linux-kernel - Re: [PATCH 0/3 v2] arm64/ptrace: allow to get all registers on syscall traps

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Date:   Thu, 4 Feb 2021 14:53:46 +0000
From:   Will Deacon <will@...nel.org>
To:     Andrei Vagin <avagin@...il.com>
Cc:     Catalin Marinas <catalin.marinas@....com>,
        Oleg Nesterov <oleg@...hat.com>,
        linux-arm-kernel@...ts.infradead.org, linux-kernel@...r.kernel.org,
        linux-api@...r.kernel.org,
        Anthony Steinhauser <asteinhauser@...gle.com>,
        Dave Martin <Dave.Martin@....com>,
        Keno Fischer <keno@...iacomputing.com>
Subject: Re: [PATCH 0/3 v2] arm64/ptrace: allow to get all registers on
 syscall traps

Hi Andrei,

On Mon, Feb 01, 2021 at 11:40:09AM -0800, Andrei Vagin wrote:
> Right now, ip/r12 for AArch32 and x7 for AArch64 is used to indicate
> whether or not the stop has been signalled from syscall entry or syscall
> exit. This means that:
> 
> - Any writes by the tracer to this register during the stop are
>   ignored/discarded.
> 
> - The actual value of the register is not available during the stop,
>   so the tracer cannot save it and restore it later.
> 
> For applications like the user-mode Linux or gVisor, it is critical to
> have access to the full set of registers in any moment. For example,
> they need to change values of all registers to emulate rt_sigreturn or
> execve and they need to have the full set of registers to build a signal
> frame.
> 
> This series introduces the PTRACE_O_ARM64_RAW_REGS option. If it is set,
> PTRACE_GETREGSET returns values of all registers, and PTRACE_SETREGSET
> allows to change any of them.

I haven't had a chance to go through this properly yet, but I spotted a
couple of things worth mentioning off the bat:

  - Please drop all of the compat changes here. The compat layer is intended
    to be compatible with arch/arm/, so if you want to introduce new ptrace
    behaviours for 32-bit applications, you need to make the changes there
    and then update our compat layer accordingly.

  - When Keno mentioned this before [1,2], he also talked about making
    orig_x0 available. Since extending the ABI is a giant pain, I think
    this should be seriously considered.

[1] https://lore.kernel.org/r/CABV8kRzkLiVuqxT3+8c1o8m_OuROtXgfowQcrMVnrxu=CiGB=w@mail.gmail.com
[2] https://lore.kernel.org/r/CABV8kRzg1BaKdAhqXU3hONhfPAHj6Nbw0wLBC1Lo7PN1UA0CoA@mail.gmail.com

Will