[PATCH] libstdc++: Use realpath for /etc/localtime symlink [PR125467]
Jonathan Wakely
jwakely.gcc@gmail.com
Mon Jun 29 10:31:45 GMT 2026
On Mon, 29 Jun 2026 at 10:21, Tomasz Kaminski <tkaminsk@redhat.com> wrote:
>
>
>
> On Mon, Jun 29, 2026 at 10:34 AM Jonathan Wakely <jwakely.gcc@gmail.com> wrote:
>>
>>
>>
>> On Mon, 29 Jun 2026, 07:23 Tomasz Kaminski, <tkaminsk@redhat.com> wrote:
>>>
>>>
>>>
>>> On Fri, Jun 26, 2026 at 7:02 PM Jonathan Wakely <jwakely@redhat.com> 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,
>>
>>
>> Will we? We might resolve a name, but it still needs to be looked up in tzdb to find a time_zone. If there's no tzdata that will fail (unless it's one of the zones that is always present like UTC and GMT).
>
> From my reading, we fail first to the embeded static tzdb data if opening tzdb file fails:
> tzdata_stream() : istream(nullptr)
> {
> if (string path = zoneinfo_file(tzdata_file); !path.empty())
> {
> filebuf fbuf;
> if (fbuf.open(path, std::ios::in))
> {
> std::construct_at(&fb, std::move(fbuf));
> this->init(&fb);
> return;
> }
> }
> std::construct_at(&sb);
> this->init(&sb);
> }
> We can successfully look up the zone if we know about it, even if we cannot read the zone database.
Right - but that seems orthogonal to chrono::current_zone(), and I
don't think it is an argument against using weakly_canonical to deal
with dangling /etc/localtime symlinks.
The semantics of current_zone are to decide on a name, then use
locate_zone(str) to look that name up (and maybe retry if the lookup
fails and we can try a different name). The presence or absence of a
valid tzdb object affects the locate_zone(str) step but I don't think
it should affect the process for deciding on a name. With the code on
trunk today, we can still extract a name from a dangling symlink (but
we can't extract a name from a chain of two or more symlinks). With my
proposed patch we would be able to extract a name from a chain of
symlinks, but would not be able to get anything from dangling
symlinks.
> Maybe I am missing something here.
>
>>
>>
>>> and then only
>>> If the data requires updating, the update will fail. From that perspective I much
>>> more prefer loud failure.
>>
>>
>> A dangling symlink to the Etc/GMT or UTC zone would always work OK even if there is no tzdata, and maybe that's the expected behaviour for some system.
>
> This system would be better served by having TZDB disabled, if that is the consistent behavior.
That requires a custom build of libstdc++.so whereas a minimal
container could use the system libstdc++.so and remove all the tzdata
files to save disk space.
>>
>>
>>>
>>> As the fact that this works is libstdc++ specific
>>
>>
>> Which part?
>
> Not using the binary format.
>>
>>
>>> (other libraries and clients
>>> may use the content of the file),
>>
>>
>> They should not do that on Linux. Maybe on other systems, and for ancient versions of Linux where /etc/localtime is a real file containing a copy of a Zif file. But there nothing we can do in that case, we can't resolve it to a chrino::time_zone object without comparing the file with every file under the zoneinfo directory.
>
> Why, if there use binary format, and they do not allow lookups for other zones, you may simply load the content
> of the file pointed out by current_zone. At least as far as I understand.
>
> For example in such case, the ancient version of linux could be supported, by loading the content
> of the /etc/localtime with time zone database, and storing it under distinct name in tzdb.
Yes, that's true. There's no guarantee that current_zone() returns one
of the values in get_tzdb()->zones. And even if it does, it could be a
special entry that is created by the /etc/localtime file and not one
of the files under /usr/share/zoneinfo. But it wouldn't return
anything useful for current_zone()->name() in that case, because
there's no name in the binary TZif file. You would have to gTive it a
fake name like "localtime".
But even in implementations which use the binary TZif files (libc++
and HowardHinnant/date) they don't support /etc/localtime being a TZif
file, as Mark de Wever said in
https://github.com/llvm/llvm-project/issues/105634#issuecomment-2660976683
Libc++ and date both assume a hardcoded path such as
/usr/share/zoneinfo and then simply remove that prefix from the target
of /etc/localtime to get the zone name. Libc++ uses
filesystem::read_symlink optionally followed by filesystem::canonical
(I don't know why they don't just use filesystem::canonical in the
first place) and then filesystem::relative to remove the prefix.
Howard uses realpath, like my proposed patch, and then removes
"*zoneinfo/" from the result, and so that also doesn't support
dangling symlinks.
I'll just add a comment noting that we don't support dangling
symlinks, and then as you suggested we can postpone supporting them
until somebody complains.
>
>>> 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?
>>
>>
>> It was reported directly to me by a friend, as this behaviour is preventing their codebase from replacing Howard's date library with C++20 chrono.
>>
>>
>>>
>>>>
>>>> * 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.
>>
>>
>> Will do.
>>
>>>
>>>>
>>>> + // 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())
I'm going to also check str != "/etc/localtime" here, so that if it's
not a symlink we don't bother trying to extract a name from it.
>>>> {
>>>> - // 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
>>>>
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