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Improve 'last_write_time(...)' accuracy and detect overflow errors.

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The ::stat struct on Linux, FreeBSD, and OS X  provides the access and
modification times as an instance of 'timespec', which has a nanosecond
resolution. The 'st_mtime' and 'st_atime' members simply reference the 'tv_sec'
value of the timespec struct. This patch changes 'last_write_time(...)' so that
it extracts both the seconds and nanoseconds values of the last modification
time, providing a more accurate implementation of 'last_write_time(...)'.

Additionally this patch fixes a possible signed integer overflow bug. The
'file_time_type' type cannot represent all possible values returned by
the filesystem. Attempting to construct a 'file_time_type' from one of these
values is undefined behavior. This patch avoids that UB by detecting possible
overflows before the conversion.


git-svn-id: https://llvm.org/svn/llvm-project/libcxx/trunk@282634 91177308-0d34-0410-b5e6-96231b3b80d8
This commit is contained in:
Eric Fiselier
2016-09-28 21:16:58 +00:00
parent c3e00f6d37
commit d5fc5ca37f
1 changed files with 35 additions and 5 deletions
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40
src/experimental/filesystem/operations.cpp
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@@ -509,7 +509,6 @@ bool set_times_checked(time_t* sec_out, SubSecT* subsec_out, file_time_type tp)
// The tv_nsec and tv_usec fields must not be negative so adjust accordingly
if (subsec_dur.count() < 0) {
if (sec_dur.count() > min_seconds) {
sec_dur -= seconds(1);
subsec_dur += seconds(1);
} else {
@@ -520,11 +519,24 @@ bool set_times_checked(time_t* sec_out, SubSecT* subsec_out, file_time_type tp)
&& checked_set(subsec_out, subsec_dur.count());
}
using TimeSpec = struct ::timespec;
using StatT = struct ::stat;
#if defined(__APPLE__)
TimeSpec extract_mtime(StatT const& st) { return st.st_mtimespec; }
TimeSpec extract_atime(StatT const& st) { return st.st_atimespec; }
#else
TimeSpec extract_mtime(StatT const& st) { return st.st_mtim; }
__attribute__((unused)) // Suppress warning
TimeSpec extract_atime(StatT const& st) { return st.st_atim; }
#endif
}} // end namespace detail
file_time_type __last_write_time(const path& p, std::error_code *ec)
{
using namespace ::std::chrono;
std::error_code m_ec;
struct ::stat st;
detail::posix_stat(p, st, &m_ec);
@@ -532,8 +544,25 @@ file_time_type __last_write_time(const path& p, std::error_code *ec)
set_or_throw(m_ec, ec, "last_write_time", p);
return file_time_type::min();
}
if (ec) ec->clear();
return file_time_type::clock::from_time_t(st.st_mtime);
#ifndef _LIBCPP_HAS_NO_INT128
using IntMax = __int128_t;
#else
using IntMax = intmax_t;
#endif
// FIXME: The value may not be representable as file_time_type. Fix
// file_time_type so it can represent all possible values returned by the
// filesystem. For now we do the calculation with the largest possible types
// and then truncate, this prevents signed integer overflow bugs.
auto ts = detail::extract_mtime(st);
const auto NsDur = duration<IntMax, nano>(ts.tv_nsec) + seconds(ts.tv_sec);
if (NsDur > file_time_type::max().time_since_epoch() ||
NsDur < file_time_type::min().time_since_epoch()) {
set_or_throw(error_code(EOVERFLOW, generic_category()), ec,
"last_write_time", p);
return file_time_type::min();
}
if (ec) ec->clear();
return file_time_type(duration_cast<file_time_type::duration>(NsDur));
}
void __last_write_time(const path& p, file_time_type new_time,
@@ -554,9 +583,10 @@ void __last_write_time(const path& p, file_time_type new_time,
set_or_throw(m_ec, ec, "last_write_time", p);
return;
}
auto atime = detail::extract_atime(st);
struct ::timeval tbuf[2];
tbuf[0].tv_sec = st.st_atime;
tbuf[0].tv_usec = 0;
tbuf[0].tv_sec = atime.tv_sec;
tbuf[0].tv_usec = duration_cast<microseconds>(nanoseconds(atime.tv_nsec)).count();
const bool overflowed = !detail::set_times_checked<microseconds>(
&tbuf[1].tv_sec, &tbuf[1].tv_usec, new_time);