From e7ca3a43842129084426faba865503ee7d42ce48 Mon Sep 17 00:00:00 2001 From: Jonathan Wakely Date: Fri, 13 Nov 2020 15:19:04 +0000 Subject: [PATCH] libstdc++: Avoid 32-bit time_t overflows in futex calls The existing code doesn't check whether the chrono::seconds value is out of range of time_t. When using a timeout before the epoch (with a negative value) subtracting the current time (as time_t) and then assigning it to a time_t can overflow to a large positive value. This means that we end up waiting several years even though the specific timeout was in the distant past. We do have a check for negative timeouts, but that happens after the conversion to time_t so happens after the overflow. libstdc++-v3/ChangeLog: * src/c++11/futex.cc (relative_timespec): New function to create relative time from two absolute times. (__atomic_futex_unsigned_base::_M_futex_wait_until): Use relative_timespec. (cherry picked from commit b54bd045ae908b8ace5c50ce1bdc8d472d48a514) --- libstdc++-v3/src/c++11/futex.cc | 56 ++++++++++++++++++++++++++------- 1 file changed, 45 insertions(+), 11 deletions(-) diff --git a/libstdc++-v3/src/c++11/futex.cc b/libstdc++-v3/src/c++11/futex.cc index c9de11a7ec77..5d68dafa052e 100644 --- a/libstdc++-v3/src/c++11/futex.cc +++ b/libstdc++-v3/src/c++11/futex.cc @@ -31,6 +31,7 @@ #include #include #include +#include #include // Constants for the wait/wake futex syscall operations @@ -41,10 +42,48 @@ namespace std _GLIBCXX_VISIBILITY(default) { _GLIBCXX_BEGIN_NAMESPACE_VERSION +namespace +{ + // Return the relative duration from (now_s + now_ns) to (abs_s + abs_ns) + // as a timespec. + struct timespec + relative_timespec(chrono::seconds abs_s, chrono::nanoseconds abs_ns, + time_t now_s, long now_ns) + { + struct timespec rt; + + // Did we already time out? + if (now_s > abs_s.count()) + { + rt.tv_sec = -1; + return rt; + } + + auto rel_s = abs_s.count() - now_s; + + // Avoid overflows + if (rel_s > __gnu_cxx::__int_traits::__max) + rel_s = __gnu_cxx::__int_traits::__max; + else if (rel_s < __gnu_cxx::__int_traits::__min) + rel_s = __gnu_cxx::__int_traits::__min; + + // Convert the absolute timeout value to a relative timeout + rt.tv_sec = rel_s; + rt.tv_nsec = abs_ns.count() - now_ns; + if (rt.tv_nsec < 0) + { + rt.tv_nsec += 1000000000; + --rt.tv_sec; + } + + return rt; + } +} // namespace + bool - __atomic_futex_unsigned_base::_M_futex_wait_until(unsigned *__addr, - unsigned __val, - bool __has_timeout, chrono::seconds __s, chrono::nanoseconds __ns) + __atomic_futex_unsigned_base:: + _M_futex_wait_until(unsigned *__addr, unsigned __val, bool __has_timeout, + chrono::seconds __s, chrono::nanoseconds __ns) { if (!__has_timeout) { @@ -60,15 +99,10 @@ _GLIBCXX_BEGIN_NAMESPACE_VERSION { struct timeval tv; gettimeofday (&tv, NULL); + // Convert the absolute timeout value to a relative timeout - struct timespec rt; - rt.tv_sec = __s.count() - tv.tv_sec; - rt.tv_nsec = __ns.count() - tv.tv_usec * 1000; - if (rt.tv_nsec < 0) - { - rt.tv_nsec += 1000000000; - --rt.tv_sec; - } + auto rt = relative_timespec(__s, __ns, tv.tv_sec, tv.tv_usec * 1000); + // Did we already time out? if (rt.tv_sec < 0) return false; -- 2.47.2