linux-kernel - Re: [PATCH] x86/coredump: always use user_regs_struct for compat_elf_gregset

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Message-ID: <CALCETrUQDBX_QqHGeozQ3Q+9pF3SeyE9XyPqX4M6k3XOV8Zd=Q@mail.gmail.com>
Date:   Thu, 8 Dec 2016 15:14:53 -0800
From:   Andy Lutomirski <luto@...capital.net>
To:     Dmitry Safonov <dsafonov@...tuozzo.com>
Cc:     Thomas Gleixner <tglx@...utronix.de>,
        Dmitry Safonov <0x7f454c46@...il.com>,
        Ingo Molnar <mingo@...hat.com>,
        "linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>,
        Oleg Nesterov <oleg@...hat.com>,
        "linux-mm@...ck.org" <linux-mm@...ck.org>, X86 ML <x86@...nel.org>,
        "H. Peter Anvin" <hpa@...or.com>
Subject: Re: [PATCH] x86/coredump: always use user_regs_struct for compat_elf_gregset_t

On Nov 23, 2016 10:16 AM, "Dmitry Safonov" <dsafonov@...tuozzo.com> wrote:
>
> From commit 90954e7b9407 ("x86/coredump: Use pr_reg size, rather that
> TIF_IA32 flag") elf coredump file is constructed according to register
> set size - and that's good: if binary crashes with 32-bit code selector,
> generate 32-bit ELF core, otherwise - 64-bit core.
> That was made for restoring 32-bit applications on x86_64: we want
> 32-bit application after restore to generate 32-bit ELF dump on crash.
> All was quite good and recently I started reworking 32-bit applications
> dumping part of CRIU: now it has two parasites (32 and 64) for seizing
> compat/native tasks, after rework it'll have one parasite, working in
> 64-bit mode, to which 32-bit prologue long-jumps during infection.
>
> And while it has worked for my work machine, in VM with
> !CONFIG_X86_X32_ABI during reworking I faced that segfault in 32-bit
> binary, that has long-jumped to 64-bit mode results in dereference
> of garbage:

Can you point to the actual line that's crashing?  I'm wondering if we
have code that should be made more robust.

--Andy