Re: arm64: Register modification during syscall entry/exit stop

On Mon, Jun 01, 2020 at 05:40:28AM -0400, Keno Fischer wrote:

On Mon, Jun 1, 2020 at 5:23 AM Dave Martin [off-list ref] wrote:

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Can't PTRACE_SYSEMU be emulated by using PTRACE_SYSCALL, cancelling the
syscall at the syscall enter stop, then modifying the regs at the
syscall exit stop?

Yes, it can. The idea behind SYSEMU is to be able to save half the
ptrace traps that would require, in theory making the ptracer
a decent amount faster. That said, the x7 issue is orthogonal to
SYSEMU, you'd have the same issues if you used PTRACE_SYSCALL.

Right, I just wondered whether there was some deeper difference between
the two approaches.

You're asking about a new regset vs trying to do it via ptrace option?

I meant SYSEMU versus SYSCALL + cancellation and emulating the syscall
at the syscall exit stop.

i.e., I was trying to understand whether SYSEMU is just a convenience,
or does some magic that can't be reproduced by other means.

I don't think there's anything a ptrace option can do that a new regset
that replicates the same registers (I'm gonna propose adding orig_x0,
while we're at it and changing the x0 semantics a bit, will have
those details with the patch) wouldn't be able to do . The reason I
originally thought it might have to be a ptrace option is because
the register modification currently gets applied in the syscall entry
code to the actual regs struct, so I thought you might have to know
to preserve those registers. However, then I realized that you could
just change the regset accessors to emulate the old behavior, since
we do already store all the required information (what kind of stop
we're currently at) in order to be able to answer the ptrace
informational queries. So doing that it probably just all around
easier. I guess NT_PRSTATUS might also rot, but I guess strace
doesn't really have to stop using it, since it doesn't care about
the x7 value nor does it need to modify it.

I think NT_PRSTATUS probably doesn't need to change.

Having a duplicate regset feels like a worse outcome that having a new
ptrace option.  Undocumentedly different things already happen to the
regs depending on how the tracee stopped, so adding a new special case
doesn't seem to justify creating a new regset.

Cheers
---Dave

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