linux-kernel - Re: [PATCH 5.15 00/78] 5.15.55-rc1 review

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Message-ID: <CAHk-=wj5cOA+fbGeV15kvwe6YGT54Wsk8F2UGoekVQLTPJz_pw@mail.gmail.com>
Date:   Wed, 13 Jul 2022 11:33:22 -0700
From:   Linus Torvalds <torvalds@...ux-foundation.org>
To:     Guenter Roeck <linux@...ck-us.net>,
        Peter Zijlstra <peterz@...radead.org>,
        Borislav Petkov <bp@...e.de>
Cc:     Naresh Kamboju <naresh.kamboju@...aro.org>,
        Greg Kroah-Hartman <gregkh@...uxfoundation.org>,
        kvm list <kvm@...r.kernel.org>,
        Linux Kernel Mailing List <linux-kernel@...r.kernel.org>,
        stable <stable@...r.kernel.org>,
        Andrew Morton <akpm@...ux-foundation.org>,
        Shuah Khan <shuah@...nel.org>, patches@...nelci.org,
        lkft-triage@...ts.linaro.org, Pavel Machek <pavel@...x.de>,
        Jon Hunter <jonathanh@...dia.com>,
        Florian Fainelli <f.fainelli@...il.com>,
        Sudip Mukherjee <sudipm.mukherjee@...il.com>,
        Slade Watkins <slade@...dewatkins.com>,
        Sean Christopherson <seanjc@...gle.com>,
        Paolo Bonzini <pbonzini@...hat.com>,
        Thomas Gleixner <tglx@...utronix.de>,
        Ingo Molnar <mingo@...hat.com>,
        Dave Hansen <dave.hansen@...ux.intel.com>,
        X86 ML <x86@...nel.org>, "H. Peter Anvin" <hpa@...or.com>,
        Alex Bennée <alex.bennee@...aro.org>,
        Anders Roxell <anders.roxell@...aro.org>
Subject: Re: [PATCH 5.15 00/78] 5.15.55-rc1 review

On Wed, Jul 13, 2022 at 6:34 AM Guenter Roeck <linux@...ck-us.net> wrote:
>
> Looking into the log, I don't think that message is related to the crash.
>
> ...
> [  105.653777] Modules linked in: x86_pkg_temp_thermal
> [  105.902123] ---[ end trace cec99cae36bcbfd7 ]---
> [  105.902124] RIP: 0010:xaddw_ax_dx+0x9/0x10    <--- crash
> [  105.902126] Code: 00 0f bb d0 c3 cc cc cc cc 48 0f bb d0 c3 cc cc

Yeah, the code you snipped, shows

  20: 66 0f c1 d0          xadd   %dx,%ax
  24: c3                    ret
  25: cc                    int3
  26: cc                    int3
  27: cc                    int3
  28: cc                    int3
  29:* 0f 1f 80 00 00 00 00 nopl   0x0(%rax) <-- trapping instruction
  30: 0f c1 d0              xadd   %edx,%eax
  33: c3                    ret
  34: cc                    int3
  35: cc                    int3
  36: cc                    int3
  37: cc                    int3
  38: 48 0f c1 d0          xadd   %rdx,%rax
  3c: c3                    ret
  3d: cc                    int3

and that's a bit odd.

It says "xaddw_ax_dx+0x9/0x10", but I think somebody jumped to
"xaddw_ax_dx+8", hit the 'int3', and the RIP points to the next
instruction (because that's how int3 works).

And the fastop code says:

 * fastop functions have a special calling convention:
...
 * Moreover, they are all exactly FASTOP_SIZE bytes long,

but that is clearly *NOT* the case for xaddw_ax_dx, because it's 16
bytes in size, and the other ones are 8 bytes. That's where the "nopl"
comes from: it's the alignment instruction to the next fastop
function.

Compare that to the word-sized 'xaddl' case rigth afterwards: that one
*is* just 8 bytes in size, so the 64-byte 'xaddq' comes 8 bytes aftrer
it, and there's no 7-byte padding nop-instruction.

So I think that that is where the "xaddw_ax_dx+8" comes from: some
code assumes that FASTOP_SIZE is 8, but that xaddw_ax_dx case was
actually 9 bytes and thus got that "int3 + padding" in the next 8
bytes.

The whole kvm x86 emulation thiing is quite complicated and has lots
of instruction size #defines and magic.

I'm not familiar enough with it to go "Ahh, it's obviously XYZ", but
I'm sure PeterZ and Borislav know exactly what's going on.

                 Linus