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

On Thu, Jan 14, 2021 at 11:39:28AM +0100, Borislav Petkov wrote:

On Wed, Jan 13, 2021 at 09:56:04AM -0800, Nick Desaulniers wrote:

quoted

Apologies, that was not my intention.  I've sent a follow up in
https://lore.kernel.org/lkml/20210113174620.958429-1-ndesaulniers@google.com/T/#u (local)
since BP picked up v3 in tip x86/entry:
https://git.kernel.org/pub/scm/linux/kernel/git/tip/tip.git/commit/?h=x86/entry&id=bde718b7e154afc99e1956b18a848401ce8e1f8e

It is the topmost patch so I can rebase...

Also, I replicated that text into linkage.h and removed the change over
SYM_CODE_START and I've got the below.

Further complaints?

---
From: Nick Desaulniers <ndesaulniers@google.com>
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. ]

Acked-by: Josh Poimboeuf <redacted>

-- 
Josh