[PATCH] libstdc++: Use realpath for /etc/localtime symlink [PR125467]
Jonathan Wakely
jwakely.gcc@gmail.com
Wed Jul 1 08:04:25 GMT 2026
On Wed, 1 Jul 2026, 08:51 Tomasz Kaminski, <tkaminsk@redhat.com> wrote:
>
>
> On Wed, Jul 1, 2026 at 9:30 AM Jonathan Wakely <jwakely.gcc@gmail.com>
> wrote:
>
>>
>>
>> On Wed, 1 Jul 2026, 07:45 Tomasz Kaminski, <tkaminsk@redhat.com> wrote:
>>
>>>
>>>
>>> On Tue, Jun 30, 2026 at 7:48 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 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.
>>
> OK.
> You would need to observe that from the destructor of the object created
> before this #ifdef,
> but it is some argument.
>
Yeah, in this case it makes no difference because no dangling uses are
possible. It's just a general habit that I prefer to create the view so
that its lifetime is shorter than the underlying storage.
I can change it here.
>>
>
>>
>>>
>>>> + // 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
>>>>
>>>>
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