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?
- 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.
---
Tested x86_64-linux.
Should we handle the dangling symlink case, maybe by using
filesystem::weakly_canonical?
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;
+ // 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();
+#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
