[PATCH v2] hurd: Implement sigreturn for x86_64

[Sergey Bugaev <bugaevc@gmail.com>]

Here's a better version that does not do the stupid __builtin_frame_address
thing (instead, we call the function for real, and pass usp as an arg), and
should actually ensure the 16-byte alignment.

-- >8 --

Signed-off-by: Sergey Bugaev <bugaevc@gmail.com>
---
 sysdeps/mach/hurd/x86_64/sigreturn.c | 151 +++++++++++++++++++++++++++
 1 file changed, 151 insertions(+)
 create mode 100644 sysdeps/mach/hurd/x86_64/sigreturn.c

diff --git a/sysdeps/mach/hurd/x86_64/sigreturn.c b/sysdeps/mach/hurd/x86_64/sigreturn.c
new file mode 100644
index 00000000..429cee30
--- /dev/null
+++ b/sysdeps/mach/hurd/x86_64/sigreturn.c
@@ -0,0 +1,151 @@
+/* Copyright (C) 1991-2023 Free Software Foundation, Inc.
+   This file is part of the GNU C Library.
+
+   The GNU C Library is free software; you can redistribute it and/or
+   modify it under the terms of the GNU Lesser General Public
+   License as published by the Free Software Foundation; either
+   version 2.1 of the License, or (at your option) any later version.
+
+   The GNU C Library is distributed in the hope that it will be useful,
+   but WITHOUT ANY WARRANTY; without even the implied warranty of
+   MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE.  See the GNU
+   Lesser General Public License for more details.
+
+   You should have received a copy of the GNU Lesser General Public
+   License along with the GNU C Library; if not, see
+   <https://www.gnu.org/licenses/>.  */
+
+#include <hurd.h>
+#include <hurd/signal.h>
+#include <hurd/msg.h>
+#include <stdlib.h>
+
+/* This is run on the thread stack after restoring it, to be able to
+   unlock SS off sigstack.  */
+void
+__sigreturn2 (struct hurd_sigstate *ss, uintptr_t *usp)
+{
+  _hurd_sigstate_unlock (ss);
+
+  asm volatile (
+                /* Point the stack to the register dump.  */
+                "movq %0, %%rsp\n"
+                /* Pop off the registers.  */
+                "popq %%r8\n"
+                "popq %%r9\n"
+                "popq %%r10\n"
+                "popq %%r11\n"
+                "popq %%r12\n"
+                "popq %%r13\n"
+                "popq %%r14\n"
+                "popq %%r15\n"
+                "popq %%rdi\n"
+                "popq %%rsi\n"
+                "popq %%rbp\n"
+                "popq %%rbx\n"
+                "popq %%rdx\n"
+                "popq %%rcx\n"
+                "popq %%rax\n"
+                "popfq\n"
+                /* Restore %rip and %rsp with a single instruction.  */
+                "retq $128" :
+                : "rm" (usp));
+  __builtin_unreachable ();
+}
+
+int
+__sigreturn (struct sigcontext *scp)
+{
+  struct hurd_sigstate *ss;
+  struct hurd_userlink *link = (void *) &scp[1];
+  mach_port_t reply_port;
+
+  if (scp == NULL || (scp->sc_mask & _SIG_CANT_MASK))
+    {
+      errno = EINVAL;
+      return -1;
+    }
+
+  ss = _hurd_self_sigstate ();
+  _hurd_sigstate_lock (ss);
+
+  /* Remove the link on the `active resources' chain added by
+     _hurd_setup_sighandler.  Its purpose was to make sure
+     that we got called; now we have, it is done.  */
+  _hurd_userlink_unlink (link);
+
+  /* Restore the set of blocked signals, and the intr_port slot.  */
+  ss->blocked = scp->sc_mask;
+  ss->intr_port = scp->sc_intr_port;
+
+  /* Check for pending signals that were blocked by the old set.  */
+  if (_hurd_sigstate_pending (ss) & ~ss->blocked)
+    {
+      /* There are pending signals that just became unblocked.  Wake up the
+	 signal thread to deliver them.  But first, squirrel away SCP where
+	 the signal thread will notice it if it runs another handler, and
+	 arrange to have us called over again in the new reality.  */
+      ss->context = scp;
+      _hurd_sigstate_unlock (ss);
+      __msg_sig_post (_hurd_msgport, 0, 0, __mach_task_self ());
+      /* If a pending signal was handled, sig_post never returned.
+	 If it did return, the pending signal didn't run a handler;
+	 proceed as usual.  */
+      _hurd_sigstate_lock (ss);
+      ss->context = NULL;
+    }
+
+  if (scp->sc_onstack)
+    ss->sigaltstack.ss_flags &= ~SS_ONSTACK;
+
+  /* Destroy the MiG reply port used by the signal handler, and restore the
+     reply port in use by the thread when interrupted.  */
+  reply_port = THREAD_GETMEM (THREAD_SELF, reply_port);
+  THREAD_SETMEM (THREAD_SELF, reply_port, scp->sc_reply_port);
+  __mach_port_mod_refs (__mach_task_self (), reply_port,
+                        MACH_PORT_RIGHT_RECEIVE, -1);
+
+  if (scp->sc_fpused)
+    /* Restore the FPU state.  Mach conveniently stores the state
+       in the format the i387 `frstor' instruction uses to restore it.  */
+    asm volatile ("frstor %0" : : "m" (scp->sc_fpsave));
+
+  {
+    /* There are convenient instructions to pop state off the stack, so we
+       copy the registers onto the user's stack, switch there, pop and
+       return.  */
+
+    uintptr_t *usp = (uintptr_t *) scp->sc_ursp - 128;
+
+    *--usp = scp->sc_rip;
+    *--usp = scp->sc_rfl;
+    *--usp = scp->sc_rax;
+    *--usp = scp->sc_rcx;
+    *--usp = scp->sc_rdx;
+    *--usp = scp->sc_rbx;
+    *--usp = scp->sc_rbp;
+    *--usp = scp->sc_rsi;
+    *--usp = scp->sc_rdi;
+    *--usp = scp->sc_r15;
+    *--usp = scp->sc_r14;
+    *--usp = scp->sc_r13;
+    *--usp = scp->sc_r12;
+    *--usp = scp->sc_r11;
+    *--usp = scp->sc_r10;
+    *--usp = scp->sc_r9;
+    *--usp = scp->sc_r8;
+
+    /* Switch to the user's stack that we have just prepared, and call
+       __sigreturn2.  Clobber "memory" to make sure GCC flushes the stack
+       setup to actual memory.  We align the stack as per the ABI, but pass
+       the original usp to __sigreturn2 as an argument.  */
+    asm volatile ("movq %1, %%rsp\n"
+                  "andq $-16, %%rsp\n"
+                  "call __sigreturn2" :
+                  : "D" (ss), "S" (usp)
+                  : "memory");
+    __builtin_unreachable ();
+  }
+}
+
+weak_alias (__sigreturn, sigreturn)
-- 
2.39.2