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

[jhenderson]

Summary

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.

Depends on D97660.

[MaskRay]

Thanks!

llvm/test/tools/llvm-objcopy/ELF/auto-remove-add-symtab-shndx.test

+# This test makes sure that the .symtab_shndx section is automatically removed
+# when it is not needed and added when it is.
+
+RUN: %python %p/../Inputs/ungzip.py %p/Inputs/many-sections.o.gz > %t.0
+RUN: echo 'foo' > %t
+# many-sections.o contains sections from s1 to s65535, each with a symbol.
+# Remove enough so that the last section with a symbol has index
+# SHN_LORESERVE (i.e. s65280), and show the section hasn't been removed.
+RUN: llvm-objcopy -R 's65[3-9][0-9][0-9]' -R 's6529[0-9]' -R 's6528[1-9]' %t.0 %t2
+RUN: llvm-readobj --sections %t2 | FileCheck --check-prefix=SHNDX %s
+# Remove one more to cause removal of the section.
+RUN: llvm-objcopy -R s65280 %t2 %t3
+RUN: llvm-readobj --sections %t3 | FileCheck --check-prefix=NOSHNDX %s
+
+# Add additional sections. No .symtab_shndx should be added, as they contain no
+# symbols.
+RUN: llvm-objcopy --add-section=newsec1=%t --add-section=newsec2=%t --add-section=newsec3=%t %t3 %t4
+RUN: llvm-readobj --sections %t4 | FileCheck --check-prefix=NOSHNDX %s
+
+# Add a symbol to one of these sections and show that the .symtab_shndx
+# section has been created and that the new symbol has the right section index.
+RUN: llvm-objcopy --add-symbol=newsym=newsec1:0 %t4 %t5
+RUN: llvm-readobj --symbols --sections %t5 | \
+RUN:   FileCheck --check-prefixes=SHNDX,SHNDX-SYM -DSECTION=newsec1 %s
+
+# Show that removing and adding symbols, but not their sections, has the same
+# effect regarding removal/addition of the .symtab_shndx section.
+# FIXME: llvm-objcopy does not currently remove this section in these
+# circumstances. See https://bugs.llvm.org/show_bug.cgi?id=49364
+# Test is left here to document the current behaviour.
+RUN: llvm-objcopy -N 'sym65[3-9][0-9][0-9]' -N 'sym652[89][0-9]' -w %t.0 %t6
+RUN: llvm-readobj --sections %t6 | not FileCheck --check-prefix=NOSHNDX %s
+# FIXME: Remove these test lines once the bug has been fixed. They simply show
+# that the section is removed by a run without any further operations.
+RUN: llvm-objcopy %t6 %t6b
+RUN: llvm-readobj --sections %t6b | FileCheck --check-prefix=NOSHNDX %s
+
+# Add a symbol to ensure the section is reinstated.
+RUN: llvm-objcopy --add-symbol=newsym=s65280:0 -w %t6 %t7
+RUN: llvm-readobj --sections --symbols %t7 | \
+RUN:   FileCheck --check-prefixes=SHNDX,SHNDX-SYM -DSECTION=s65280 %s
+
+NOSHNDX-NOT:    Name: .symtab_shndx
+SHNDX:          Name: .symtab_shndx
+SHNDX-SYM:      Name: newsym
+SHNDX-SYM:      Section:
+SHNDX-SYM-SAME:           [[SECTION]]

llvm/test/tools/llvm-objcopy/ELF/auto-remove-shndx.test

-# RUN: %python %p/../Inputs/ungzip.py %p/Inputs/many-sections.o.gz > %t
-# RUN: llvm-objcopy -R 's65[3-9][0-9][0-9]' -R 's6529[0-9]' -R 's6528[1-9]' %t %t2
-# RUN: llvm-readobj --sections %t2 | FileCheck --check-prefix=SECS %s
-
-# SECS-NOT: Name: .symtab_shndx

llvm/test/tools/llvm-objcopy/ELF/strict-no-add.test

-# This test makes sure that sections added at the end that don't have symbols
-# defined in them don't trigger the creation of a large index table.
-
-RUN: %python %p/../Inputs/ungzip.py %p/Inputs/many-sections.o.gz > %t.0
-RUN: echo 'foo' > %t
-RUN: llvm-objcopy -R 's65[3-9][0-9][0-9]' -R 's6529[0-9]' -R 's652[89][0-9]' %t.0 %t2
-RUN: llvm-objcopy --add-section=.s0=%t --add-section=.s1=%t --add-section=.s2=%t %t2 %t2
-RUN: llvm-readobj --sections %t2 | FileCheck --check-prefix=SECS %s
-
-SECS-NOT: Name: .symtab_shndx

llvm/tools/llvm-objcopy/ELF/Object.cpp

template <class ELFT> Error ELFWriter<ELFT>::finalize() {
   Obj.sortSections();
 
   // We need to assign indexes before we perform layout because we need to know
   // if we need large indexes or not. We can assign indexes first and check as
   // we go to see if we will actully need large indexes.
   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.
   }
 
   if (NeedsLargeIndexes) {
     // This means we definitely need to have a section index table but if we
     // already have one then we should use it instead of making a new one.