ia64: fix ia64_syscall_get_set_arguments() for break-based syscalls

In https://bugs.gentoo.org/769614 Dmitry noticed that
`ptrace(PTRACE_GET_SYSCALL_INFO)` does not work for syscalls called via
glibc's syscall() wrapper.

ia64 has two ways to call syscalls from userspace: via `break` and via
`eps` instructions.

The difference is in stack layout:

1. `eps` creates simple stack frame: no locals, in{0..7} == out{0..8}
2. `break` uses userspace stack frame: may be locals (glibc provides
   one), in{0..7} == out{0..8}.

Both work fine in syscall handling cde itself.

But `ptrace(PTRACE_GET_SYSCALL_INFO)` uses unwind mechanism to
re-extract syscall arguments but it does not account for locals.

The change always skips locals registers. It should not change `eps`
path as kernel's handler already enforces locals=0 and fixes `break`.

Tested on v5.10 on rx3600 machine (ia64 9040 CPU).

Link: https://lkml.kernel.org/r/20210221002554.333076-1-slyfox@gentoo.org
Link: https://bugs.gentoo.org/769614
Signed-off-by: Sergei Trofimovich <slyfox@gentoo.org>
Reported-by: Dmitry V. Levin <ldv@altlinux.org>
Cc: Oleg Nesterov <oleg@redhat.com>
Cc: John Paul Adrian Glaubitz <glaubitz@physik.fu-berlin.de>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>

Author: Sergei Trofimovich

arch/ia64/kernel/ptrace.c

@@ -2013,27 +2013,39 @@ static void syscall_get_set_args_cb(struct unw_frame_info *info, void *data)
 {
 	struct syscall_get_set_args *args = data;
 	struct pt_regs *pt = args->regs;
-	unsigned long *krbs, cfm, ndirty;
+	unsigned long *krbs, cfm, ndirty, nlocals, nouts;
 	int i, count;
 
 	if (unw_unwind_to_user(info) < 0)
 		return;
 
+	/*
+	 * We get here via a few paths:
+	 * - break instruction: cfm is shared with caller.
+	 *   syscall args are in out= regs, locals are non-empty.
+	 * - epsinstruction: cfm is set by br.call
+	 *   locals don't exist.
+	 *
+	 * For both cases argguments are reachable in cfm.sof - cfm.sol.
+	 * CFM: [ ... | sor: 17..14 | sol : 13..7 | sof : 6..0 ]
+	 */
 	cfm = pt->cr_ifs;
+	nlocals = (cfm >> 7) & 0x7f; /* aka sol */
+	nouts = (cfm & 0x7f) - nlocals; /* aka sof - sol */
 	krbs = (unsigned long *)info->task + IA64_RBS_OFFSET/8;
 	ndirty = ia64_rse_num_regs(krbs, krbs + (pt->loadrs >> 19));
 
 	count = 0;
 	if (in_syscall(pt))
-		count = min_t(int, args->n, cfm & 0x7f);
+		count = min_t(int, args->n, nouts);
 
+	/* Iterate over outs. */
 	for (i = 0; i < count; i++) {
+		int j = ndirty + nlocals + i + args->i;
 		if (args->rw)
-			*ia64_rse_skip_regs(krbs, ndirty + i + args->i) =
-				args->args[i];
+			*ia64_rse_skip_regs(krbs, j) = args->args[i];
 		else
-			args->args[i] = *ia64_rse_skip_regs(krbs,
-				ndirty + i + args->i);
+			args->args[i] = *ia64_rse_skip_regs(krbs, j);
 	}
 
 	if (!args->rw) {