[committed] libstdc++: Fix out-of-bounds read in format string "{:{}." [PR110974]

[Jonathan Wakely <jwakely@redhat.com>]

Tested x86_64-linux. Pushed to trunk. Backport to gcc-13 to follow.

-- >8 --

libstdc++-v3/ChangeLog:

	PR libstdc++/110974
	* include/std/format (_Spec::_S_parse_width_or_precision): Check
	for empty range before dereferencing iterator.
	* testsuite/std/format/string.cc: Check for expected exception.
	Fix expected exception message in test_pr110862() and actually
	call it.
---
 libstdc++-v3/include/std/format             |  7 ++++---
 libstdc++-v3/testsuite/std/format/string.cc | 21 ++++++++++++++++++++-
 2 files changed, 24 insertions(+), 4 deletions(-)

diff --git a/libstdc++-v3/include/std/format b/libstdc++-v3/include/std/format
index 5d7af53fc94..2fe430f75f6 100644
--- a/libstdc++-v3/include/std/format
+++ b/libstdc++-v3/include/std/format
@@ -520,10 +520,11 @@ namespace __format
 	if (__first[0] != '.')
 	  return __first;
 
-	++__first;
+	iterator __next = ++__first;
 	bool __arg_id = false;
-	auto __next = _S_parse_width_or_precision(__first, __last, _M_prec,
-						  __arg_id, __pc);
+	if (__next != __last)
+	  __next = _S_parse_width_or_precision(__first, __last, _M_prec,
+					       __arg_id, __pc);
 	if (__next == __first)
 	  __throw_format_error("format error: missing precision after '.' in "
 			       "format string");
diff --git a/libstdc++-v3/testsuite/std/format/string.cc b/libstdc++-v3/testsuite/std/format/string.cc
index 6a45237b8c4..fef55b9bcd9 100644
--- a/libstdc++-v3/testsuite/std/format/string.cc
+++ b/libstdc++-v3/testsuite/std/format/string.cc
@@ -137,7 +137,24 @@ test_pr110862()
     VERIFY( false );
   } catch (const std::format_error& e) {
     std::string_view what = e.what();
-    VERIFY( what.find("unmatched left brace") != what.npos );
+    VERIFY( what.find("unmatched '{'") != what.npos );
+  }
+}
+
+void
+test_pr110974()
+{
+  try {
+    // PR libstdc++/110974 out of bounds read on invalid format string "{:{}."
+    std::string_view fmt{"{:{}.0", 5}; // "0" is not part of the format string.
+    (void) std::vformat(fmt, std::make_format_args(1.0, 1));
+    VERIFY( false );
+  } catch (const std::format_error& e) {
+    std::string_view what = e.what();
+    // GCC 13.2 throws "invalid width or precision in format-spec" after
+    // trying to parse the "0" past-the-end of the format string.
+    // There should be an exception before even trying that:
+    VERIFY( what.find("missing precision after '.'") != what.npos );
   }
 }
 
@@ -146,4 +163,6 @@ int main()
   test_no_args();
   test_indexing();
   test_format_spec();
+  test_pr110862();
+  test_pr110974();
 }
-- 
2.41.0