[PATCH 1/2] hurd: Remove the ecx kludge

[Sergey Bugaev <bugaevc@gmail.com>]

"We don't need it any more"

The INTR_MSG_TRAP macro in intr-msg.h used to play little trick with
the stack pointer: it would temporarily save the "real" stack pointer
into ecx, while setting esp to point to just before the message buffer,
and then invoke the mach_msg trap. This way, INTR_MSG_TRAP reused the
on-stack arguments laid out for the containing call of
_hurd_intr_rpc_mach_msg (), passing them to the mach_msg trap directly.

This, however, required special support in hurdsig.c and trampoline.c,
since they now had to recognize when a thread is inside the piece of
code where esp doesn't point to the real tip of the stack, and handle
this situation specially.

Commit 1d20f33ff4fb634310f27493b7b87d0b20f4a0b0 has removed the actual
temporary change of esp by actually re-pushing mach_msg arguments onto
the stack, and popping them back at end. It did not, however, deal with
the rest of "the ecx kludge" code in other files, resulting in potential
crashes if a signal arrives in the middle of pushing arguments onto the
stack.

Fix that by removing "the ecx kludge". Instead, when we want a thread
to skip the RPC, but cannot make just make it jump to after the trap
since it's not done adjusting the stack yet, set the SYSRETURN register
to MACH_SEND_INTERRUPTED (as we do anyway), and rely on the thread
itself for detecting this case and skipping the RPC.

This simplifies things somewhat and paves the way for a future x86_64
port of this code.

Signed-off-by: Sergey Bugaev <bugaevc@gmail.com>
---
 hurd/hurdsig.c                      | 18 +++++++++----
 sysdeps/mach/hurd/i386/intr-msg.h   | 40 +++++++++++++----------------
 sysdeps/mach/hurd/i386/trampoline.c | 21 ---------------
 3 files changed, 31 insertions(+), 48 deletions(-)

diff --git a/hurd/hurdsig.c b/hurd/hurdsig.c
index ea79ffb5..5ff0a91f 100644
--- a/hurd/hurdsig.c
+++ b/hurd/hurdsig.c
@@ -415,6 +415,7 @@ _hurdsig_abort_rpcs (struct hurd_sigstate *ss, int signo, int sigthread,
 		     void (*reply) (void))
 {
   extern const void _hurd_intr_rpc_msg_about_to;
+  extern const void _hurd_intr_rpc_msg_setup_done;
   extern const void _hurd_intr_rpc_msg_in_trap;
   mach_port_t rcv_port = MACH_PORT_NULL;
   mach_port_t intr_port;
@@ -434,11 +435,18 @@ _hurdsig_abort_rpcs (struct hurd_sigstate *ss, int signo, int sigthread,
       && state->basic.PC < (uintptr_t) &_hurd_intr_rpc_msg_in_trap)
     {
       /* The thread is about to do the RPC, but hasn't yet entered
-	 mach_msg.  Mutate the thread's state so it knows not to try
-	 the RPC.  */
-      INTR_MSG_BACK_OUT (&state->basic);
-      MACHINE_THREAD_STATE_SET_PC (&state->basic,
-				   &_hurd_intr_rpc_msg_in_trap);
+         mach_msg.  Importantly, it may have already checked ss->cancel for
+         the last time before doing the RPC, so setting that is not enough
+         to make it not enter mach_msg.  Instead, mutate the thread's state
+         so it knows not to try the RPC.
+
+         If the thread is past _hurd_intr_rpc_msg_setup_done, just make it
+         jump to after the trap, since we know it's safe to do so.  Otherwise,
+         we know that the thread is yet to check for the MACH_SEND_INTERRUPTED
+         value we set below, and will skip the trap by itself.  */
+      if (state->basic.PC >= (uintptr_t) &_hurd_intr_rpc_msg_setup_done)
+        MACHINE_THREAD_STATE_SET_PC (&state->basic,
+                                     &_hurd_intr_rpc_msg_in_trap);
       state->basic.SYSRETURN = MACH_SEND_INTERRUPTED;
       *state_change = 1;
     }
diff --git a/sysdeps/mach/hurd/i386/intr-msg.h b/sysdeps/mach/hurd/i386/intr-msg.h
index 29cb4620..21088fa8 100644
--- a/sysdeps/mach/hurd/i386/intr-msg.h
+++ b/sysdeps/mach/hurd/i386/intr-msg.h
@@ -25,10 +25,13 @@
 ({									      \
   error_t err;								      \
   asm (".globl _hurd_intr_rpc_msg_about_to\n"				      \
-       ".globl _hurd_intr_rpc_msg_cx_sp\n"				      \
-       ".globl _hurd_intr_rpc_msg_do_trap\n" 				      \
+       ".globl _hurd_intr_rpc_msg_setup_done\n"				      \
        ".globl _hurd_intr_rpc_msg_in_trap\n"				      \
-       ".globl _hurd_intr_rpc_msg_sp_restored\n"			      \
+       /* Clear eax before we do the check for cancel below.  This is to
+          detect eax being set to non-zero (actually MACH_SEND_INTERRUPTED)
+          from the outside (namely, _hurdsig_abort_rpcs), which signals us
+          to skip the trap we were about to enter.  */			      \
+       "				xorl %0, %0\n"			      \
        "_hurd_intr_rpc_msg_about_to:"					      \
        /* We need to make a last check of cancel, in case we got interrupted
           right before _hurd_intr_rpc_msg_about_to.  */			      \
@@ -36,10 +39,10 @@
        "				jz _hurd_intr_rpc_msg_do\n"	      \
        /* We got interrupted, note so and return EINTR.  */		      \
        "				movl $0, %3\n"			      \
-       "				movl %6, %%eax\n"		      \
+       "				movl %6, %0\n"			      \
        "				jmp _hurd_intr_rpc_msg_sp_restored\n" \
        "_hurd_intr_rpc_msg_do:"						      \
-       /* Ok, push the mach_msg_trap arguments.  */			      \
+       /* Ok, push the mach_msg_trap arguments and a fake return address.  */ \
        "				pushl 24(%4)\n"			      \
        "				pushl %2\n"			      \
        "				pushl 16(%4)\n"			      \
@@ -48,9 +51,14 @@
        "				pushl %1\n"			      \
        "				pushl (%4)\n"			      \
        "				pushl $0\n"			      \
-       /* TODO: remove this ecx kludge, we don't need it any more */	      \
-       "				movl %%esp, %%ecx\n"		      \
-       "_hurd_intr_rpc_msg_cx_sp:	movl $-25, %%eax\n"		      \
+       "_hurd_intr_rpc_msg_setup_done:"					      \
+       /* From here on, it is safe to make us jump over the syscall.  Now
+          check if we have been told to skip the syscall while running
+          the above.  */						      \
+       "				test %0, %0\n"			      \
+       "				jnz _hurd_intr_rpc_msg_in_trap\n"     \
+       /* Do the actual syscall.  */					      \
+       "				movl $-25, %%eax\n"		      \
        "_hurd_intr_rpc_msg_do_trap:	lcall $7, $0 # status in %0\n"	      \
        "_hurd_intr_rpc_msg_in_trap:"					      \
        /* Ok, clean the arguments and update OPTION and TIMEOUT.  */	      \
@@ -60,22 +68,10 @@
        "				popl %2\n"			      \
        "				addl $4, %%esp\n"		      \
        "_hurd_intr_rpc_msg_sp_restored:"				      \
-       : "=a" (err), "+r" (option), "+r" (timeout), "=m" (*intr_port_p)	      \
-       : "r" (&msg), "m" (*cancel_p), "i" (EINTR)			      \
-       : "ecx");							      \
+       : "=&a" (err), "+r" (option), "+r" (timeout), "=m" (*intr_port_p)      \
+       : "r" (&msg), "m" (*cancel_p), "i" (EINTR));			      \
   err;									      \
 })
-
-
-static void inline
-INTR_MSG_BACK_OUT (struct i386_thread_state *state)
-{
-  extern const void _hurd_intr_rpc_msg_cx_sp;
-  if (state->eip >= (natural_t) &_hurd_intr_rpc_msg_cx_sp)
-    state->uesp = state->ecx;
-  else
-    state->ecx = state->uesp;
-}
 \f
 #include "hurdfault.h"
 
diff --git a/sysdeps/mach/hurd/i386/trampoline.c b/sysdeps/mach/hurd/i386/trampoline.c
index 42c9d732..8f481e79 100644
--- a/sysdeps/mach/hurd/i386/trampoline.c
+++ b/sysdeps/mach/hurd/i386/trampoline.c
@@ -89,8 +89,6 @@ _hurd_setup_sighandler (struct hurd_sigstate *ss, const struct sigaction *action
   void trampoline (void);
   void rpc_wait_trampoline (void);
   void firewall (void);
-  extern const void _hurd_intr_rpc_msg_cx_sp;
-  extern const void _hurd_intr_rpc_msg_sp_restored;
   void *volatile sigsp;
   struct sigcontext *scp;
   struct
@@ -146,25 +144,6 @@ _hurd_setup_sighandler (struct hurd_sigstate *ss, const struct sigaction *action
      interrupted RPC frame.  */
   state->basic.esp = state->basic.uesp;
 
-  /* This code has intimate knowledge of the special mach_msg system call
-     done in intr-msg.c; that code does (see intr-msg.h):
-					movl %esp, %ecx
-					leal ARGS, %esp
-	_hurd_intr_rpc_msg_cx_sp:	movl $-25, %eax
-	_hurd_intr_rpc_msg_do_trap:	lcall $7, $0
-	_hurd_intr_rpc_msg_in_trap:	movl %ecx, %esp
-	_hurd_intr_rpc_msg_sp_restored:
-     We must check for the window during which %esp points at the
-     mach_msg arguments.  The space below until %ecx is used by
-     the _hurd_intr_rpc_mach_msg frame, and must not be clobbered.  */
-  if (state->basic.eip >= (int) &_hurd_intr_rpc_msg_cx_sp
-      && state->basic.eip < (int) &_hurd_intr_rpc_msg_sp_restored)
-  /* The SP now points at the mach_msg args, but there is more stack
-     space used below it.  The real SP is saved in %ecx; we must push the
-     new frame below there (if not on the altstack), and restore that value as
-     the SP on sigreturn.  */
-    state->basic.uesp = state->basic.ecx;
-
   if ((action->sa_flags & SA_ONSTACK)
       && !(ss->sigaltstack.ss_flags & (SS_DISABLE|SS_ONSTACK)))
     {
-- 
2.39.2