linux-kernel - Re: [PATCH 5/9] x86/dumpstack: Improve opcodes dumping in the Code: section

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Date:   Wed, 7 Mar 2018 11:13:14 +0100
From:   Borislav Petkov <bp@...en8.de>
To:     Linus Torvalds <torvalds@...ux-foundation.org>
Cc:     X86 ML <x86@...nel.org>, Andy Lutomirski <luto@...capital.net>,
        Josh Poimboeuf <jpoimboe@...hat.com>,
        Peter Zijlstra <peterz@...radead.org>,
        LKML <linux-kernel@...r.kernel.org>
Subject: Re: [PATCH 5/9] x86/dumpstack: Improve opcodes dumping in the Code:
 section

On Tue, Mar 06, 2018 at 10:47:56AM -0800, Linus Torvalds wrote:
> Are these always serialized? For oopses, I think we end up serializing
> with die_lock, but is that always the case?

Hmm, good question.

> Maybe at least a comment about why a static allocation is ok?

Well, I'm afraid it is not ok but let me show what I'm seeing - maybe
I'm wrong somewhere:

Normally, when something calls die() we do this:

die
|-> oops_begin
|-> arch_spin_lock(&die_lock)		<-- grab die_lock
|-> __die
|-> show_regs
|-> __show_regs
|-> show_iret_regs
|-> show_ip
|-> show_opcodes

and we dump fine here.

But, if, for example, a #PF happens while we die(), we could do this:

do_page_fault
|-> __do_page_fault
|-> bad_area_nosemaphore
|-> __bad_area_nosemaphore
|-> show_signal_msg
|-> show_opcodes

that's the catch-all case in:

        if (unlikely(fault_in_kernel_space(address))) {

and that doesn't sync with the die_lock, AFAICT, and we're walking all
over the opcodes buffer.

Unless I'm missing something, that is.

If I'm not, then I guess I need to think about a better way to solve
this. Because I like the improvement of not having to probe_kernel_read()
byte-by-byte but read it all at once.

And that is fine if I do a 64-byte default, on-stack buffer but that
code_bytes= thing can be 2 pages max which is yuck. No way I'm doing
on-stack buffers then.

Hmm, I need to think about it.

Thanks!

-- 
Regards/Gruss,
    Boris.

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