Index: lldb/packages/Python/lldbsuite/test/functionalities/data-formatter/data-formatter-stl/libcxx/string/main.cpp
===================================================================
--- lldb/packages/Python/lldbsuite/test/functionalities/data-formatter/data-formatter-stl/libcxx/string/main.cpp
+++ lldb/packages/Python/lldbsuite/test/functionalities/data-formatter/data-formatter-stl/libcxx/string/main.cpp
@@ -1,5 +1,12 @@
 #include <string>
 
+// A corrupt libcxx string which hits the SSO code path, but has an invalid
+// size.
+static unsigned char garbage_string_sso[] = {232, 32, 175, 29, 1, 0, 0, 0, 32, 30, 175, 29, 1, 0, 0, 0, 184, 30, 175, 29, 1, 0, 0, 0};
+
+// A corrupt libcxx string which points to garbage and has a crazy length.
+static unsigned char garbage_string_long[] = {185, 52, 168, 29, 1, 0, 0, 0, 168, 61, 175, 29, 1, 0, 0, 0, 104, 222, 174, 29, 1, 0, 0, 0};
+
 int main()
 {
     std::wstring wempty(L"");
@@ -17,6 +24,11 @@
     std::u32string u32_string(U"🍄🍅🍆🍌");
     std::u32string u32_empty(U"");
     std::basic_string<unsigned char> uchar(5, 'a');
+    std::string garbage1, garbage2;
+    if (sizeof(std::string) == sizeof(garbage_string_sso))
+      memcpy((void *)&garbage1, &garbage_string_sso, sizeof(std::string));
+    if (sizeof(std::string) == sizeof(garbage_string_long))
+      memcpy((void *)&garbage2, &garbage_string_long, sizeof(std::string));
     S.assign(L"!!!!!"); // Set break point at this line.
     return 0;
 }
Index: lldb/source/DataFormatters/StringPrinter.cpp
===================================================================
--- lldb/source/DataFormatters/StringPrinter.cpp
+++ lldb/source/DataFormatters/StringPrinter.cpp
@@ -59,11 +59,19 @@
   return true;
 }
 
+/// Return true if \p Needle lies within the range [\p Start, \p End).
+static bool isInHalfOpenRange(uint8_t *Needle, uint8_t *Start, uint8_t *End) {
+  return uintptr_t(Needle) >= uintptr_t(Start) &&
+         uintptr_t(Needle) < uintptr_t(End);
+}
+
 template <>
 StringPrinter::StringPrinterBufferPointer<>
 GetPrintableImpl<StringPrinter::StringElementType::ASCII>(uint8_t *buffer,
                                                           uint8_t *buffer_end,
                                                           uint8_t *&next) {
+  assert(isInHalfOpenRange(buffer, buffer, buffer_end) &&
+         "Cannot read the first byte of ASCII string buffer");
   StringPrinter::StringPrinterBufferPointer<> retval = {nullptr};
 
   switch (*buffer) {
@@ -129,16 +137,17 @@
 GetPrintableImpl<StringPrinter::StringElementType::UTF8>(uint8_t *buffer,
                                                          uint8_t *buffer_end,
                                                          uint8_t *&next) {
+  assert(isInHalfOpenRange(buffer, buffer, buffer_end) &&
+         "Cannot read the first byte of UTF8 string buffer");
   StringPrinter::StringPrinterBufferPointer<> retval{nullptr};
 
   unsigned utf8_encoded_len = llvm::getNumBytesForUTF8(*buffer);
 
-  if (1u + std::distance(buffer, buffer_end) < utf8_encoded_len) {
-    // I don't have enough bytes - print whatever I have left
-    retval = {buffer, static_cast<size_t>(1 + buffer_end - buffer)};
-    next = buffer_end + 1;
+  // If the utf8 encoded length is invalid, or if there aren't enough bytes to
+  // print, this is some kind of corrupted string.
+  if ((utf8_encoded_len == 0 || utf8_encoded_len > 4) ||
+      !isInHalfOpenRange(buffer + (utf8_encoded_len - 1), buffer, buffer_end))
     return retval;
-  }
 
   char32_t codepoint = 0;
   switch (utf8_encoded_len) {
@@ -160,12 +169,6 @@
         (unsigned char)*buffer, (unsigned char)*(buffer + 1),
         (unsigned char)*(buffer + 2), (unsigned char)*(buffer + 3));
     break;
-  default:
-    // this is probably some bogus non-character thing just print it as-is and
-    // hope to sync up again soon
-    retval = {buffer, 1};
-    next = buffer + 1;
-    return retval;
   }
 
   if (codepoint) {
@@ -215,9 +218,7 @@
     return retval;
   }
 
-  // this should not happen - but just in case.. try to resync at some point
-  retval = {buffer, 1};
-  next = buffer + 1;
+  // We couldn't figure out how to print this string.
   return retval;
 }
 
@@ -354,13 +355,11 @@
             escaping_callback(utf8_data_ptr, utf8_data_end_ptr, next_data);
         auto printable_bytes = printable.GetBytes();
         auto printable_size = printable.GetSize();
-        if (!printable_bytes || !next_data) {
-          // GetPrintable() failed on us - print one byte in a desperate resync
-          // attempt
-          printable_bytes = utf8_data_ptr;
-          printable_size = 1;
-          next_data = utf8_data_ptr + 1;
-        }
+
+        // We failed to figure out how to print this string.
+        if (!printable_bytes || !next_data)
+          return false;
+
         for (unsigned c = 0; c < printable_size; c++)
           stream.Printf("%c", *(printable_bytes + c));
         utf8_data_ptr = (uint8_t *)next_data;
Index: lldb/source/Plugins/Language/CPlusPlus/LibCxx.cpp
===================================================================
--- lldb/source/Plugins/Language/CPlusPlus/LibCxx.cpp
+++ lldb/source/Plugins/Language/CPlusPlus/LibCxx.cpp
@@ -527,6 +527,16 @@
     size = (layout == eLibcxxStringLayoutModeDSC)
                ? size_mode_value
                : ((size_mode_value >> 1) % 256);
+
+    // When the small-string optimization takes place, the data must fit in the
+    // inline string buffer (23 bytes on x86_64/Darwin). If it doesn't, it's
+    // likely that the string isn't initialized and we're reading garbage.
+    ExecutionContext exe_ctx(location_sp->GetExecutionContextRef());
+    auto max_bytes = location_sp->GetCompilerType().GetByteSize(
+        exe_ctx.GetBestExecutionContextScope());
+    if (size > max_bytes)
+      return false;
+
     return (location_sp.get() != nullptr);
   } else {
     ValueObjectSP l(D->GetChildAtIndex(0, true));
@@ -657,9 +667,7 @@
   options.SetQuote('"');
   options.SetSourceSize(size);
   options.SetBinaryZeroIsTerminator(false);
-  StringPrinter::ReadBufferAndDumpToStream<element_type>(options);
-
-  return true;
+  return StringPrinter::ReadBufferAndDumpToStream<element_type>(options);
 }
 
 bool lldb_private::formatters::LibcxxStringSummaryProviderASCII(