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Date:   Fri, 11 Sep 2020 17:10:52 -0700
From:   Kees Cook <keescook@chromium.org>
To:     Michael Ellerman <mpe@ellerman.id.au>
Cc:     Thomas Gleixner <tglx@linutronix.de>,
        Robert O'Callahan <rocallahan@gmail.com>,
        LKML <linux-kernel@vger.kernel.org>,
        "maintainer:X86 ARCHITECTURE (32-BIT AND 64-BIT)" <x86@kernel.org>,
        linux-arch@vger.kernel.org, Will Deacon <will@kernel.org>,
        Arnd Bergmann <arnd@arndb.de>,
        Mark Rutland <mark.rutland@arm.com>,
        Keno Fischer <keno@juliacomputing.com>,
        Paolo Bonzini <pbonzini@redhat.com>,
        kvm list <kvm@vger.kernel.org>,
        Gabriel Krisman Bertazi <krisman@collabora.com>,
        Sean Christopherson <sean.j.christopherson@intel.com>,
        Kyle Huey <me@kylehuey.com>
Subject: Re: [REGRESSION] x86/entry: Tracer no longer has opportunity to
 change the syscall number at entry via orig_ax
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On Wed, Sep 09, 2020 at 11:53:42PM +1000, Michael Ellerman wrote:
> I can observe the difference between v5.8 and mainline, using the
> raw_syscall trace event and running the seccomp_bpf selftest which turns
> a getpid (39) into a getppid (110).
> 
> With v5.8 we see getppid on entry and exit:
> 
>      seccomp_bpf-1307  [000] .... 22974.874393: sys_enter: NR 110 (7ffff22c46e0, 40a350, 4, fffffffffffff7ab, 7fa6ee0d4010, 0)
>      seccomp_bpf-1307  [000] .N.. 22974.874401: sys_exit: NR 110 = 1304
> 
> Whereas on mainline we see an enter for getpid and an exit for getppid:
> 
>      seccomp_bpf-1030  [000] ....    21.806766: sys_enter: NR 39 (7ffe2f6d1ad0, 40a350, 7ffe2f6d1ad0, 0, 0, 407299)
>      seccomp_bpf-1030  [000] ....    21.806767: sys_exit: NR 110 = 1027

For my own notes, this is how I reproduced it:

# ./perf-$VER record -e raw_syscalls:sys_enter -e raw_syscalls:sys_exit &
# ./seccomp_bpf
# fg
ctrl-c
# ./perf-$VER script | grep seccomp_bpf | awk '{print $7}' | sort | uniq -c > $VER.log
*repeat*
# diff -u old.log new.log
...

(Is there an easier way to get those results?)

I will go see if I can figure out the best way to correct this.

-- 
Kees Cook