[llvm-objcopy/strip] Fix off-by-one error in SYMTAB_SHNDX need check

The check for whether an extended symbol index table was required
dropped the first SHN_LORESERVE sections from the sections array before
checking whether the remaining sections had symbols. Unfortunately, the
null section header is not present in this list, so the check was
skipping the first section that might be important. If that section
contained a symbol, and no subsequent ones did, the .symtab_shndx
section would not be emitted, leading to a corrupt object.

Also consolidate and expand test coverage in the area to cover this bug
and other aspects of the SYMTAB_SHNDX section.

Reviewed by: alexshap, MaskRay

Differential Revision: https://reviews.llvm.org/D97661

1 files changed, 3 insertions, 1 deletions

diff --git a/llvm/tools/llvm-objcopy/ELF/Object.cpp b/llvm/tools/llvm-objcopy/ELF/Object.cpp
index 7faf415..fe68c51 100644
--- a/llvm/tools/llvm-objcopy/ELF/Object.cpp
+++ b/llvm/tools/llvm-objcopy/ELF/Object.cpp
@@ -2450,8 +2450,10 @@ template <class ELFT> Error ELFWriter<ELFT>::finalize() {
   bool NeedsLargeIndexes = false;
   if (Obj.sections().size() >= SHN_LORESERVE) {
     SectionTableRef Sections = Obj.sections();
+    // Sections doesn't include the null section header, so account for this
+    // when skipping the first N sections.
     NeedsLargeIndexes =
-        any_of(drop_begin(Sections, SHN_LORESERVE),
+        any_of(drop_begin(Sections, SHN_LORESERVE - 1),
                [](const SectionBase &Sec) { return Sec.HasSymbol; });
     // TODO: handle case where only one section needs the large index table but
     // only needs it because the large index table hasn't been removed yet.