[RISCV] check pointer before dereference

[BeMg]

Summary

Encountered ASAN crash and found it dereference without check pointer.

[• pcwang-thead]

llvm/lib/Support/RISCVISAInfo.cpp

705

What about:

auto GoToNextExt = [&](StringRef::iterator &I, unsigned ConsumeLength) {
  I += 1 + ConsumeLength;
  if (I != Exts.end() && *I == '_')
    ++I;
};

So that there is no need to make changes below.

[kito-cheng]

LGTM, I guess I am old school guys, I would like to explicitly pass values instead of capture ref. from outside.

llvm/lib/Support/RISCVISAInfo.cpp

RISCVISAInfo::parseArchString(StringRef Arch, bool EnableExperimentalExtension,
 
   // Consume the base ISA version number and any '_' between rvxxx and the
   // first extension
   Exts = Exts.drop_front(ConsumeLength);
   Exts.consume_front("_");
 
   auto StdExtsItr = StdExts.begin();
   auto StdExtsEnd = StdExts.end();
-  auto GoToNextExt = [](StringRef::iterator &I, unsigned ConsumeLength) {
+  auto GoToNextExt = [](StringRef::iterator &I, unsigned ConsumeLength,
+                        StringRef::iterator E) {
     I += 1 + ConsumeLength;
-    if (*I == '_')
+    if (I != E && *I == '_')
       ++I;
   };
   for (auto I = Exts.begin(), E = Exts.end(); I != E;) {
     char C = *I;
 
     // Check ISA extensions are specified in the canonical order.
     while (StdExtsItr != StdExtsEnd && *StdExtsItr != C)
       ++StdExtsItr;
for (auto I = Exts.begin(), E = Exts.end(); I != E;) {
     unsigned Major, Minor, ConsumeLength;
     if (std::next(I) != E)
       Next = StringRef(std::next(I), E - std::next(I));
     if (auto E = getExtensionVersion(StringRef(&C, 1), Next, Major, Minor,
                                      ConsumeLength, EnableExperimentalExtension,
                                      ExperimentalExtensionVersionCheck)) {
       if (IgnoreUnknown) {
         consumeError(std::move(E));
-        GoToNextExt(I, ConsumeLength);
+        GoToNextExt(I, ConsumeLength, Exts.end());
         continue;
       }
       return std::move(E);
     }
 
     // The order is OK, then push it into features.
     // Currently LLVM supports only "mafdcvh".
     if (!isSupportedExtension(StringRef(&C, 1))) {
       if (IgnoreUnknown) {
-        GoToNextExt(I, ConsumeLength);
+        GoToNextExt(I, ConsumeLength, Exts.end());
         continue;
       }
       return createStringError(errc::invalid_argument,
                                "unsupported standard user-level extension '%c'",
                                C);
     }
     ISAInfo->addExtension(StringRef(&C, 1), Major, Minor);
 
     // Consume full extension name and version, including any optional '_'
     // between this extension and the next
-    GoToNextExt(I, ConsumeLength);
+    GoToNextExt(I, ConsumeLength, Exts.end());
   }
 
   // Handle other types of extensions other than the standard
   // general purpose and standard user-level extensions.
   // Parse the ISA string containing non-standard user-level
   // extensions, standard supervisor-level extensions and
   // non-standard supervisor-level extensions.
   // These extensions start with 'z', 's', 'x' prefixes, follow a