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

Will Deacon
2021-02-04T14:55:29Z

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