.plt section in modules with IBT #1606
Description
Peter Zijlstra reported that attempting to run objtool on the .o part of modules before linking the .o and .mod.o into .ko shows a warning about a .plt section:
warning: objtool: .plt+0x6: indirect jump found in RETPOLINE build
As it turns out, this can be reproduced with an "empty" .o file:
$ echo | clang -fcf-protection=branch -O2 -x c -c -o foo.o -
$ ld.lld -m elf_x86_64 -r -o foo-mod.o foo.o
$ objtool orc generate --module --lto --ibt --ibt-fix-direct --ibt-seal --no-fp --retpoline --uaccess foo-mod.o
foo-mod.o: warning: objtool: .plt+0x6: indirect jump found in RETPOLINE build
$ llvm-objdump -d --section=.plt foo-mod.o
foo-mod.o: file format elf64-x86-64
Disassembly of section .plt:
0000000000000000 <.plt>:
0: ff 35 02 00 00 00 pushq 2(%rip) # 0x8 <.plt+0x8>
6: ff 25 04 00 00 00 jmpq *4(%rip) # 0x10 <.plt+0x10>
c: 0f 1f 40 00 nopl (%rax)
ld.bfd does not exhibit this same behavior:
$ ld -m elf_x86_64 -r -o foo-mod.o foo.o
$ objtool orc generate --module --lto --ibt --ibt-fix-direct --ibt-seal --no-fp --retpoline --uaccess foo-mod.o
$ llvm-objdump -d --section=.plt foo-mod.o
foo-mod.o: file format elf64-x86-64
llvm-objdump: warning: section '.plt' mentioned in a -j/--section option, but not found in any input file
It is not reproducible without -fcf-protection=branch:
$ echo | clang -O2 -x c -c -o foo.o -
$ ld.lld -m elf_x86_64 -r -o foo-mod.o foo.o
$ objtool orc generate --module --lto --ibt --ibt-fix-direct --ibt-seal --no-fp --retpoline --uaccess foo-mod.o
$ llvm-objdump -d --section=.plt foo-mod.o
foo-mod.o: file format elf64-x86-64
llvm-objdump: warning: section '.plt' mentioned in a -j/--section option, but not found in any input file
It does not appear to depend on compiler, I could reproduce this with GCC + ld.lld as well:
$ echo | gcc -fcf-protection=branch -O2 -x c -c -o foo.o -
$ ld.lld -m elf_x86_64 -r -o foo-mod.o foo.o
$ objtool orc generate --module --lto --ibt --ibt-fix-direct --ibt-seal --no-fp --retpoline --uaccess foo-mod.o
foo-mod.o: warning: objtool: .plt+0x6: indirect jump found in RETPOLINE build
$ llvm-objdump -d --section=.plt foo-mod.o
foo-mod.o: file format elf64-x86-64
Disassembly of section .plt:
0000000000000000 <.plt>:
0: ff 35 02 00 00 00 pushq 2(%rip) # 0x8 <.plt+0x8>
6: ff 25 04 00 00 00 jmpq *4(%rip) # 0x10 <.plt+0x10>
c: 0f 1f 40 00 nopl (%rax)
I have uploaded objtool here, based on Peter's x86/wip.ibt branch.