[LLD] Fix padding of .eh_frame when in executable segment

[andrewng]

Summary

The default padding for an executable segment is the target trap
instruction which for x86_64 is 0xCC. However, the .eh_frame section
requires the padding to be zero. The code that writes the .eh_frame
section assumes that its segment is zero initialized and does not
explicitly write the zero padding. This does not work when the .eh_frame
section is in the executable segment (for example when using
-no-rosegment).

This patch changes the .eh_frame writing code to explicitly write the
zero padding.

[andrewng]

A similar fix was required for the string table, see https://reviews.llvm.org/D36267.

[ruiu]

ELF/SyntheticSections.cpp

525 ↗

(On Diff #113815)

I think you can simplify the code by adding this piece of code here.

constexpr size_t Wordsize = sizeof(typename ELFT::uint);

// Zero-clear trailing padding if exists.
memset(Buf + D.size(), 0, Wordsize - D.size() % Wordsize);

[andrewng]

Updated based on suggestion. I still think that keeping the common "AlignSize" is better and safer.

[ruiu]

LGTM

ELF/SyntheticSections.cpp

526 ↗

(On Diff #113975)

I'd name this Aligned as AlignSize sounds like a verb (and AlignedSize is too long).

lld/trunk/ELF/SyntheticSections.cpp

void EhFrameSection<ELFT>::addSection(InputSectionBase *C) {
   }
   addSectionAux(Sec, makeArrayRef<Elf_Rela>(nullptr, nullptr));
 }
 
 template <class ELFT>
 static void writeCieFde(uint8_t *Buf, ArrayRef<uint8_t> D) {
   memcpy(Buf, D.data(), D.size());
 
+  size_t Aligned = alignTo(D.size(), sizeof(typename ELFT::uint));
+
+  // Zero-clear trailing padding if it exists.
+  memset(Buf + D.size(), 0, Aligned - D.size());
+
   // Fix the size field. -4 since size does not include the size field itself.
   const endianness E = ELFT::TargetEndianness;
-  write32<E>(Buf, alignTo(D.size(), sizeof(typename ELFT::uint)) - 4);
+  write32<E>(Buf, Aligned - 4);
 }
 
 template <class ELFT> void EhFrameSection<ELFT>::finalizeContents() {
   if (this->Size)
     return; // Already finalized.
 
   size_t Off = 0;
   for (CieRecord *Cie : Cies) {

lld/trunk/test/ELF/eh-frame-padding-no-rosegment.s

+// REQUIRES: x86
+
+.cfi_startproc
+.cfi_personality 0x1b, bar
+.cfi_endproc
+
+.global bar
+.hidden bar
+bar:
+
+// RUN: llvm-mc -filetype=obj -triple=x86_64-pc-linux %s -o %t.o
+
+// Check the size of the CIE (0x18 + 4) and FDE (0x10 + 4)
+// RUN: llvm-readobj -s -section-data %t.o | FileCheck --check-prefix=OBJ %s
+
+// OBJ:      Name: .eh_frame
+// OBJ-NEXT: Type:
+// OBJ-NEXT: Flags [
+// OBJ-NEXT:   SHF_ALLOC
+// OBJ-NEXT: ]
+// OBJ-NEXT: Address:
+// OBJ-NEXT: Offset:
+// OBJ-NEXT: Size:
+// OBJ-NEXT: Link:
+// OBJ-NEXT: Info:
+// OBJ-NEXT: AddressAlignment:
+// OBJ-NEXT: EntrySize:
+// OBJ-NEXT: SectionData (
+// OBJ-NEXT:   0000: 18000000 00000000 017A5052 00017810
+// OBJ-NEXT:   0010: 061B0000 00001B0C 07089001 10000000
+// OBJ-NEXT:   0020: 20000000 00000000 00000000 00000000
+// OBJ-NEXT: )
+
+// RUN: ld.lld %t.o -no-rosegment -o %t -shared
+
+// Check that .eh_frame is in the same segment as .text
+// RUN: llvm-readobj -l --elf-output-style=GNU %t | FileCheck --check-prefix=PHDR %s
+
+// PHDR: Segment Sections
+// PHDR: .text
+// PHDR-SAME: .eh_frame
+
+// Check that the CIE and FDE are padded with 0x00 and not 0xCC when the
+// .eh_frame section is placed in the executable segment
+// RUN: llvm-readobj -s -section-data %t | FileCheck %s
+
+// CHECK:      Name: .eh_frame
+// CHECK-NEXT: Type:
+// CHECK-NEXT: Flags
+// CHECK-NEXT:   SHF_ALLOC
+// CHECK-NEXT: ]
+// CHECK-NEXT: Address:
+// CHECK-NEXT: Offset:
+// CHECK-NEXT: Size:
+// CHECK-NEXT: Link:
+// CHECK-NEXT: Info:
+// CHECK-NEXT: AddressAlignment:
+// CHECK-NEXT: EntrySize:
+// CHECK-NEXT: SectionData (
+// CHECK-NEXT:   0000: 1C000000 00000000 017A5052 00017810
+// CHECK-NEXT:   0010: 061BBEFF FFFF1B0C 07089001 00000000
+// CHECK-NEXT:   0020: 14000000 24000000 A8FFFFFF 00000000
+// CHECK-NEXT:   0030: 00000000 00000000
+// CHECK-NEXT: )