Excerpts from Dmitry V. Levin's message of May 19, 2021 8:24 pm:

On Wed, May 19, 2021 at 12:50:24PM +1000, Nicholas Piggin wrote:
[...]

quoted

With this patch, I think the ptrace ABI should mostly be fixed. I think 
a problem remains with applications that look at system call return 
registers directly and have powerpc specific error cases. Those probably
will just need to be updated unfortunately. Michael thought it might be
possible to return an indication via ptrace somehow that the syscall is
using a new ABI, so such apps can be updated to test for it. I don't 
know how that would be done.

Is there any sane way for these applications to handle the scv case?
How can they tell that the scv semantics is being used for the given
syscall invocation?  Can this information be obtained e.g. from struct
pt_regs?

Not that I know of. Michael suggested there might be a way to add 
something. ptrace_syscall_info has some pad bytes, could
we use one for flags bits and set a bit for "new system call ABI"?

As a more hacky thing you could make a syscall with -1 and see how
the error looks, and then assume all syscalls will be the same.

Thanks,
Nick

For example, in strace we have the following powerpc-specific code used
for syscall tampering:

$ cat src/linux/powerpc/set_error.c
/*
 * Copyright (c) 2016-2021 The strace developers.
 * All rights reserved.
 *
 * SPDX-License-Identifier: LGPL-2.1-or-later
 */

static int
arch_set_r3_ccr(struct tcb *tcp, const unsigned long r3,
		const unsigned long ccr_set, const unsigned long ccr_clear)
{
	if (ptrace_syscall_info_is_valid() &&
	    upeek(tcp, sizeof(long) * PT_CCR, &ppc_regs.ccr))
                return -1;
	const unsigned long old_ccr = ppc_regs.ccr;
	ppc_regs.gpr[3] = r3;
	ppc_regs.ccr |= ccr_set;
	ppc_regs.ccr &= ~ccr_clear;
	if (ppc_regs.ccr != old_ccr &&
	    upoke(tcp, sizeof(long) * PT_CCR, ppc_regs.ccr))
		return -1;
	return upoke(tcp, sizeof(long) * (PT_R0 + 3), ppc_regs.gpr[3]);
}

static int
arch_set_error(struct tcb *tcp)
{
	return arch_set_r3_ccr(tcp, tcp->u_error, 0x10000000, 0);
}

static int
arch_set_success(struct tcb *tcp)
{
	return arch_set_r3_ccr(tcp, tcp->u_rval, 0, 0x10000000);
}


-- 
ldv