On Fri, Jun 26, 2026 at 7:02 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 will be . 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.
> Maybe we could fall back to filesystem::weakly_canonical for this
> case?
>
I would be concerned about doing that silently, as this could lead to
non-local errors in the deployed environment are extremely hard to debug.
What I mean is the situation where tzdata should be mounted.
If the mounting fails, we will resolve the zone without issue, and then only
If the data requires updating, the update will fail. From that perspective
I much
more prefer loud failure.
As the fact that this works is libstdc++ specific (other libraries and
clients
may use the content of the file), I think this case (if appears) may be
better
served by a dedicated enviroment variable.
>
> - 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)
Yes, and the link name is not accessible to the program: zone->name
returns the target zone, so it is not observable if we located Asia/Istanbul
(that was symlink to Europe/Istanbul) or Asia/Istanbul. I think that is OK.
> but where one of them is defined on-disk
> using a symlink to the other.
>
> libstdc++-v3/ChangeLog:
>
> PR libstdc++/125467
>
This PR is created by you. Does it come from real user complain, or this
was found by you?
> * src/c++20/tzdb.cc (tzdb::current_zone): Use realpath to
> resolve the /etc/localtime symlink instead of readlink.
> ---
>
> Tested x86_64-linux.
>
> Should we handle the dangling symlink case, maybe by using
> filesystem::weakly_canonical?
I would wait to see if anybody is actually impacted by it in non-error case.
I would got with this with only small modification suggested bellow.
>
> libstdc++-v3/src/c++20/tzdb.cc | 65 +++++++++++-----------------------
> 1 file changed, 20 insertions(+), 45 deletions(-)
>
> diff --git a/libstdc++-v3/src/c++20/tzdb.cc
> b/libstdc++-v3/src/c++20/tzdb.cc
> index 0158659f79e1..acfc57f76437 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
> @@ -2094,58 +2099,28 @@ 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)
> +
> +#if defined _GLIBCXX_USE_REALPATH && _XOPEN_VERSION >= 700
> + unique_ptr<char[], void(*)(void*)> buf{ nullptr, &::free };
> + string_view str;
>
A slight stylistic note: I would prefer `str` to always be string_view
(substr will do
different thing otherwise, that may bite us), and declared before the #if.
> + // 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;
> - }
> + buf.reset(p);
> + str = p;
> }
> +#else
> + string str = std::filesystem::canonical("/etc/localtime").string();
>
And here we would have:
string buf
= std::filesystem::canonical("/etc/localtime").string();
str = buf;
> +#endif
>
> if (!str.empty())
> {
> - // 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".
> @@ -2171,7 +2146,7 @@ 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
> --
> 2.54.0
>
>
