linux-kernel - clang memcpy calls

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Date:   Thu, 24 Mar 2022 12:19:19 +0100
From:   Borislav Petkov <bp@...en8.de>
To:     Nick Desaulniers <ndesaulniers@...gle.com>,
        Nathan Chancellor <nathan@...nel.org>
Cc:     x86-ml <x86@...nel.org>, lkml <linux-kernel@...r.kernel.org>
Subject: clang memcpy calls

Hi folks,

so I've been looking at a recent objtool noinstr warning from clang
builds:

vmlinux.o: warning: objtool: sync_regs()+0x20: call to memcpy() leaves .noinstr.text section

The issue is that clang generates a memcpy() call when a struct copy
happens:

        if (regs != eregs)
                *regs = *eregs;

see below for asm output.

While gcc does simply generate an actual "rep; movsq".

So, how hard would it be to make clang do that too pls?

Oh, and another thing while we're comparing asm: I'd love for clang's
-fverbose-asm to issue interleaved C source lines too, like gcc does.

That's it - no pink pony - just "normal" wishes. :-)

GCC:
====

sync_regs:
.LASANPC4246:
# arch/x86/kernel/traps.c:770: {
        movq    %rdi, %rsi      # tmp91, eregs
# arch/x86/kernel/traps.c:771:  struct pt_regs *regs = (struct pt_regs *)this_cpu_read(cpu_current_top_of_stack) - 1;
#APP
# 771 "arch/x86/kernel/traps.c" 1
        movq %gs:cpu_current_top_of_stack(%rip), %rax   # cpu_current_top_of_stack, pfo_val__
# 0 "" 2
# arch/x86/kernel/traps.c:771:  struct pt_regs *regs = (struct pt_regs *)this_cpu_read(cpu_current_top_of_stack) - 1;
#NO_APP
        subq    $168, %rax      #, <retval>
# arch/x86/kernel/traps.c:772:  if (regs != eregs)
        cmpq    %rdi, %rax      # eregs, <retval>
        je      .L387   #,
# arch/x86/kernel/traps.c:773:          *regs = *eregs;
        movl    $21, %ecx       #, tmp89
        movq    %rax, %rdi      # <retval>, <retval>
        rep movsq
.L387:
# arch/x86/kernel/traps.c:775: }
        ret

CLANG:
======

        .section        .noinstr.text,"ax",@progbits
        .globl  sync_regs                       # -- Begin function sync_regs
        .p2align        6, 0x90
        .type   sync_regs,@function
sync_regs:                              # @sync_regs
# %bb.0:                                # %entry
        pushq   %rbx
        #APP
        movq    %gs:cpu_current_top_of_stack(%rip), %rbx
        #NO_APP
        addq    $-168, %rbx
        cmpq    %rdi, %rbx
        je      .LBB19_2
# %bb.1:                                # %if.then
        movq    %rdi, %rsi
        movl    $168, %edx
        movq    %rbx, %rdi
        callq   memcpy@PLT
.LBB19_2:                               # %if.end
        movq    %rbx, %rax
        popq    %rbx
        retq


-- 
Regards/Gruss,
    Boris.

https://people.kernel.org/tglx/notes-about-netiquette