On Wed, 1 Jul 2026, 07:45 Tomasz Kaminski, <[email protected]> wrote:
> > > On Tue, Jun 30, 2026 at 7:48 PM Jonathan Wakely <[email protected]> > wrote: > >> Although the systemd docs say that /etc/localtime should be a symlink to >> one of the zoneinfo files, some systems make it a symlink to another >> path, where that second path is a symlink to a zoneinfo file (e.g. if >> /etc is mounted read-only then /etc/localtime can be a symlink to >> another symlink on a writable disk, so that the system timezone can be >> altered by re-pointing the symlink on the writable disk). >> >> In that case, using readlink would only tell us the location of the >> second symlink, not which zoneinfo file it points to. Therefore, we >> would not be able to extract a valid time zone name from the path, and >> chrono::current_zone() would fail. >> >> To support multiple symlinks we could recursively keep resolving >> symlinks with readlink until we reach a path from which we can extract a >> zone name. Alternatively, we can just use realpath to resolve all >> symlinks to a physical file (which is what HowardHinnant/date does). >> This means we only need one system call and don't need the extra >> complexity of calling readlink in a loop. >> >> The realpath system call also removes redunant slashes, so we can remove >> the code that did that manually. >> >> The possible downsides of this approach that I'm aware of are: >> >> - When /etc/localtime is a symlink to /invalid/Europe/London but that >> file doesn't exist. With the previous implementation we would have >> resolved that symlink to the zone "Europe/London" as long as that name >> is known to the current chrono::tzdb object. With this change, we >> won't get a valid zone name and current_zone() will fail. I'm not sure >> how realistic this case is. It might be plausible if libstdc++ is >> using the embedded static copy of tzdata.zi and there are no zoneinfo >> files on disk at all. In that case the system might still use >> /etc/localtime to name a zone, even though the symlink is dangling. >> We could fall back to filesystem::weakly_canonical for this case, but >> this patch leaves that for a future change, if it turns out to be >> needed by any users. >> >> - When /etc/localtime is a symlink to /usr/share/zoneinfo/Foo/Bar where >> "Foo/Bar" is a valid zone in the chrono::tzdb object, but the Bar file >> is another symlink to ./Baz where "Foo/Bar" is also a valid zone. >> With the previous implementation current_zone() would have returned >> the "Foo/Bar" zone. With this change it would return "Foo/Baz". I >> don't think it's realistic to have two zones which are distinct zones >> (not a Zone and a Link to it) but where one of them is defined on-disk >> using a symlink to the other. >> >> libstdc++-v3/ChangeLog: >> >> PR libstdc++/125467 >> * src/c++20/tzdb.cc (tzdb::current_zone): Use realpath to >> resolve the /etc/localtime symlink instead of readlink. >> --- >> >> v2: Make the type of 'str' always std::string_view. Check str != >> "/etc/localtime" so that we don't bother trying to extract a zone name >> from the symlink target if it isn't even a symlink. >> >> Tested x86_64-linux. >> > LGTM with very small suggestion. > >> >> libstdc++-v3/src/c++20/tzdb.cc | 76 +++++++++++++--------------------- >> 1 file changed, 28 insertions(+), 48 deletions(-) >> >> diff --git a/libstdc++-v3/src/c++20/tzdb.cc >> b/libstdc++-v3/src/c++20/tzdb.cc >> index 9e601fc176f3..c658e0c9cd37 100644 >> --- a/libstdc++-v3/src/c++20/tzdb.cc >> +++ b/libstdc++-v3/src/c++20/tzdb.cc >> @@ -41,8 +41,13 @@ >> # include <ext/concurrence.h> // __gnu_cxx::__mutex >> #endif >> >> -#if defined(_GLIBCXX_HAVE_READLINK) && defined(_GLIBCXX_HAVE_UNISTD_H) >> -# include <unistd.h> // readlink >> +#ifdef _GLIBCXX_HAVE_UNISTD_H >> +# include <unistd.h> // _XOPEN_VERSION >> +#endif >> +#if defined _GLIBCXX_USE_REALPATH && _XOPEN_VERSION >= 700 >> +# include <stdlib.h> // malloc, free, realpath >> +#else >> +# include <filesystem> // filesystem::canonicalize >> #endif >> >> #ifdef _AIX >> @@ -2098,58 +2103,33 @@ constinit tzdb_list::_Node::NumLeapSeconds >> tzdb_list::_Node::num_leap_seconds; >> // to have a way to force a re-read. >> >> #if !defined(_AIX) && !defined(_GLIBCXX_HAVE_WINDOWS_H) >> -#if defined(_GLIBCXX_HAVE_READLINK) && defined(_GLIBCXX_HAVE_UNISTD_H) >> - string_view str; >> - char buf[128]; // strlen("../usr/share/zoneinfo/...") is usually < 55 >> - string dynbuf; >> // /etc/localtime should be a symlink that ends with a zone name, >> // e.g. /etc/localtime -> /usr/share/zoneinfo/Europe/London >> // >> https://www.freedesktop.org/software/systemd/man/latest/localtime.html >> // This should work on GNU/Linux, macOS, NetBSD, and OpenBSD. >> - // Some FreeBSD systems also use a symlink for /etc/localtime. >> - // Use readlink directly to avoid std::filesystem overhead. >> - if (auto n = ::readlink("/etc/localtime", buf, sizeof(buf)); n > 0) >> + // Some FreeBSD systems also use a symlink for /etc/localtime (since >> 15.0). >> + >> + // N.B. we do not support dangling symlinks here. If that becomes >> necessary >> + // then after realpath fails we could fallback to using >> + // >> filesystem::weakly_canonical(filesystem::read_symlink("etc/localtime")). >> + >> +#if defined _GLIBCXX_USE_REALPATH && _XOPEN_VERSION >= 700 >> + unique_ptr<char[], void(*)(void*)> cbuf{ nullptr, &::free }; >> + string_view str; >> > This is very subjective and not necessary, but I would prefer this > string_view to be > declared before #if here. I find it easier to follow that way. > The order I used means that the string view lifetime is shorter than the buffer lifetime so it can't dangle. > >> + // Use realpath directly to avoid std::filesystem overhead. >> + // We use realpath not readlink to resolve multiple levels of >> symlinks. >> + if (char* p = ::realpath("/etc/localtime", nullptr)) >> { >> - if (static_cast<size_t>(n) < sizeof(buf)) >> - str = string_view(buf, n); >> - else [[unlikely]] >> - { >> - // We read the symlink but it didn't fit in buf[], use dynbuf. >> - do >> - { >> - n *= 2; >> - dynbuf.__resize_and_overwrite(n, [](char* p, size_t len) { >> - auto n2 = ::readlink("/etc/localtime", p, len); >> - if (n2 == -1) // symlink removed or replaced by file?! >> - __throw_runtime_error("tzdb: error reading >> /etc/localtime"); >> - const size_t r = n2; >> - return r < len ? r : 0; >> - }); >> - } >> - while (dynbuf.empty()); >> - str = dynbuf; >> - } >> + cbuf.reset(p); >> + str = p; >> } >> +#else >> + string sbuf = std::filesystem::canonical("/etc/localtime").string(); >> + string_view str = sbuf; >> > And this will become an assignment. > >> +#endif >> >> - if (!str.empty()) >> + if (!str.empty() && str != "/etc/localtime") >> { >> - // Remove any redundant slashes so we can match zone names. >> - // e.g. /usr/share/zoneinfo/Europe//London is a valid symlink, >> - // but won't match against "Europe/London". >> - if (auto pos = str.rfind("//"); pos != str.npos) [[unlikely]] >> - { >> - if (str.data() != dynbuf.data()) >> - dynbuf = str; >> - string::size_type spos = pos; >> - do >> - { >> - dynbuf.erase(spos, 1); >> - spos = dynbuf.rfind("//", spos); >> - } >> - while (spos != dynbuf.npos); >> - str = dynbuf; >> - } >> - >> // Check the trailing components of the path against known zone >> names. >> // Valid IANA times zones can have one, two, or three parts, e.g. >> // "UTC", "Europe/London", and "America/Indiana/Indianapolis". >> @@ -2175,10 +2155,10 @@ constinit tzdb_list::_Node::NumLeapSeconds >> tzdb_list::_Node::num_leap_seconds; >> str.substr(pos + 1))) >> return tz; >> } >> -#endif >> + >> // Otherwise, look for a file naming the time zone. >> string_view files[] { >> - "/etc/timezone", // Debian derivates >> + "/etc/timezone", // Debian derivates, non-systemd Gentoo >> "/var/db/zoneinfo", // FreeBSD >> }; >> for (auto f : files) >> -- >> 2.54.0 >> >>
