64-bit archs, there are only superficial changes to the code after
preprocessing. on current 32-bit archs, the timeout is passed via an
intermediate copy to remove the assumption that time_t is a 32-bit
type.

time64 syscall is used only if it's the only one defined for the arch,
or if the requested absolute timeout does not fit in 32 bits. on
current 32-bit archs where time_t is a 32-bit type, this makes it
statically unreachable.

on 64-bit archs, there is no change to the code after preprocessing.
on current 32-bit archs, the timeout is passed via an intermediate
copy to remove the assumption that time_t is a 32-bit type.

time64 syscall is used only if it's the only one defined for the arch,
or if the requested time does not fit in 32 bits. on current 32-bit
archs where time_t is a 32-bit type, this makes it statically
unreachable.

on 64-bit archs, there is no change to the code after preprocessing.
on current 32-bit archs, the time is moved through an intermediate
copy to remove the assumption that time_t is a 32-bit type.

this is yet another place where special handling of time syscalls can
and should be avoided by implementing legacy functions in terms of
their modern replacements. in theory a fallback to SYS_settimeofday
could be added to clock_settime, but SYS_clock_settime has been
available since Linux 2.6.0 or earlier, i.e. all the way back to the
minimum supported version.

this commit has no effect whatsoever right now, but is in preparation
for a future riscv32 port and other future 32-bit archs that will be
"time64-only" from the start on the kernel side.

together with the previous x32 changes, this commit ensures that
syscall call points that don't care about time (passing null timeouts,
etc.) can continue to do so without having to special-case time64-only
archs, and allows code using the time64 syscalls to uniformly test for
the need to fallback with SYS_foo != SYS_foo_time64, rather than
needing to check defined(SYS_foo) && SYS_foo != SYS_foo_time64.

x32 is odd in that it's the only ILP32 arch/ABI we have where time_t
is 64-bit rather than (32-bit) long, and this has always been
problematic in that it results in struct timespec having unused
padding space, since tv_nsec has type long, which the kernel insists
be zero- or sign-extended (due to negative tv_nsec being invalid, it
doesn't matter which) to match the x86_64 type.

up til now, we've had really ugly hacks in x32/syscall_arch.h to patch
up the timespecs passed to the kernel. but the same requirement to
zero- or sign-extend tv_nsec also applies to all the new time64
syscalls on true 32-bit archs. so let's take advantage of this to
clean things up.

this patch defines all of the time64 syscalls for x32 as aliases for
the existing syscalls by the same name. this establishes the following
invariants:

- if the time64 form is defined, it takes time arguments as 64-bit
  objects, and tv_nsec inputs must be zero-/sign-extended to 64-bit.

- if the time64 form is not defined, or if the time64 form is defined
  and is not equal to the "plain" form, the plain form takes time
  arguments as longs.

this will avoid the need for protocols for archs to define appropriate
types for each family of syscalls, and for the reader of the code to
have to be aware of such type definitions.

in some sense it might be simpler if the plain syscall form were
undefined for x32, so that it would always take longs if defined.
however, a number of these syscalls are used in contexts with a null
time argument, or (e.g. futex) for commands that don't involve time at
all, and having to introduce time64-specific logic to all those call
points does not make sense. thus, while the "plain" forms are kept now
just because they're needed until the affected code is converted over,
they'll also almost surely be kept in the future as well.

kernel support for x32 was added long after the utimensat syscall was
already available, so having a fallback is just wasted code size.

also, for changes related to time64 support on 32-bit archs, I want to
be able to assume the old futimesat syscall always works with longs,
which is true except for x32. by ensuring that it's not used on x32,
the needed invariant is established.

previously the fallback wrongly failed with EINVAL rather than ENOSYS
when UTIME_NOW was used with one component but not both. commit
dd5f50da introduced this behavior when
initially adding the fallback support.

instead, detect the case where both are UTIME_NOW early and replace
with a null times pointer; this may improve performance slightly (less
copy from user), and removes the complex logic from the fallback case.
it also makes things slightly simpler for adding time64 code paths.

for namespace-safety with thrd_sleep, this requires an alias, which is
also added. this eliminates all but one direct call point for
nanosleep syscalls, and arranges that 64-bit time_t conversion logic
will only need to exist in one file rather than three.

as a bonus, clock_nanosleep with CLOCK_REALTIME and empty flags is now
implemented as SYS_nanosleep, thereby working on older kernels that
may lack POSIX clocks functionality.

commit 01ae3fc6 modified fstatat to
translate the kernel's struct stat ("kstat") into the libc struct stat.
To do this, it created a local kstat object, and copied its contents
into the user-provided object.

However, the commit neglected to update the fstat compatibility path and
its fallbacks. They continued to pass the user-supplied object to the
kernel, later overwiting it with the uninitialized memory in the local
temporary.

this removes the assumption that userspace struct timex matches the
syscall type and sets the stage for 64-bit time_t on 32-bit archs.

this sets the stage for having the conversion logic for 64-bit time_t
all in one file, and as a bonus makes clock_adjtime for CLOCK_REALTIME
work even on kernels too old to have the clock_adjtime syscall.

commit dfc81828 accidentally defined
an instance of struct statx along with the struct declaration.

this commit adds a new backend for fstatat (and thereby the whole stat
family) using the SYS_statx syscall, but conditions the new code on
the kernel stat structure's time fields being smaller than time_t. in
principle that should make it all dead code at present, but mips64 has
a broken stat structure with 32-bit time fields despite having 64-bit
time_t elsewhere, so on mips64 it is a functional change that makes
post-Y2038 filesystem timestamps accessible.

whenever the 32-bit archs end up getting 64-bit time_t, regardless of
how that happens, the changes in this commit will automatically take
effect for them too.

AT_FDCWD is not a valid file descriptor, so POSIX requires fstat to
fail with EBADF. if passed to fstatat, the call would spuriously
succeed and return results for the working directory.

now that we have a kstat structure decoupled from the public struct
stat, we can just use the broken kernel structures directly and let
the code in fstatat do the translation.

presently, all archs/ABIs have struct stat matching the kernel
stat[64] type, except mips/mipsn32/mips64 which do conversion hacks in
syscall_arch.h to work around bugs in the kernel type. this patch
completely decouples them and adds a translation step to the success
path of fstatat. at present, this is just a gratuitous copying, but it
opens up multiple possibilities for future support for 64-bit time_t
on 32-bit archs and for cleaned-up/unified ABIs.

for clarity, the mips hacks are not yet removed in this commit, so the
mips kstat structs still correspond to the output of the hacks in
their syscall_arch.h files, not the raw kernel type. a subsequent
commit will fix this.

equivalent logic for fstat+O_PATH fallback and direct use of
stat/lstat syscalls where appropriate is kept, now in the fstatat
function. this change both improves functionality (now, fstatat forms
equivalent to fstat/lstat/stat will work even on kernels too old to
have the at functions) and localizes direct interfacing with the
kernel stat structure to one file.

these were overlooked during review. bits headers are not allowed to
pull in additional headers (note: that rule is currently broken in
other places but just for endian.h). string.h has no place here
anyway, and including bits/alltypes.h without defining macros to
request types from it is a nop.

the "A" constraint is simply for an address expression that's a single
register, but it's not yet supported by clang, and has no advantage
here over just using a register operand for the address. the latter is
actually preferable in the a_cas_p case because it avoids aliasing an
lvalue onto the memory.

most egregious problem was the lack of memory clobber and lack of
volatile asm; this made the atomics memory barriers but not compiler
barriers. use of "+r" rather than "=r" for a clobbered temp was also
wrong, since the initial value is indeterminate.

having "+r"(a0) is redundant with "0"(a0) in syscalls with at least 1
arg, which is arguably a constraint violation (clang treats it as
such), and an invalid input with indeterminate value in the 0-arg
case. use the "=r"(a0) form instead.

this is analogous to commit 918c5fa0
which fixed the corresponding issue for mips n32.

mips n32 has 32-bit long, and generally uses long syscall arguments
and return values, but provides only SYS_lseek, not SYS_llseek. we
have some framework (syscall_arg_t, added for x32) to make syscall
arguments 64-bit in such a setting, but it's not clear whether this
could match the sign-extension semantics needed for 32-bit args to all
the other syscalls, and we don't have any existing mechanism to allow
the return value of syscalls to be something other than long.

instead, just provide a custom mipsn32 version of the lseek function
doing its own syscall asm with 64-bit arguments. as a result of commit
03919b26, stdio will also get the new
code, fixing fseeko/ftello too.

ever since inline syscalls were added for (o32) mips in commit
328810d3, the asm has nonsensically
loaded the syscall number, rather than taking $2 as an input
constraint to let the compiler load it. commit
cfc09b1e improved on this somewhat by
allowing a constant syscall number to propagate into an immediate, but
missed that the whole operation made no sense.

now, only $4, $5, $6, $8, and $9 are potential input-only registers.
$2 is always input and output, and $7 is both when it's an argument,
otherwise output-only. previously, $7 was treated as an input (with a
"1" constraint matching its output position) even when it was not an
input, which was arguably undefined behavior (asm input from
indeterminate value). this is corrected.

this patch is not purely non-functional changes, since before, $8 and
$9 were wrongly in the clobberlist for syscalls with fewer than 5 or 6
arguments. of course it's impossible for syscalls to have different
clobbers depending on their number of arguments. the clobberlist for
the recently-added 5- and 6-argument forms was correct, and for the 0-
to 4-argument forms was erroneously copied from the mips o32 ABI where
the additional arguments had to be passed on the stack.

in making this change, I reviewed the kernel sources, and $8 and $9
are always saved for 64-bit kernels since they're part of the syscall
argument list for n32 and n64 ABIs.

this probably saves a few bytes, avoids duplicating the clunky
lseek/_llseek syscall convention in two places, and sets the stage for
fixing broken seeks on x32 and mipsn32.

these additions were made by scanning git log since the last major
update in commit 1366b3c5.

as before my aim was adding everyone with either substantial code
contributions or a pattern of ongoing simple patch submission; any
omissions are unintentional.

a fully thumb1 build is not supported because some asm files are
incompatible with thumb1, but apparently it works to compile the C
code as thumb1

commit 06fbefd1 caused this regression
but introducing use of the clz instruction, which is not supported in
arm mode prior to v5, and not supported in thumb prior to thumb2
(v6t2). commit 1b9406b0 fixed the
issue only for arm mode pre-v5 but left thumb1 broken.