libstdc++: Use realpath for /etc/localtime symlink [PR125467]

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.

Reviewed-by: Tomasz Kamiński <tkaminsk@redhat.com>

diff --git a/libstdc++-v3/src/c++20/tzdb.cc b/libstdc++-v3/src/c++20/tzdb.cc
index 9e601fc176f3341eee887639dc5e6d6af724dfe1..83aca103a5f65b2adc2c5a30b02836770dfe43cd 100644 (file)
--- 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")).
+
+    string_view str;
+#if defined _GLIBCXX_USE_REALPATH && _XOPEN_VERSION >= 700
+    unique_ptr<char[], void(*)(void*)> cbuf{ nullptr, &::free };
+    // 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();
+    str = sbuf;
+#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)