Re: [PATCH v4] x86/entry: emit a symbol for register restoring thunk

[Peter Zijlstra]

On Thu, Jan 14, 2021 at 11:39:28AM +0100, Borislav Petkov wrote:
> From: Nick Desaulniers <ndesaulniers@xxxxxxxxxx>
> Date: Tue, 12 Jan 2021 11:46:24 -0800
> Subject: [PATCH] x86/entry: Emit a symbol for register restoring thunk
> 
> Arnd found a randconfig that produces the warning:
> 
>   arch/x86/entry/thunk_64.o: warning: objtool: missing symbol for insn at
>   offset 0x3e
> 
> when building with LLVM_IAS=1 (Clang's integrated assembler). Josh
> notes:
> 
>   With the LLVM assembler not generating section symbols, objtool has no
>   way to reference this code when it generates ORC unwinder entries,
>   because this code is outside of any ELF function.
> 
>   The limitation now being imposed by objtool is that all code must be
>   contained in an ELF symbol.  And .L symbols don't create such symbols.
> 
>   So basically, you can use an .L symbol *inside* a function or a code
>   segment, you just can't use the .L symbol to contain the code using a
>   SYM_*_START/END annotation pair.
> 
> Fangrui notes that this optimization is helpful for reducing image size
> when compiling with -ffunction-sections and -fdata-sections. I have
> observed on the order of tens of thousands of symbols for the kernel
> images built with those flags.
> 
> A patch has been authored against GNU binutils to match this behavior
> of not generating unused section symbols ([1]), so this will
> also become a problem for users of GNU binutils once they upgrade to 2.36.
> 
> Omit the .L prefix on a label so that the assembler will emit an entry
> into the symbol table for the label, with STB_LOCAL binding. This
> enables objtool to generate proper unwind info here with LLVM_IAS=1 or
> GNU binutils 2.36+.
> 
>  [ bp: Massage commit message. ]
> 
> Reported-by: Arnd Bergmann <arnd@xxxxxxxx>
> Suggested-by: Josh Poimboeuf <jpoimboe@xxxxxxxxxx>
> Suggested-by: Borislav Petkov <bp@xxxxxxxxx>
> Suggested-by: Mark Brown <broonie@xxxxxxxxxx>
> Signed-off-by: Nick Desaulniers <ndesaulniers@xxxxxxxxxx>
> Signed-off-by: Borislav Petkov <bp@xxxxxxx>

Acked-by: Peter Zijlstra (Intel) <peterz@xxxxxxxxxxxxx>

And while looking, I suppose we can delete the put_ret_addr_in_rdi crud,
but that's another patch.