[ELF] - Check that section alignment is a power of 2.

[grimar]

Summary

I found that this check still may be useful in some cases.
At fact since we use uint32_t alignment, then maximum value
that is valid for us is 0x80000000. But some broken files,
for example file from testcase may have greater value.
Because of that offset calculation overflow and crash happens.

[ruiu]

ELF/InputSection.cpp

59

Technically, an alignment greater than 2^32 is not invalid, so this error message is wrong. Please separate the two conditions and print out different messages

sh_addralign too large
sh_addralign is not a power of 2

[ruiu]

ELF/InputSection.cpp

57

Also, please leave a comment here to say that we reject object files having insanely large alignment requirements and may want to relax this limitation in the future.

binutils-ish tools have incredible long lifetime -- GNU ld has been used for decades now for example. We want to leave a hint why we are doing this, so that people who look at this code 10 years later won't have to wonder why we reject 4GB-aligned sections when they are creating 10 terabyte executable.

[grimar]

• Addressed review comments.

ELF/InputSection.cpp

57

Done.

59

Done.

[• rafael]

LGTM

[grimar]

r283544

ELF/InputSection.cpp

 template <class ELFT>
 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)
+  // no alignment constraits. Also we reject object files having insanely large
+  // alignment requirements and may want to relax this limitation in the future.
+  uintX_t V = std::max<uintX_t>(Header->sh_addralign, 1);
+  if (!isPowerOf2_64(V))
+    fatal(getFilename(File) + ": section sh_addralign is not a power of 2");
+  if (V > UINT32_MAX)
     fatal(getFilename(File) + ": section sh_addralign is too large");
-  Alignment = std::max<uintX_t>(Header->sh_addralign, 1);
+  Alignment = V;
 }
 
 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.test

 # RUN: yaml2obj %s -o %t
 # RUN: not ld.lld %t -o %tout 2>&1 | FileCheck %s
 
 ## In current lld implementation, we do not accept sh_addralign
 ## larger than UINT32_MAX.
 !ELF
 FileHeader:
   Class:           ELFCLASS64
   Data:            ELFDATA2LSB
   Type:            ET_REL
   Machine:         EM_X86_64
 Sections:
   - Name:            .text
     Type:            SHT_PROGBITS
     Flags:           [ SHF_ALLOC, SHF_EXECINSTR ]
-    AddressAlign:    0x1000000000000001
+    AddressAlign:    0x1000000000000000
     Content:         "00000000"
 
 # CHECK: section sh_addralign is too large

test/ELF/invalid/section-alignment2.s

+## section-alignment-notpow2.elf has section alignment 
+## 0xFFFFFFFF which is not a power of 2.
+# RUN: not ld.lld %p/Inputs/section-alignment-notpow2.elf -o %t2 2>&1 | \
+# RUN:   FileCheck %s
+# CHECK: section sh_addralign is not a power of 2