Messages in this thread | | | Date | Thu, 24 Mar 2022 12:19:19 +0100 | From | Borislav Petkov <> | Subject | clang memcpy calls |
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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
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