[AArch64] Avoid crashing on invalid -Wa,-march= values

[dim]

Summary

As reported in https://bugs.freebsd.org/260078, the gnutls Makefiles
pass -Wa,-march=all to compile a number of assembly files. Clang does
not support this -march value, but because of a mistake in handling
the arguments, an unitialized Arg pointer is dereferenced, which can
cause a segfault.

Work around this by adding a check if the local WaMArch variable is
initialized, and if so, using its value in the diagnostic message.

[tschuett]

clang/lib/Driver/ToolChains/Arch/AArch64.cpp

262–270

Would an if statement be more readable and actually show what the issue is?

Bonus points for a comment.

[tschuett]

There is precedence for checking WaMArch.size() in the file. I would expected !WaMArch.empty().

[dim]

Address review comments.

[tschuett]

LGTM

clang/lib/Driver/ToolChains/Arch/AArch64.cpp

void aarch64::getAArch64TargetFeatures(const Driver &D,
                                        const llvm::Triple &Triple,
                                        const ArgList &Args,
                                        std::vector<StringRef> &Features,
                                        bool ForAS) {
   Arg *A;
   bool success = true;
   // Enable NEON by default.
   Features.push_back("+neon");
-  llvm::StringRef WaMArch = "";
+  llvm::StringRef WaMArch;
   if (ForAS)
     for (const auto *A :
          Args.filtered(options::OPT_Wa_COMMA, options::OPT_Xassembler))
       for (StringRef Value : A->getValues())
         if (Value.startswith("-march="))
           WaMArch = Value.substr(7);
   // Call getAArch64ArchFeaturesFromMarch only if "-Wa,-march=" or
   // "-Xassembler -march" is detected. Otherwise it may return false
   // and causes Clang to error out.
-  if (WaMArch.size())
+  if (!WaMArch.empty())
     success = getAArch64ArchFeaturesFromMarch(D, WaMArch, Args, Features);
   else if ((A = Args.getLastArg(options::OPT_march_EQ)))
     success = getAArch64ArchFeaturesFromMarch(D, A->getValue(), Args, Features);
   else if ((A = Args.getLastArg(options::OPT_mcpu_EQ)))
     success = getAArch64ArchFeaturesFromMcpu(D, A->getValue(), Args, Features);
   else if (Args.hasArg(options::OPT_arch) || isCPUDeterminedByTriple(Triple))
     success = getAArch64ArchFeaturesFromMcpu(
         D, getAArch64TargetCPU(Args, Triple, A), Args, Features);
   else
     // Default to 'A' profile if the architecture is not specified.
     success = getAArch64ArchFeaturesFromMarch(D, "armv8-a", Args, Features);
 
   if (success && (A = Args.getLastArg(clang::driver::options::OPT_mtune_EQ)))
     success =
         getAArch64MicroArchFeaturesFromMtune(D, A->getValue(), Args, Features);
   else if (success && (A = Args.getLastArg(options::OPT_mcpu_EQ)))
     success =
         getAArch64MicroArchFeaturesFromMcpu(D, A->getValue(), Args, Features);
   else if (success &&
            (Args.hasArg(options::OPT_arch) || isCPUDeterminedByTriple(Triple)))
     success = getAArch64MicroArchFeaturesFromMcpu(
         D, getAArch64TargetCPU(Args, Triple, A), Args, Features);
 
-  if (!success)
-    D.Diag(diag::err_drv_clang_unsupported) << A->getAsString(Args);
+  if (!success) {
+    auto Diag = D.Diag(diag::err_drv_clang_unsupported);
+    // If "-Wa,-march=" is used, 'WaMArch' will contain the argument's value,
+    // while 'A' is uninitialized. Only dereference 'A' in the other case.
+    if (!WaMArch.empty())
+      Diag << "-march=" + WaMArch.str();
+    else
+      Diag << A->getAsString(Args);
+  }
 
   if (Args.getLastArg(options::OPT_mgeneral_regs_only)) {
     Features.push_back("-fp-armv8");
     Features.push_back("-crypto");
     Features.push_back("-neon");
   }
 
   if (Arg *A = Args.getLastArg(options::OPT_mtp_mode_EQ)) {

clang/test/Driver/aarch64-target-as-march.s

 // RUN: FileCheck --check-prefix=TARGET-FEATURE-3 %s
 // RUN: %clang --target=aarch64-linux-gnueabi -### -c -Wa,-march=armv8.3-a -march=armv8.4-a \
 // RUN: %S/Inputs/wildcard1.c 2>&1 | FileCheck --check-prefix=TARGET-FEATURE-4 %s
 
 // TARGET-FEATURE-3: "-target-feature" "+v8.3a"
 // TARGET-FEATURE-3-NOT: "-target-feature" "+v8.4a"
 // TARGET-FEATURE-4: "-target-feature" "+v8.4a"
 // TARGET-FEATURE-4-NOT: "-target-feature" "+v8.3a"
+
+// Invalid -march settings
+// RUN: %clang --target=aarch64-linux-gnueabi -### -c -Wa,-march=all %s 2>&1 | \
+// RUN: FileCheck --check-prefix=INVALID-ARCH-1 %s
+// RUN: %clang --target=aarch64-linux-gnueabi -### -c -Wa,-march=foobar %s 2>&1 | \
+// RUN: FileCheck --check-prefix=INVALID-ARCH-2 %s
+
+// INVALID-ARCH-1: error: the clang compiler does not support '-march=all'
+// INVALID-ARCH-2: error: the clang compiler does not support '-march=foobar'