[ELF] - Do not crash on invalid section alignment.

[grimar]

Summary

Case was revealed by id_000010,sig_08,src_000000,op_havoc,rep_4 from PR30540.

Out implementation uses uint32 for storing section alignment value,
what seems reasonable, though if value exceeds 32 bits bounds we have
truncation and final value of 0.

afl-min was applied to input.

[evgeny777]

ELF/InputSection.cpp

48

This doesn't make any sense for ELF32, does it? What about using MaxPageSize?

[grimar]

ELF/InputSection.cpp

48

For ELF32 it does not, I see no problem here.
About MaxPageSize - it seems there is no dependency in GNU linkers between section alignment and MaxPageSize,
why do you think this would be correct ?

[evgeny777]

ELF/InputSection.cpp

48

Just checked: both gas and llvm-mc require alignment to be 32-bit value which is a power of 2. So better check for alignment can be:

bool NotPower2 = Header->sh_addralign & (Header->sh_addralign - 1);
if (Header->sh_addralign > 0x80000000 || NotPower2)
  error("...");

[grimar]

ELF/InputSection.cpp

48

It is not just gas/llvm-mc, it is requirement for this field: "Only 0 and positive integral powers of 2 are currently allowed as values for this field. A value of 0 or 1 indicates no address alignment constraints.",

though I do not know do we want to check such details in linker or not. As far I understand main aim to fix wrong inputs is "just please do not crash" and not to verify if the ELF object is correct. As you mentioned, llvm-mc already do this check for us.

[• rafael]

This is fine since it fixes a crash and we can always improve on it afterwards.

Can you write the test with yaml2obj or it too requires the value to be 32 bits?

[grimar]

In D25082#557514, @rafael wrote:

This is fine since it fixes a crash and we can always improve on it afterwards.

Can you write the test with yaml2obj or it too requires the value to be 32 bits?

It is possible to write test using yapl2obj for this, will commit in a while.

[grimar]

r283097

ELF/InputSection.cpp

 InputSectionBase<ELFT>::InputSectionBase(elf::ObjectFile<ELFT> *File,
                                          const Elf_Shdr *Hdr, StringRef Name,
                                          Kind SectionKind)
     : InputSectionData(SectionKind, Name, getSectionContents(File, Hdr),
                        Hdr->sh_flags & SHF_COMPRESSED, !Config->GcSections),
       Header(Hdr), File(File), Repl(this) {
   // The ELF spec states that a value of 0 means the section has
   // no alignment constraits.
+  if (Header->sh_addralign > UINT32_MAX)
+    fatal(getFilename(File) + ": section sh_addralign is too large");
   Alignment = std::max<uintX_t>(Header->sh_addralign, 1);
 }
 
 template <class ELFT> size_t InputSectionBase<ELFT>::getSize() const {
   if (auto *D = dyn_cast<InputSection<ELFT>>(this))
     if (D->getThunksSize() > 0)
       return D->getThunkOff() + D->getThunksSize();
   return Header->sh_size;

test/ELF/invalid/section-alignment.s

+# REQUIRES: x86
+
+## In current lld implementation, we do not accept sh_addralign
+## larger than UINT32_MAX.
+# RUN: not ld.lld %S/Inputs/section-alignment.elf \
+# RUN:   -o %t 2>&1 | FileCheck %s
+# CHECK: section sh_addralign is too large