Linux: warning: objtool: missing symbol for insn at offset 0x3e

[arndb]

This happens on x86 randconfigs, currently looking at linux-next-20201203:

arch/x86/entry/thunk_64.o: warning: objtool: missing symbol for insn at offset 0x3e

Example .config: https://pastebin.com/wwwhUL8L

[ihalip]

I assume this happens with LLVM_IAS=1, it's how I reproduced it. Looks like in this case, some section symbols are not being output.

Symbol table '.symtab' contains 8 entries:
   Num:    Value          Size Type    Bind   Vis      Ndx Name
     0: 0000000000000000     0 NOTYPE  LOCAL  DEFAULT  UND 
     1: 0000000000000000     0 SECTION LOCAL  DEFAULT    1                <---- missing with LLVM_IAS=1
     2: 0000000000000000     0 SECTION LOCAL  DEFAULT    3                <---- missing with LLVM_IAS=1
     3: 0000000000000000     0 SECTION LOCAL  DEFAULT    4                <---- missing with LLVM_IAS=1
     4: 0000000000000000    24 FUNC    GLOBAL DEFAULT    1 preempt_schedule_thunk
     5: 0000000000000000     0 NOTYPE  GLOBAL DEFAULT  UND preempt_schedule
     6: 0000000000000018    24 FUNC    GLOBAL DEFAULT    1 preempt_schedule_notrace_
     7: 0000000000000000     0 NOTYPE  GLOBAL DEFAULT  UND preempt_schedule_notrace

[nickdesaulniers]

Looks like in this case, some section symbols are not being output.

Yes, this is a known difference where objtool has been patched in the past. So then sounds like something may have changed possibly in objtool to rely on the section symbols, again. But the warning is about a symbol for an instruction; I wouldn't expect section symbols to be related to instructions. See also: https://github.com/ClangBuiltLinux/linux/issues/669#issuecomment-534303910

[nickdesaulniers]

cc @jcai19 @groeck looks like CrOS just hit this as well.

[groeck]

Yes. Unfirtunately is all but impossible for "outsiders" to determine if this is a llvm assembler problem or an objtool problem or maybe both.

[nickdesaulniers]

in the bad case, arch/x86/entry/thunk_64.s does not define:
STATIC_JUMP_IF_TRUE
STATIC_JUMP_IF_FALSE
SET_NOFLUSH_BIT
ADJUST_KERNEL_CR3
SWITCH_TO_KERNEL_CR3
SWITCH_TO_USER_CR3_NOSTACK
SWITCH_TO_USER_CR3_STACK
SAVE_AND_SWITCH_TO_KERNEL_CR3
RESTORE_CR3
LOAD_CPU_AND_NODE_SEG_LIMIT

GET_PERCPU_BASE looks like just pcpu_unit_offsets(%rip), \reg. ___EXPORT_SYMBOL looks different. there's thunks for preempt_schedule_thunk and preempt_schedule_notrace_thunk and a .L_restore symbol.

So CONFIG_PREEMPTION=y (bad), CONFIG_SMP=n (bad), CONFIG_PAGE_TABLE_ISOLATION=n (bad). But those alone on top of defconfig are not enough to repro. I'm having trouble pinning down the configs via tools/testing/ktest/config-bisect.pl.

[nickdesaulniers]

$ clang -Wp,-MMD,arch/x86/entry/.thunk_64.o.d -nostdinc -isystem /android0/llvm-project/llvm/build/lib/clang/12.0.0/include -I./arch/x86/include -I./arch/x86/include/generated  -I./include -I./arch/x86/include/uapi -I./arch/x86/include/generated/uapi -I./include/uapi -I./include/generated/uapi -include ./include/linux/kconfig.h -D__KERNEL__ -Qunused-arguments -fmacro-prefix-map=./= -D__ASSEMBLY__ -fno-PIE -Werror=unknown-warning-option -m64 -DCONFIG_X86_X32_ABI    -c -o arch/x86/entry/thunk_64.o arch/x86/entry/thunk_64.S
$ ./tools/objtool/objtool orc generate  --no-fp --no-unreachable --retpoline arch/x86/entry/thunk_64.o
arch/x86/entry/thunk_64.o: warning: objtool: missing symbol for insn at offset 0x3e
$ llvm-objdump -Dr arch/x86/entry/thunk_64.o
0000000000000018 <preempt_schedule_notrace_thunk>:
...
      3e: c3                            retq

Even with Josh's patch from https://groups.google.com/g/clang-built-linux/c/1C6YoJKBsQQ/m/a8IS1NjGAgAJ applied.

[nickdesaulniers]

insn_to_reloc_sym_addend() calls find_symbol_containing twice; once with offset, once with offset - 1. Both calls return NULL in the bad config.

[jpoimboe]

Can somebody attach the .o file? Not sure why my mailer's rejecting it.

[nickdesaulniers]

thunk_64.o.txt
cc @jpoimboe

[jpoimboe]

The problem is this code segment at the bottom of arch/x86/entry/thunk_64.S. With the LLVM assembler stripping the .text section symbol, objtool has no way to reference this code when it generates ORC unwinder entries, because this code is outside of any ELF function.

SYM_CODE_START_LOCAL_NOALIGN(.L_restore)
        popq %r11
        popq %r10
        popq %r9
        popq %r8
        popq %rax
        popq %rcx
        popq %rdx
        popq %rsi
        popq %rdi
        popq %rbp
        ret
        _ASM_NOKPROBE(.L_restore)
SYM_CODE_END(.L_restore)

[jpoimboe]

I don't have access to this assembler at the moment, but is there a way to encourage it to not strip the ".text" section symbol in this file, either with a flag or some trick to create a section reference?

[jpoimboe]

Something like the below would probably fix it, by forcing the code into an ELF symbol.

Still, it would be really nice to have a flag to turn off the section symbol stripping, as I don't think it has any benefits for the kernel (and it definitely has drawbacks for cases like this).

diff --git a/arch/x86/entry/thunk_64.S b/arch/x86/entry/thunk_64.S
index ccd32877a3c4..4920037445fb 100644
--- a/arch/x86/entry/thunk_64.S
+++ b/arch/x86/entry/thunk_64.S
@@ -44,7 +44,7 @@ SYM_FUNC_END(\name)
 #endif

 #ifdef CONFIG_PREEMPTION
-SYM_CODE_START_LOCAL_NOALIGN(.L_restore)
+SYM_CODE_START_GLOBAL_NOALIGN(__thunk_restore)
        popq %r11
        popq %r10
        popq %r9
@@ -56,6 +56,6 @@ SYM_CODE_START_LOCAL_NOALIGN(.L_restore)
        popq %rdi
        popq %rbp
        ret
-       _ASM_NOKPROBE(.L_restore)
-SYM_CODE_END(.L_restore)
+       _ASM_NOKPROBE(__thunk_restore)
+SYM_CODE_END(__thunk_restore)
 #endif

[nickdesaulniers]

I don't have access to this assembler at the moment, but is there a way to encourage it to not strip the ".text" section symbol in this file, either with a flag or some trick to create a section reference?

@MaskRay didn't we have a thread about this previously? I don't think adding support for emitting symbols for sections would be too bad.

[nickdesaulniers]

https://github.com/llvm/llvm-project/commit/a401eee22fabea8d214ab604037c937277477c38 seems relevant.

```diff
diff --git a/llvm/lib/MC/ELFObjectWriter.cpp b/llvm/lib/MC/ELFObjectWriter.cpp
index 10c61fc8b453..bb9089ebe85b 100644
--- a/llvm/lib/MC/ELFObjectWriter.cpp
+++ b/llvm/lib/MC/ELFObjectWriter.cpp
@@ -598,9 +598,6 @@ bool ELFWriter::isInSymtab(const MCAsmLayout &Layout, const MCSymbolELF &Symbol,
if (Symbol.isTemporary())
return false;

• if (Symbol.getType() == ELF::STT_SECTION)
• return false;
  -
  return true;
  }
  ```
  is enough to restore the behavior of emitting section symbols; would just need to clean up the resulting test failures.

Failed Tests (17):
LLVM :: CodeGen/PowerPC/pcrel-tls-general-dynamic.ll
LLVM :: CodeGen/PowerPC/pcrel-tls-initial-exec.ll
LLVM :: ExecutionEngine/JITLink/X86/ELF_x86-64_relocations.s
LLVM :: MC/AArch64/size-directive.s
LLVM :: MC/ELF/alias.s
LLVM :: MC/ELF/cgprofile.s
LLVM :: MC/ELF/comdat.s
LLVM :: MC/ELF/empty.s
LLVM :: MC/ELF/reloc-same-name-section.s
LLVM :: MC/ELF/relocation-alias.s
LLVM :: MC/ELF/section-sym.s
LLVM :: MC/ELF/undef.s
LLVM :: tools/llvm-objdump/X86/demangle.s
LLVM :: tools/llvm-readobj/ELF/many-sections2.s
lld :: ELF/error-handling-script-linux.test
lld :: ELF/icf-safe.s
lld :: ELF/lto/thinlto-single-module.ll

[nickdesaulniers]

WIP: https://reviews.llvm.org/D93783 let's see what the feedback is. Fixes the reported issue.

[nickdesaulniers]

Also submitted: https://lore.kernel.org/lkml/[email protected]/T/#u.

[MaskRay]

Replied on https://reviews.llvm.org/D93783#2470728
I understand it is a (continuous) pain for objtool but we probably should take it. The LLVM integrated assembler behavior is very much desired and emitting unneeded STT_SECTION can cause unneeded .o size bloat for many users. GNU as feature request: https://sourceware.org/bugzilla/show_bug.cgi?id=27109 (I thought I had reported this earlier but it turned out that I hadn't. oops)

[jpoimboe]

Fair enough, it does make sense for '-ffunction-sections' at least.

[arndb]

I see a second produce a similar warning

arch/x86/kernel/ftrace_64.o: warning: objtool: missing symbol for insn at offset 0x16

0xE5F582C9-config.gz

[nickdesaulniers]

HJL posted a patch for GAS to omit STT_SECTION symbols as well: https://sourceware.org/pipermail/binutils/2020-December/114671.html

[jpoimboe]

I see a second produce a similar warning

arch/x86/kernel/ftrace_64.o: warning: objtool: missing symbol for insn at offset 0x16

0xE5F582C9-config.gz

This one's slightly different.

Symbol table '.symtab' contains 7 entries:
   Num:    Value          Size Type    Bind   Vis      Ndx Name
     0: 0000000000000000     0 NOTYPE  LOCAL  DEFAULT  UND 
     1: 0000000000000000     0 NOTYPE  LOCAL  DEFAULT    4 __ksym_marker___[...]
     2: 000000000000000e     0 NOTYPE  LOCAL  DEFAULT    2 fgraph_trace
     3: 000000000000000f     0 NOTYPE  LOCAL  DEFAULT    2 trace
     4: 0000000000000000   165 FUNC    GLOBAL DEFAULT    2 __fentry__
     5: 000000000000000e     0 NOTYPE  GLOBAL DEFAULT    2 ftrace_stub
     6: 0000000000000000     0 NOTYPE  GLOBAL DEFAULT  UND ftrace_trace_function

The instruction at 0x16 is inside the __fentry__ function, but objtool can't figure that out because its red-black tree of symbols is sorted by offset and it gets confused by the fact that the NOTYPE symbols overlap the function.

This will probably need an objtool fix.

[jpoimboe]

Fix for this 2nd issue posted:
https://lkml.kernel.org/r/[e-mail]

[nickdesaulniers]

v3: https://lore.kernel.org/lkml/[email protected]/T/#u

[nickdesaulniers]

accepted: https://git.kernel.org/pub/scm/linux/kernel/git/tip/tip.git/commit/?h=x86/entry&id=bde718b7e154afc99e1956b18a848401ce8e1f8e

[nickdesaulniers]

Looks like CrOS just hit this issue.