[MachO] Report error when there are too many sections (#167418)

When there are more than 255 sections, MachO object writer allows
creation of object files which are potentially malformed. Currently,
there are assertions in object writer code that prevents this behavior. 
But for distributions where assertions are turned off this still results
in
creation of malformed object files. Turning assertions into explicit
errors.

Author: Prabhuk

llvm/lib/MC/MachObjectWriter.cpp

@@ -28,6 +28,7 @@
 #include "llvm/Support/Debug.h"
 #include "llvm/Support/ErrorHandling.h"
 #include "llvm/Support/MathExtras.h"
+#include "llvm/Support/SMLoc.h"
 #include "llvm/Support/raw_ostream.h"
 #include <algorithm>
 #include <cassert>
@@ -584,7 +585,12 @@ void MachObjectWriter::computeSymbolTable(
   unsigned Index = 1;
   for (MCSection &Sec : Asm)
     SectionIndexMap[&Sec] = Index++;
-  assert(Index <= 256 && "Too many sections!");
+
+  // Section indices begin from 1 in MachO. Only sections 1-255 can be indexed
+  // into section symbols. Referencing a section with index larger than 255 will
+  // not set n_sect for these symbols.
+  if (Index > 255)
+    getContext().reportError(SMLoc(), "Too many sections!");
 
   // Build the string table.
   for (const MCSymbol &Symbol : Asm.symbols()) {
@@ -621,7 +627,8 @@ void MachObjectWriter::computeSymbolTable(
       ExternalSymbolData.push_back(MSD);
     } else {
       MSD.SectionIndex = SectionIndexMap.lookup(&Symbol.getSection());
-      assert(MSD.SectionIndex && "Invalid section index!");
+      if (!MSD.SectionIndex)
+        getContext().reportError(SMLoc(), "Invalid section index!");
       ExternalSymbolData.push_back(MSD);
     }
   }
@@ -645,7 +652,8 @@ void MachObjectWriter::computeSymbolTable(
       LocalSymbolData.push_back(MSD);
     } else {
       MSD.SectionIndex = SectionIndexMap.lookup(&Symbol.getSection());
-      assert(MSD.SectionIndex && "Invalid section index!");
+      if (!MSD.SectionIndex)
+        getContext().reportError(SMLoc(), "Invalid section index!");
       LocalSymbolData.push_back(MSD);
     }
   }