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 (local)
[2] https://lore.kernel.org/r/CABV8kRzg1BaKdAhqXU3hONhfPAHj6Nbw0wLBC1Lo7PN1UA0CoA@mail.gmail.com (local)

Will

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