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    SubjectRe: [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

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