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Date: Thu, 18 Mar 2021 08:17:06 +0100
From: Dmitry Vyukov <dvyukov@...gle.com>
To: Alexander Lochmann <info@...xander-lochmann.de>
Cc: Andrey Konovalov <andreyknvl@...gle.com>,
Jonathan Corbet <corbet@....net>,
Andrew Morton <akpm@...ux-foundation.org>,
Wei Yongjun <weiyongjun1@...wei.com>,
Maciej Grochowski <maciej.grochowski@...me>,
kasan-dev <kasan-dev@...glegroups.com>,
"open list:DOCUMENTATION" <linux-doc@...r.kernel.org>,
LKML <linux-kernel@...r.kernel.org>,
syzkaller <syzkaller@...glegroups.com>
Subject: Re: [PATCH] KCOV: Introduced tracing unique covered PCs
On Wed, Mar 17, 2021 at 10:10 PM Alexander Lochmann
<info@...xander-lochmann.de> wrote:
> On 15.03.21 09:02, Dmitry Vyukov wrote:
> >>> Does this introduce an additional real of t->kcov_mode?
> >>> If yes, please reuse the value read in check_kcov_mode.
> >> Okay. How do I get that value from check_kcov_mode() to the caller?
> >> Shall I add an additional parameter to check_kcov_mode()?
> >
> > Yes, I would try to add an additional pointer parameter for mode. I
> > think after inlining the compiler should be able to regestrize it.
> First, I'll go for the extra argument. However, the compiler doesn't
> seem to inline check_kcov_mode(). Can I enforce inlining?
> I'm using GCC 9.3 on Debian Testing.
That's very strange and wrong. Maybe you use something like
CONFIG_CC_OPTIMIZE_FOR_SIZE=y?
With gcc-10 I am getting:
ffffffff81529ba0 <__sanitizer_cov_trace_pc>:
ffffffff81529ba0: 65 8b 05 59 53 af 7e mov
%gs:0x7eaf5359(%rip),%eax # 1ef00 <__preempt_count>
ffffffff81529ba7: 89 c1 mov %eax,%ecx
ffffffff81529ba9: 48 8b 34 24 mov (%rsp),%rsi
ffffffff81529bad: 81 e1 00 01 00 00 and $0x100,%ecx
ffffffff81529bb3: 65 48 8b 14 25 40 ef mov %gs:0x1ef40,%rdx
ffffffff81529bba: 01 00
ffffffff81529bbc: a9 00 01 ff 00 test $0xff0100,%eax
ffffffff81529bc1: 74 0e je
ffffffff81529bd1 <__sanitizer_cov_trace_pc+0x31>
ffffffff81529bc3: 85 c9 test %ecx,%ecx
ffffffff81529bc5: 74 35 je
ffffffff81529bfc <__sanitizer_cov_trace_pc+0x5c>
ffffffff81529bc7: 8b 82 d4 14 00 00 mov 0x14d4(%rdx),%eax
ffffffff81529bcd: 85 c0 test %eax,%eax
ffffffff81529bcf: 74 2b je
ffffffff81529bfc <__sanitizer_cov_trace_pc+0x5c>
ffffffff81529bd1: 8b 82 b0 14 00 00 mov 0x14b0(%rdx),%eax
ffffffff81529bd7: 83 f8 02 cmp $0x2,%eax
ffffffff81529bda: 75 20 jne
ffffffff81529bfc <__sanitizer_cov_trace_pc+0x5c>
ffffffff81529bdc: 48 8b 8a b8 14 00 00 mov 0x14b8(%rdx),%rcx
ffffffff81529be3: 8b 92 b4 14 00 00 mov 0x14b4(%rdx),%edx
ffffffff81529be9: 48 8b 01 mov (%rcx),%rax
ffffffff81529bec: 48 83 c0 01 add $0x1,%rax
ffffffff81529bf0: 48 39 c2 cmp %rax,%rdx
ffffffff81529bf3: 76 07 jbe
ffffffff81529bfc <__sanitizer_cov_trace_pc+0x5c>
ffffffff81529bf5: 48 89 34 c1 mov %rsi,(%rcx,%rax,8)
ffffffff81529bf9: 48 89 01 mov %rax,(%rcx)
ffffffff81529bfc: c3 retq
Oh, wait gcc-9 indeed does not inline:
0000000000000070 <__sanitizer_cov_trace_pc>:
70: 65 48 8b 0c 25 00 00 mov %gs:0x0,%rcx
77: 00 00
79: bf 02 00 00 00 mov $0x2,%edi
7e: 48 89 ce mov %rcx,%rsi
81: 4c 8b 04 24 mov (%rsp),%r8
85: e8 76 ff ff ff callq 0 <check_kcov_mode>
8a: 84 c0 test %al,%al
8c: 74 20 je ae
<__sanitizer_cov_trace_pc+0x3e>
8e: 48 8b 91 b8 14 00 00 mov 0x14b8(%rcx),%rdx
95: 8b 89 b4 14 00 00 mov 0x14b4(%rcx),%ecx
9b: 48 8b 02 mov (%rdx),%rax
9e: 48 83 c0 01 add $0x1,%rax
a2: 48 39 c1 cmp %rax,%rcx
a5: 76 07 jbe ae
<__sanitizer_cov_trace_pc+0x3e>
a7: 4c 89 04 c2 mov %r8,(%rdx,%rax,8)
ab: 48 89 02 mov %rax,(%rdx)
ae: c3 retq
This looks like a bug in gcc-8/9. gcc-6 inlines again as well as
clang-11/12 inline.
Please add __always_inline for check_kcov_mode.
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