From: Mark Rutland <mark.rutland@arm.com>
To: Borislav Petkov <bp@alien8.de>
Cc: Nick Desaulniers <ndesaulniers@google.com>,
Nathan Chancellor <nathan@kernel.org>, x86-ml <x86@kernel.org>,
lkml <linux-kernel@vger.kernel.org>,
Peter Zijlstra <peterz@infradead.org>,
Josh Poimboeuf <jpoimboe@redhat.com>
Subject: Re: clang memcpy calls
Date: Thu, 24 Mar 2022 15:29:56 +0000 [thread overview]
Message-ID: <YjyOdJlda3JcK3he@lakrids> (raw)
In-Reply-To: <YjxTt3pFIcV3lt8I@zn.tnic>
On Thu, Mar 24, 2022 at 12:19:19PM +0100, Borislav Petkov wrote:
> Hi folks,
Hi Boris,
> 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".
I think there's a more general soundness problem with noinstr here,
because with the options we pass today it's entirely legitimate for the
compiler to generate out-of-line calls to a number of support functions
(e.g. memcpy, but also memset and others), and we either need to inhibit
out-of-line calls to *any* of those, or ensure the out-of-line copies
used are never instrumented.
I'm not entirely sure how to prevent this on arm64 short of some
whole-compilation-unit shennanigans -- we don't have short sequence like
"rep movsq" that can be easily inlined, and we explicitly instrument
mem*() when certain KASAN options are selected.
I think we need more compiler help to make noinstr sound generally,
and/or may need to rethink the way we use noinstr.
Thanks,
Mark.
> 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
next prev parent reply other threads:[~2022-03-24 15:30 UTC|newest]
Thread overview: 19+ messages / expand[flat|nested] mbox.gz Atom feed top
2022-03-24 11:19 clang memcpy calls Borislav Petkov
2022-03-24 15:29 ` Mark Rutland [this message]
2022-03-24 18:43 ` Nick Desaulniers
2022-03-24 22:54 ` David Laight
2022-03-25 12:15 ` Mark Rutland
2022-03-25 14:13 ` Peter Zijlstra
2022-03-25 15:12 ` Segher Boessenkool
2022-03-28 9:52 ` Mark Rutland
2022-03-28 10:20 ` Jakub Jelinek
2022-03-28 11:54 ` Peter Zijlstra
2022-03-28 12:55 ` Mark Rutland
2022-03-28 13:12 ` Jakub Jelinek
2022-03-28 13:44 ` Mark Rutland
2022-03-30 14:45 ` Marco Elver
2022-03-28 14:22 ` Segher Boessenkool
2022-03-28 14:58 ` Mark Rutland
2022-03-28 15:59 ` Segher Boessenkool
2022-03-28 16:16 ` Peter Zijlstra
2022-03-28 16:58 ` Segher Boessenkool
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