hurd: Remove the ecx kludge

"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>
Message-Id: <20230301162355.426887-1-bugaevc@gmail.com>

1 files changed, 13 insertions, 5 deletions

diff --git a/hurd/hurdsig.c b/hurd/hurdsig.c
index ea79ffb..5ff0a91 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;
     }