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libstdc++: Use custom timespec in system calls [PR 93421]

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On 32-bit targets where userspace has switched to 64-bit time_t, we
cannot pass struct timespec to SYS_futex or SYS_clock_gettime, because
the userspace definition of struct timespec will not match what the
kernel expects.

We use the existence of the SYS_futex_time64 or SYS_clock_gettime_time64
macros to imply that userspace *might* have switched to the new timespec
definition.  This is a conservative assumption. It's possible that the
new syscall numbers are defined in the libc headers but that timespec
hasn't been updated yet (as is the case for glibc currently). But using
the alternative struct with two longs is still OK, it's just redundant
if userspace timespec still uses a 32-bit time_t.

We also check that SYS_futex_time64 != SYS_futex so that we don't try
to use a 32-bit tv_sec on modern targets that only support the 64-bit
system calls and define the old macro to the same value as the new one.

We could possibly check #ifdef __USE_TIME_BITS64 to see whether
userspace has actually been updated, but it's not clear if user code
is meant to inspect that or if it's only for libc internal use.

libstdc++-v3/ChangeLog:

	PR libstdc++/93421
	* src/c++11/chrono.cc [_GLIBCXX_USE_CLOCK_GETTIME_SYSCALL]
	(syscall_timespec): Define a type suitable for SYS_clock_gettime
	calls.
	(system_clock::now(), steady_clock::now()): Use syscall_timespec
	instead of timespec.
	* src/c++11/futex.cc (syscall_timespec): Define a type suitable
	for SYS_futex and SYS_clock_gettime calls.
	(relative_timespec): Use syscall_timespec instead of timespec.
	(__atomic_futex_unsigned_base::_M_futex_wait_until): Likewise.
	(__atomic_futex_unsigned_base::_M_futex_wait_until_steady):
	Likewise.
This commit is contained in:
Jonathan Wakely 2020-09-24 17:35:52 +01:00
parent b8d36dcc91
commit 4d039cb9a1
2 changed files with 32 additions and 8 deletions
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17
libstdc++-v3/src/c++11/chrono.cc
View File

@ -35,6 +35,17 @@
#ifdef _GLIBCXX_USE_CLOCK_GETTIME_SYSCALL
#include <unistd.h>
#include <sys/syscall.h>
# if defined(SYS_clock_gettime_time64) \
&& SYS_clock_gettime_time64 != SYS_clock_gettime
// Userspace knows about the new time64 syscalls, so it's possible that
// userspace has also updated timespec to use a 64-bit tv_sec.
// The SYS_clock_gettime syscall still uses the old definition
// of timespec where tv_sec is 32 bits, so define a type that matches that.
struct syscall_timespec { long tv_sec; long tv_nsec; };
# else
using syscall_timespec = ::timespec;
# endif
#endif
namespace std _GLIBCXX_VISIBILITY(default)
@ -52,11 +63,12 @@ _GLIBCXX_BEGIN_NAMESPACE_VERSION
system_clock::now() noexcept
{
#ifdef _GLIBCXX_USE_CLOCK_REALTIME
timespec tp;
// -EINVAL, -EFAULT
#ifdef _GLIBCXX_USE_CLOCK_GETTIME_SYSCALL
syscall_timespec tp;
syscall(SYS_clock_gettime, CLOCK_REALTIME, &tp);
#else
timespec tp;
clock_gettime(CLOCK_REALTIME, &tp);
#endif
return time_point(duration(chrono::seconds(tp.tv_sec)
@ -80,11 +92,12 @@ _GLIBCXX_BEGIN_NAMESPACE_VERSION
steady_clock::now() noexcept
{
#ifdef _GLIBCXX_USE_CLOCK_MONOTONIC
timespec tp;
// -EINVAL, -EFAULT
#ifdef _GLIBCXX_USE_CLOCK_GETTIME_SYSCALL
syscall_timespec tp;
syscall(SYS_clock_gettime, CLOCK_MONOTONIC, &tp);
#else
timespec tp;
clock_gettime(CLOCK_MONOTONIC, &tp);
#endif
return time_point(duration(chrono::seconds(tp.tv_sec)

23
libstdc++-v3/src/c++11/futex.cc
View File

@ -58,13 +58,23 @@ namespace
std::atomic<bool> futex_clock_realtime_unavailable;
std::atomic<bool> futex_clock_monotonic_unavailable;
#if defined(SYS_futex_time64) && SYS_futex_time64 != SYS_futex
// Userspace knows about the new time64 syscalls, so it's possible that
// userspace has also updated timespec to use a 64-bit tv_sec.
// The SYS_futex and SYS_clock_gettime syscalls still use the old definition
// of timespec where tv_sec is 32 bits, so define a type that matches that.
struct syscall_timespec { long tv_sec; long tv_nsec; };
#else
using syscall_timespec = ::timespec;
#endif
// Return the relative duration from (now_s + now_ns) to (abs_s + abs_ns)
// as a timespec.
struct timespec
// as a timespec suitable for syscalls.
syscall_timespec
relative_timespec(chrono::seconds abs_s, chrono::nanoseconds abs_ns,
time_t now_s, long now_ns)
{
struct timespec rt;
syscall_timespec rt;
// Did we already time out?
if (now_s > abs_s.count())
@ -119,7 +129,7 @@ namespace
if (__s.count() < 0)
return false;
struct timespec rt;
syscall_timespec rt;
if (__s.count() > __int_traits<time_t>::__max) [[unlikely]]
rt.tv_sec = __int_traits<time_t>::__max;
else
@ -195,7 +205,7 @@ namespace
if (__s.count() < 0) [[unlikely]]
return false;
struct timespec rt;
syscall_timespec rt;
if (__s.count() > __int_traits<time_t>::__max) [[unlikely]]
rt.tv_sec = __int_traits<time_t>::__max;
else
@ -224,10 +234,11 @@ namespace
// We only get to here if futex_clock_monotonic_unavailable was
// true or has just been set to true.
struct timespec ts;
#ifdef _GLIBCXX_USE_CLOCK_GETTIME_SYSCALL
syscall_timespec ts;
syscall(SYS_clock_gettime, CLOCK_MONOTONIC, &ts);
#else
struct timespec ts;
clock_gettime(CLOCK_MONOTONIC, &ts);
#endif