Add a CIE with length 0 unconditionally.

[MaskRay]

Summary

This is not technically required, but glibc unwind-dw2-fde.c classify_object_over_fdes expects there is a CIE record length 0 as a terminator.

[ruiu]

LGTM

Thanks!

ELF/SyntheticSections.cpp

502

one -> a terminator

[grimar]

I would like tounderstand why this is needed.
You wrote: "This is not technically required, but glibc unwind-dw2-fde.c classify_object_over_fdes expects there is a CIE record length 0 as a terminator.".

Can you please point me on a line of code in glibc that needs this change in the linker? What is the current behavior of LLD linked binaries?

ELF/SyntheticSections.cpp

502

I think would be nice for that comment to contain some additional information, like:

1. A version of glibc code this code is targeted to fix.
2. Current date at which we are observing the issue.
3. Reference to a corresponding bug in their bug tracker.
4. Maybe information about what GNU linkers do can be also useful to have.

[MaskRay]

https://code.woboq.org/userspace/glibc/sysdeps/generic/unwind-dw2-fde.c.html#645

for (; ! last_fde (ob, this_fde); this_fde = next_fde (this_fde))

last_fde is defined at https://code.woboq.org/userspace/glibc/sysdeps/generic/unwind-dw2-fde.h.html#162 . It does not check whether it reaches the end of .eh_frame (DWARF2_OBJECT_END_PTR_EXTENSION seems not defined). If there is no length=0 terminator, the pointer will run beyond the end of .eh_frame and run into successive sections (out-of-bound read), segfault or stop somewhere. This issue is usually latent/benign and I can only catch this in some internal statically-linked configuration. I don't have time to read the whole source code of how glibc/libgcc_s libunwind to understand why this routine is sometimes called but sometimes not.

[MaskRay]

This is not a bug in glibc if we don't take behavior difference as a bug. bfd/gold seem to add a CIE of length=0 anyway so this piece of glibc/libgcc_s libunwind code won't go wrong.

ELF/SyntheticSections.cpp

for (CieRecord *Rec : CieRecords) {
 
     for (EhSectionPiece *Fde : Rec->Fdes) {
       Fde->OutputOff = Off;
       Off += alignTo(Fde->Size, Config->Wordsize);
     }
   }
 
   // The LSB standard does not allow a .eh_frame section with zero
-  // Call Frame Information records. Therefore add a CIE record length
-  // 0 as a terminator if this .eh_frame section is empty.
-  if (Off == 0)
-    Off = 4;
+  // Call Frame Information records. glibc unwind-dw2-fde.c
+  // classify_object_over_fdes expects there is a CIE record length 0 as a
+  // terminator. Thus we add one unconditionally.
+  Off += 4;
 
   this->Size = Off;
 }
 
 // Returns data for .eh_frame_hdr. .eh_frame_hdr is a binary search table
 // to get an FDE from an address to which FDE is applied. This function
 // returns a list of such pairs.
 std::vector<EhFrameSection::FdeData> EhFrameSection::getFdeData() const {

test/ELF/eh-frame-hdr-icf-fde.s

 # CHECK-NEXT:   Index: 2
 # CHECK-NEXT:   Name: .eh_frame
 # CHECK-NEXT:   Type: SHT_PROGBITS
 # CHECK-NEXT:   Flags [
 # CHECK-NEXT:     SHF_ALLOC
 # CHECK-NEXT:   ]
 # CHECK-NEXT:   Address: 0x200178
 # CHECK-NEXT:   Offset: 0x178
-# CHECK-NEXT:   Size: 72
+# CHECK-NEXT:   Size: 76
 # CHECK-NEXT:   Link: 0
 # CHECK-NEXT:   Info: 0
 # CHECK-NEXT:   AddressAlignment: 8
 # CHECK-NEXT:   EntrySize: 0
 # CHECK-NEXT:   SectionData (
 # CHECK-NEXT:     0000: 14000000 00000000 017A5200 01781001
 # CHECK-NEXT:     0010: 1B0C0708 90010000 14000000 1C000000
 # CHECK-NEXT:     0020: 680E0000 01000000 00000000 00000000
 # CHECK-NEXT:     0030: 14000000 34000000 520E0000 01000000
-# CHECK-NEXT:     0040: 00000000 00000000
+# CHECK-NEXT:     0040: 00000000 00000000 00000000
 # CHECK-NEXT:   )
 # CHECK-NEXT: }
 
 # CHECK:     Section {
 # CHECK:       Index:
 # CHECK:       Name: .text
 # CHECK-NEXT:  Type: SHT_PROGBITS
 # CHECK-NEXT:  Flags [

test/ELF/eh-frame-hdr.s

 // HDR-NEXT:    Index:
 // HDR-NEXT:    Name: .eh_frame
 // HDR-NEXT:    Type: SHT_PROGBITS
 // HDR-NEXT:    Flags [
 // HDR-NEXT:      SHF_ALLOC
 // HDR-NEXT:    ]
 // HDR-NEXT:    Address: 0x200180
 // HDR-NEXT:    Offset: 0x180
-// HDR-NEXT:    Size: 96
+// HDR-NEXT:    Size: 100
 // HDR-NEXT:    Link: 0
 // HDR-NEXT:    Info: 0
 // HDR-NEXT:    AddressAlignment: 8
 // HDR-NEXT:    EntrySize: 0
 // HDR-NEXT:    SectionData (
 // HDR-NEXT:      0000: 14000000 00000000 017A5200 01781001
 // HDR-NEXT:      0010: 1B0C0708 90010000 14000000 1C000000
 // HDR-NEXT:      0020: 600E0000 01000000 00000000 00000000
 // HDR-NEXT:      0030: 14000000 34000000 490E0000 01000000
 // HDR-NEXT:      0040: 00000000 00000000 14000000 4C000000
 // HDR-NEXT:      0050: 320E0000 01000000 00000000 00000000
+// HDR-NEXT:      0060: 00000000
 // HDR-NEXT:    )
 //            CIE: 14000000 00000000 017A5200 01781001 1B0C0708 90010000
 //            FDE(1): 14000000 1C000000 600E0000 01000000 00000000 00000000
 //                    address of data (starts with 0x600E0000) = 0x200180 + 0x0020 = 0x2001A0
 //                    The starting address to which this FDE applies = 0xE60 + 0x2001A0 = 0x201000
 //                    The number of bytes after the start address to which this FDE applies = 0x01000000 = 1
 //            FDE(2): 14000000 34000000 490E0000 01000000 00000000 00000000
 //                    address of data (starts with 0x490E0000) = 0x200180 + 0x0038 = 0x2001B8

test/ELF/eh-frame-merge.s

 
 // CHECK:      Name: .eh_frame
 // CHECK-NEXT: Type: SHT_PROGBITS
 // CHECK-NEXT: Flags [
 // CHECK-NEXT:   SHF_ALLOC
 // CHECK-NEXT: ]
 // CHECK-NEXT: Address:
 // CHECK-NEXT: Offset:
-// CHECK-NEXT: Size: 96
+// CHECK-NEXT: Size: 100
 // CHECK-NEXT: Link: 0
 // CHECK-NEXT: Info: 0
 // CHECK-NEXT: AddressAlignment: 8
 // CHECK-NEXT: EntrySize: 0
 // CHECK-NEXT: SectionData (
 // CHECK-NEXT: 0000: 14000000 00000000 017A5200 01781001  |
 // CHECK-NEXT: 0010: 1B0C0708 90010000 14000000 1C000000  |
 // CHECK-NEXT: 0020: E80D0000 01000000 00000000 00000000  |
 // CHECK-NEXT: 0030: 14000000 34000000 D20D0000 02000000  |
 // CHECK-NEXT: 0040: 00000000 00000000 14000000 4C000000  |
 // CHECK-NEXT: 0050: B90D0000 01000000 00000000 00000000  |
+// CHECK-NEXT: 0060: 00000000
 // CHECK-NEXT: )
 
 // CHECK:      Name: foo
 // CHECK-NEXT: Type: SHT_PROGBITS
 // CHECK-NEXT: Flags [
 // CHECK-NEXT:   SHF_ALLOC
 // CHECK-NEXT:   SHF_EXECINSTR
 // CHECK-NEXT: ]

test/ELF/ehframe-relocation.s

 // REQUIRES: x86
 // RUN: llvm-mc -filetype=obj -triple=x86_64-pc-linux %s -o %t.o
 // RUN: llvm-mc -filetype=obj -triple=x86_64-pc-linux %p/Inputs/ehframe-relocation.s  -o %t2.o
 // RUN: ld.lld %t.o %t2.o -o %t
 // RUN: llvm-readobj -s %t | FileCheck %s
 // RUN: llvm-objdump -d %t | FileCheck --check-prefix=DISASM %s
 
 // CHECK:      Name: .eh_frame
 // CHECK-NEXT: Type: SHT_PROGBITS
 // CHECK-NEXT: Flags [
 // CHECK-NEXT:   SHF_ALLOC
 // CHECK-NEXT: ]
 // CHECK-NEXT: Address: 0x200120
 // CHECK-NEXT: Offset:
-// CHECK-NEXT: Size: 48
+// CHECK-NEXT: Size: 52
 // CHECK-NOT: .eh_frame
 
 // 0x200120 = 2097440
 // 0x200120 + 5 = 2097445
 // DISASM:      Disassembly of section .text:
 // DISASM-NEXT: _start:
 // DISASM-NEXT:   201000: {{.*}} movq 2097440, %rax
 // DISASM-NEXT:   201008: {{.*}} movq 2097445, %rax
 
 .section .eh_frame,"ax",@unwind
 
 .section .text
 .globl _start
 _start:
  movq .eh_frame, %rax
  movq .eh_frame + 5, %rax

test/ELF/invalid-fde-rel.s

// the fde points to.
 
         .long 0x14
         .long 0x1c
         .long 0x0
         .long 0x0
         .long 0x0
         .long 0x0
 
-// CHECK:  1 .eh_frame     00000018
+// CHECK:  1 .eh_frame     0000001c

test/ELF/linkerscript/eh-frame-hdr.s

 # REQUIRES: x86
 # RUN: llvm-mc -filetype=obj -triple=x86_64-unknown-linux %s -o %t
 # RUN: echo "SECTIONS { .eh_frame_hdr : {} .eh_frame : {} }" > %t.script
 # RUN: ld.lld -o %t1 --eh-frame-hdr --script %t.script %t
 # RUN: llvm-objdump -s -section=".eh_frame_hdr" %t1 | FileCheck %s
 
-# CHECK:      011b033b 14000000 01000000 49000000
+# CHECK:      011b033b 14000000 01000000 4d000000
 # CHECK-NEXT: 30000000
 
 .global _start
 _start:
  nop
 
 .section .dah,"ax",@progbits
 .cfi_startproc
  nop
 .cfi_endproc

test/ELF/linkerscript/map-file2.test

 # CHECK-NEXT:       1018             3008      109     1 .ddd
 # CHECK-NEXT:       1018             3008        1     1         BYTE ( 0x11 )
 # CHECK-NEXT:       1019             3009      100     1         . += 0x100
 # CHECK-NEXT:       1119             3109        8     1         {{.*}}{{/|\\}}map-file2.test.tmp.o:(.ddd)
 # CHECK-NEXT:       1124             3114        1     4 .text
 # CHECK-NEXT:       1124             3114        1     4         {{.*}}{{/|\\}}map-file2.test.tmp.o:(.text)
 # CHECK-NEXT:       1124             3114        0     1                 f(int)
 # CHECK-NEXT:       1124             3114        0     1                 _start
-# CHECK-NEXT:       1128             3118       30     8 .eh_frame
+# CHECK-NEXT:       1128             3118       34     8 .eh_frame
 # CHECK-NEXT:       1128             3118       30     1         {{.*}}{{/|\\}}map-file2.test.tmp.o:(.eh_frame+0x0)
 # CHECK-NEXT:          0                0        8     1 .comment
 # CHECK-NEXT:          0                0        8     1         <internal>:(.comment)
 # CHECK-NEXT:          0                0       48     8 .symtab
 # CHECK-NEXT:          0                0       48     8         <internal>:(.symtab)
 # CHECK-NEXT:          0                0       48     1 .shstrtab
 # CHECK-NEXT:          0                0       48     1         <internal>:(.shstrtab)
 # CHECK-NEXT:          0                0        e     1 .strtab
 # CHECK-NEXT:          0                0        e     1         <internal>:(.strtab)

test/ELF/linkerscript/synthetic-symbols1.test

SECTIONS {
   }
 
   .eh_frame : {}
 }
 
 # CHECK:      0000000000000128         .foo    00000000 .hidden _end_sec
 # CHECK-NEXT: 0000000000000120         .foo    00000000 _begin_sec
 # CHECK-NEXT: 0000000000000128         *ABS*   00000000 _end_sec_abs
-# CHECK-NEXT: 0000000000001048         .text   00000000 _start
+# CHECK-NEXT: 000000000000104c         .text   00000000 _start
 # CHECK-NEXT: 0000000000000120         .foo    00000000 begin_foo
 # CHECK-NEXT: 0000000000000128         .foo    00000000 end_foo
 # CHECK-NEXT: 0000000000000008         *ABS*   00000000 size_foo_1
 # CHECK-NEXT: 0000000000000008         *ABS*   00000000 size_foo_1_abs
 # CHECK-NEXT: 0000000000001000         .foo    00000000 begin_bar
 # CHECK-NEXT: 0000000000001004         .foo    00000000 end_bar
 # CHECK-NEXT: 0000000000000ee4         *ABS*   00000000 size_foo_2
 # CHECK-NEXT: 0000000000000ee4         *ABS*   00000000 size_foo_3
 # CHECK-NEXT: 0000000000001004         .eh_frame_hdr     00000000 __eh_frame_hdr_start
 # CHECK-NEXT: 0000000000001010         *ABS*             00000000 __eh_frame_hdr_start2
 # CHECK-NEXT: 0000000000001018         .eh_frame_hdr     00000000 __eh_frame_hdr_end
 # CHECK-NEXT: 0000000000001020         *ABS*             00000000 __eh_frame_hdr_end2

test/ELF/map-file.s

 // CHECK-NEXT: 20026c           20026c       30     4 .hash
 // CHECK-NEXT: 20026c           20026c       30     4         <internal>:(.hash)
 // CHECK-NEXT: 20029c           20029c       31     1 .dynstr
 // CHECK-NEXT: 20029c           20029c       31     1         <internal>:(.dynstr)
 // CHECK-NEXT: 2002d0           2002d0       30     8 .rela.dyn
 // CHECK-NEXT: 2002d0           2002d0       30     8         <internal>:(.rela.dyn)
 // CHECK-NEXT: 200300           200300       30     8 .rela.plt
 // CHECK-NEXT: 200300           200300       30     8         <internal>:(.rela.plt)
-// CHECK-NEXT: 200330           200330       60     8 .eh_frame
+// CHECK-NEXT: 200330           200330       64     8 .eh_frame
 // CHECK-NEXT: 200330           200330       2c     1         {{.*}}{{/|\\}}map-file.s.tmp1.o:(.eh_frame+0x0)
 // CHECK-NEXT: 200360           200360       14     1         {{.*}}{{/|\\}}map-file.s.tmp1.o:(.eh_frame+0x2c)
 // CHECK-NEXT: 200378           200378       18     1         {{.*}}{{/|\\}}map-file.s.tmp2.o:(.eh_frame+0x18)
 // CHECK-NEXT: 201000           201000       2d     4 .text
 // CHECK-NEXT: 201000           201000       28     4         {{.*}}{{/|\\}}map-file.s.tmp1.o:(.text)
 // CHECK-NEXT: 201000           201000        0     1                 _start
 // CHECK-NEXT: 20101f           20101f        0     1                 f(int)
 // CHECK-NEXT: 201028           201028        0     1                 local

test/ELF/relocatable-eh-frame.s

 # REQUIRES: x86
 # RUN: llvm-mc -filetype=obj -triple=x86_64-pc-linux %s -o %t.o
 # RUN: ld.lld -r %t.o %t.o -o %t
 # RUN: llvm-readobj -r %t | FileCheck %s
 # RUN: ld.lld %t -o %t.so -shared
 # RUN: llvm-objdump -h %t.so | FileCheck --check-prefix=DSO %s
 
-# DSO: .eh_frame     00000030
+# DSO: .eh_frame     00000034
 
 # CHECK:      Relocations [
 # CHECK-NEXT:   Section ({{.*}}) .rela.eh_frame {
 # CHECK-NEXT:     0x20 R_X86_64_PC32 .foo 0x0
 # CHECK-NEXT:     0x50 R_X86_64_NONE - 0x0
 # CHECK-NEXT:   }
 # CHECK-NEXT: ]
 
 .section .foo,"aG",@progbits,bar,comdat
 .cfi_startproc
 .cfi_endproc