[PATCH] ia64: fix ia64_syscall_get_set_arguments() for break-based syscalls

[Sergei Trofimovich]

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).

CC: Oleg Nesterov <o...@redhat.com>
CC: linux-i...@vger.kernel.org
CC: linux-kernel@vger.kernel.org
CC: Andrew Morton <a...@linux-foundation.org>
Reported-by: Dmitry V. Levin <l...@altlinux.org>
Bug: https://bugs.gentoo.org/769614
Signed-off-by: Sergei Trofimovich <sly...@gentoo.org>
---
 arch/ia64/kernel/ptrace.c | 24 ++++++++++++++++++------
 1 file changed, 18 insertions(+), 6 deletions(-)

diff --git a/arch/ia64/kernel/ptrace.c b/arch/ia64/kernel/ptrace.c
index c3490ee2daa5..e14f5653393a 100644
--- a/arch/ia64/kernel/ptrace.c
+++ b/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) {
-- 
2.30.1