https://sourceware.org/bugzilla/show_bug.cgi?id=20830
Bug ID: 20830
Summary: .plt.got section is lacking unwind info
Product: binutils
Version: 2.27
Status: UNCONFIRMED
Severity: normal
Priority: P2
Component: ld
Assignee: unassigned at sourceware dot org
Reporter: keno at juliacomputing dot com
Target Milestone: ---
Consider a simple executable, where one function needs both a GOT and PLT
entry, e.g.
```
.text
.globl _start
.type _start, @function
_start:
.cfi_startproc
.cfi_undefined %rip
movq func1@GOTPCREL(%rip), %rax
callq func1@plt
callq func2@plt
movq func3@GOTPCREL(%rip), %rax
hlt
.cfi_endproc
```
Now, ld is smart enough to combine the vanilla GOT entry and that required by
the PLT, placing a different kind of PLT stub in the `.plt.got` section:
```
gcc -c plt.S
ld -o plt plt.o plt-dyn.so ; plt-dyn is just some shared library that defines
these symbols
objdump -d plt
plt: file format elf64-x86-64
Disassembly of section .plt:
0000000000400300 <func2@plt-0x10>:
400300: ff 35 02 0d 20 00 pushq 0x200d02(%rip) # 601008
<_GLOBAL_OFFSET_TABLE_+0x8>
400306: ff 25 04 0d 20 00 jmpq *0x200d04(%rip) # 601010
<_GLOBAL_OFFSET_TABLE_+0x10>
40030c: 0f 1f 40 00 nopl 0x0(%rax)
0000000000400310 <func2@plt>:
400310: ff 25 02 0d 20 00 jmpq *0x200d02(%rip) # 601018
<_GLOBAL_OFFSET_TABLE_+0x18>
400316: 68 00 00 00 00 pushq $0x0
40031b: e9 e0 ff ff ff jmpq 400300 <func2@plt-0x10>
Disassembly of section .plt.got:
0000000000400320 <.plt.got>:
400320: ff 25 d2 0c 20 00 jmpq *0x200cd2(%rip) # 600ff8
<_DYNAMIC+0x148>
400326: 66 90 xchg %ax,%ax
Disassembly of section .text:
0000000000400328 <_start>:
400328: 48 8b 05 c9 0c 20 00 mov 0x200cc9(%rip),%rax #
600ff8 <_DYNAMIC+0x148>
40032f: e8 ec ff ff ff callq 400320 <func2@plt+0x10>
400334: e8 d7 ff ff ff callq 400310 <func2@plt>
400339: 48 8b 05 b0 0c 20 00 mov 0x200cb0(%rip),%rax #
600ff0 <_DYNAMIC+0x140>
400340: f4 hlt
```
which is very nice. Unfortunately, unlike the for .plt section, ld does not
create an FDE for the .plt.got section:
```
objdump --dwarf=frames
a.out: file format elf64-x86-64
Contents of the .eh_frame section:
00000000 0000000000000014 00000000 CIE
Version: 1
Augmentation: "zR"
Code alignment factor: 1
Data alignment factor: -8
Return address column: 16
Augmentation data: 1b
DW_CFA_def_cfa: r7 (rsp) ofs 8
DW_CFA_offset: r16 (rip) at cfa-8
DW_CFA_undefined: r16 (rip)
00000018 0000000000000014 0000001c FDE cie=00000000
pc=0000000000400328..0000000000400341
DW_CFA_nop
DW_CFA_nop
DW_CFA_nop
DW_CFA_nop
DW_CFA_nop
DW_CFA_nop
DW_CFA_nop
00000030 0000000000000014 00000000 CIE
Version: 1
Augmentation: "zR"
Code alignment factor: 1
Data alignment factor: -8
Return address column: 16
Augmentation data: 1b
DW_CFA_def_cfa: r7 (rsp) ofs 8
DW_CFA_offset: r16 (rip) at cfa-8
DW_CFA_nop
DW_CFA_nop
00000048 0000000000000024 0000001c FDE cie=00000030
pc=0000000000400300..0000000000400320
DW_CFA_def_cfa_offset: 16
DW_CFA_advance_loc: 6 to 0000000000400306
DW_CFA_def_cfa_offset: 24
DW_CFA_advance_loc: 10 to 0000000000400310
DW_CFA_def_cfa_expression (DW_OP_breg7 (rsp): 8; DW_OP_breg16 (rip): 0;
DW_OP_lit15; DW_OP_and; DW_OP_lit11; DW_OP_ge; DW_OP_lit3; DW_OP_shl;
DW_OP_plus)
DW_CFA_nop
DW_CFA_nop
DW_CFA_nop
DW_CFA_nop
```
Of course the .plt.got needs nothing fancy, just a simple DW_CFA_nop with the
same CIE as the .plt section would be sufficient. I should note that most
debuggers still get this right due to heuristics. However, there are unwinders
(e.g. in language runtimes), that purely rely on .eh_frame, so it would be nice
if the linker created the appropriate FDE.
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