[llvm] [Object,ELF] Implement PN_XNUM extension for program headers (PR #162288)

[Fangrui Song via llvm-commits]

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@@ -889,7 +887,31 @@ Expected<uint64_t> ELFFile<ELFT>::getDynSymtabSize() const {
   return 0;
 }
 
-template <class ELFT> ELFFile<ELFT>::ELFFile(StringRef Object) : Buf(Object) {}
+template <class ELFT> ELFFile<ELFT>::ELFFile(StringRef Object) : Buf(Object) {
+  const Elf_Ehdr &Header = getHeader();
+  RealPhNum = Header.e_phnum;
+  RealShNum = Header.e_shnum;
+  RealShStrNdx = Header.e_shstrndx;
+  if (!Header.hasPhdrNumExtension())
+    return;
+
+  // An ELF binary may report `hasExtendedHeader` as true but not actually
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MaskRay wrote:

Even if we don't want to introduce a test with 0xffff program headers, we can create a test, adapted from `test/llvm-objcopy/ELF/invalid-e_phoff.test`, that shows the llvm-readelf behavior when e_phnum is 0xffff, but the section header table doesn't contain that many of sections. The diagnostic is probably different.

https://github.com/llvm/llvm-project/pull/162288