aarch64: Use br instead of ret for eh_return

The expected way to handle eh_return is to pass the stack adjustment
offset and landing pad address via

  EH_RETURN_STACKADJ_RTX
  EH_RETURN_HANDLER_RTX

to the epilogue that is shared between normal return paths and the
eh_return paths.  EH_RETURN_HANDLER_RTX is the stack slot of the
return address that is overwritten with the landing pad in the
eh_return case and EH_RETURN_STACKADJ_RTX is a register added to sp
right before return and it is set to 0 in the normal return case.

The issue with this design is that eh_return and normal return may
require different return sequence but there is no way to distinguish
the two cases in the epilogue (the stack adjustment may be 0 in the
eh_return case too).

The reason eh_return and normal return requires different return
sequence is that control flow integrity hardening may need to treat
eh_return as a forward-edge transfer (it is not returning to the
previous stack frame) and normal return as a backward-edge one.
In case of AArch64 forward-edge is protected by BTI and requires br
instruction and backward-edge is protected by PAUTH or GCS and
requires ret (or authenticated ret) instruction.

This patch resolves the issue by introducing EH_RETURN_TAKEN_RTX that
is a flag set to 1 in the eh_return path and 0 in normal return paths.
Branching on the EH_RETURN_TAKEN_RTX flag, the right return sequence
can be used in the epilogue.

The handler could be passed the old way via clobbering the return
address, but since now the eh_return case can be distinguished, the
handler can be in a different register than x30 and no stack frame
is needed for eh_return.

This patch fixes a return to anywhere gadget in the unwinder with
existing standard branch protection as well as makes EH return
compatible with the Guarded Control Stack (GCS) extension.

Some tests are adjusted because eh_return no longer prevents pac-ret
in the normal return path.

gcc/ChangeLog:

	* config/aarch64/aarch64-protos.h (aarch64_eh_return_handler_rtx):
	Remove.
	* config/aarch64/aarch64.cc (aarch64_return_address_signing_enabled):
	Sign return address even in functions with eh_return.
	(aarch64_expand_epilogue): Conditionally return with br or ret.
	(aarch64_eh_return_handler_rtx): Remove.
	* config/aarch64/aarch64.h (EH_RETURN_TAKEN_RTX): Define.
	(EH_RETURN_STACKADJ_RTX): Change to R5.
	(EH_RETURN_HANDLER_RTX): Change to R6.
	* df-scan.cc: Handle EH_RETURN_TAKEN_RTX.
	* doc/tm.texi: Regenerate.
	* doc/tm.texi.in: Document EH_RETURN_TAKEN_RTX.
	* except.cc (expand_eh_return): Handle EH_RETURN_TAKEN_RTX.

gcc/testsuite/ChangeLog:

	* gcc.target/aarch64/return_address_sign_1.c: Move func4 to ...
	* gcc.target/aarch64/return_address_sign_2.c: ... here and fix the
	scan asm check.
	* gcc.target/aarch64/return_address_sign_b_1.c: Move func4 to ...
	* gcc.target/aarch64/return_address_sign_b_2.c: ... here and fix the
	scan asm check.

gcc/config/aarch64/aarch64-protos.h

@@ -861,7 +861,6 @@ machine_mode aarch64_hard_regno_caller_save_mode (unsigned, unsigned,
 						       machine_mode);
 int aarch64_uxt_size (int, HOST_WIDE_INT);
 int aarch64_vec_fpconst_pow_of_2 (rtx);
-rtx aarch64_eh_return_handler_rtx (void);
 rtx aarch64_mask_from_zextract_ops (rtx, rtx);
 const char *aarch64_output_move_struct (rtx *operands);
 rtx aarch64_return_addr_rtx (void);

gcc/config/aarch64/aarch64.cc

@@ -6758,17 +6758,6 @@ aarch64_return_address_signing_enabled (void)
   /* This function should only be called after frame laid out.   */
   gcc_assert (cfun->machine->frame.laid_out);
 
-  /* Turn return address signing off in any function that uses
-     __builtin_eh_return.  The address passed to __builtin_eh_return
-     is not signed so either it has to be signed (with original sp)
-     or the code path that uses it has to avoid authenticating it.
-     Currently eh return introduces a return to anywhere gadget, no
-     matter what we do here since it uses ret with user provided
-     address. An ideal fix for that is to use indirect branch which
-     can be protected with BTI j (to some extent).  */
-  if (crtl->calls_eh_return)
-    return false;
-
   /* If signing scope is AARCH_FUNCTION_NON_LEAF, we only sign a leaf function
      if its LR is pushed onto stack.  */
   return (aarch_ra_sign_scope == AARCH_FUNCTION_ALL
@@ -8113,6 +8102,30 @@ aarch64_expand_epilogue (bool for_sibcall)
       RTX_FRAME_RELATED_P (insn) = 1;
     }
 
+  /* Stack adjustment for exception handler.  */
+  if (crtl->calls_eh_return && !for_sibcall)
+    {
+      /* If the EH_RETURN_TAKEN_RTX flag is set then we need
+	 to unwind the stack and jump to the handler, otherwise
+	 skip this eh_return logic and continue with normal
+	 return after the label.  We have already reset the CFA
+	 to be SP; letting the CFA move during this adjustment
+	 is just as correct as retaining the CFA from the body
+	 of the function.  Therefore, do nothing special.  */
+      rtx label = gen_label_rtx ();
+      rtx x = gen_rtx_EQ (VOIDmode, EH_RETURN_TAKEN_RTX, const0_rtx);
+      x = gen_rtx_IF_THEN_ELSE (VOIDmode, x,
+				gen_rtx_LABEL_REF (Pmode, label), pc_rtx);
+      rtx jump = emit_jump_insn (gen_rtx_SET (pc_rtx, x));
+      JUMP_LABEL (jump) = label;
+      LABEL_NUSES (label)++;
+      emit_insn (gen_add2_insn (stack_pointer_rtx,
+				EH_RETURN_STACKADJ_RTX));
+      emit_jump_insn (gen_indirect_jump (EH_RETURN_HANDLER_RTX));
+      emit_barrier ();
+      emit_label (label);
+    }
+
   /* We prefer to emit the combined return/authenticate instruction RETAA,
      however there are three cases in which we must instead emit an explicit
      authentication instruction.
@@ -8142,58 +8155,11 @@ aarch64_expand_epilogue (bool for_sibcall)
       RTX_FRAME_RELATED_P (insn) = 1;
     }
 
-  /* Stack adjustment for exception handler.  */
-  if (crtl->calls_eh_return && !for_sibcall)
-    {
-      /* We need to unwind the stack by the offset computed by
-	 EH_RETURN_STACKADJ_RTX.  We have already reset the CFA
-	 to be SP; letting the CFA move during this adjustment
-	 is just as correct as retaining the CFA from the body
-	 of the function.  Therefore, do nothing special.  */
-      emit_insn (gen_add2_insn (stack_pointer_rtx, EH_RETURN_STACKADJ_RTX));
-    }
-
   emit_use (gen_rtx_REG (DImode, LR_REGNUM));
   if (!for_sibcall)
     emit_jump_insn (ret_rtx);
 }
 
-/* Implement EH_RETURN_HANDLER_RTX.  EH returns need to either return
-   normally or return to a previous frame after unwinding.
-
-   An EH return uses a single shared return sequence.  The epilogue is
-   exactly like a normal epilogue except that it has an extra input
-   register (EH_RETURN_STACKADJ_RTX) which contains the stack adjustment
-   that must be applied after the frame has been destroyed.  An extra label
-   is inserted before the epilogue which initializes this register to zero,
-   and this is the entry point for a normal return.
-
-   An actual EH return updates the return address, initializes the stack
-   adjustment and jumps directly into the epilogue (bypassing the zeroing
-   of the adjustment).  Since the return address is typically saved on the
-   stack when a function makes a call, the saved LR must be updated outside
-   the epilogue.
-
-   This poses problems as the store is generated well before the epilogue,
-   so the offset of LR is not known yet.  Also optimizations will remove the
-   store as it appears dead, even after the epilogue is generated (as the
-   base or offset for loading LR is different in many cases).
-
-   To avoid these problems this implementation forces the frame pointer
-   in eh_return functions so that the location of LR is fixed and known early.
-   It also marks the store volatile, so no optimization is permitted to
-   remove the store.  */
-rtx
-aarch64_eh_return_handler_rtx (void)
-{
-  rtx tmp = gen_frame_mem (Pmode,
-    plus_constant (Pmode, hard_frame_pointer_rtx, UNITS_PER_WORD));
-
-  /* Mark the store volatile, so no optimization is permitted to remove it.  */
-  MEM_VOLATILE_P (tmp) = true;
-  return tmp;
-}
-
 /* Output code to add DELTA to the first argument, and then jump
    to FUNCTION.  Used for C++ multiple inheritance.  */
 static void

gcc/config/aarch64/aarch64.h

@@ -583,9 +583,12 @@ enum class aarch64_feature : unsigned char {
 /* Output assembly strings after .cfi_startproc is emitted.  */
 #define ASM_POST_CFI_STARTPROC  aarch64_post_cfi_startproc
 
-/* For EH returns X4 contains the stack adjustment.  */
-#define EH_RETURN_STACKADJ_RTX	gen_rtx_REG (Pmode, R4_REGNUM)
-#define EH_RETURN_HANDLER_RTX  aarch64_eh_return_handler_rtx ()
+/* For EH returns X4 is a flag that is set in the EH return
+   code paths and then X5 and X6 contain the stack adjustment
+   and return address respectively.  */
+#define EH_RETURN_TAKEN_RTX	gen_rtx_REG (Pmode, R4_REGNUM)
+#define EH_RETURN_STACKADJ_RTX	gen_rtx_REG (Pmode, R5_REGNUM)
+#define EH_RETURN_HANDLER_RTX	gen_rtx_REG (Pmode, R6_REGNUM)
 
 #undef TARGET_COMPUTE_FRAME_LAYOUT
 #define TARGET_COMPUTE_FRAME_LAYOUT aarch64_layout_frame

gcc/df-scan.cc

@@ -3720,6 +3720,16 @@ df_get_exit_block_use_set (bitmap exit_block_uses)
     }
 #endif
 
+#ifdef EH_RETURN_TAKEN_RTX
+  if ((!targetm.have_epilogue () || ! epilogue_completed)
+      && crtl->calls_eh_return)
+    {
+      rtx tmp = EH_RETURN_TAKEN_RTX;
+      if (tmp && REG_P (tmp))
+	df_mark_reg (tmp, exit_block_uses);
+    }
+#endif
+
   if ((!targetm.have_epilogue () || ! epilogue_completed)
       && crtl->calls_eh_return)
     {

gcc/doc/tm.texi

@@ -3507,6 +3507,18 @@ If you want to support call frame exception handling, you must
 define either this macro or the @code{eh_return} instruction pattern.
 @end defmac
 
+@defmac EH_RETURN_TAKEN_RTX
+A C expression whose value is RTL representing a location in which
+to store if the EH return path was taken instead of a normal return.
+This macro allows conditionally executing different code in the
+epilogue for the EH and normal return cases.
+
+When this macro is defined, the macros @code{EH_RETURN_STACKADJ_RTX}
+and @code{EH_RETURN_HANDLER_RTX} are only meaningful in the epilogue
+when 1 is stored to the specified location. The value 0 means normal
+return.
+@end defmac
+
 @defmac RETURN_ADDR_OFFSET
 If defined, an integer-valued C expression for which rtl will be generated
 to add it to the exception handler address before it is searched in the

gcc/doc/tm.texi.in

@@ -2743,6 +2743,18 @@ If you want to support call frame exception handling, you must
 define either this macro or the @code{eh_return} instruction pattern.
 @end defmac
 
+@defmac EH_RETURN_TAKEN_RTX
+A C expression whose value is RTL representing a location in which
+to store if the EH return path was taken instead of a normal return.
+This macro allows conditionally executing different code in the
+epilogue for the EH and normal return cases.
+
+When this macro is defined, the macros @code{EH_RETURN_STACKADJ_RTX}
+and @code{EH_RETURN_HANDLER_RTX} are only meaningful in the epilogue
+when 1 is stored to the specified location. The value 0 means normal
+return.
+@end defmac
+
 @defmac RETURN_ADDR_OFFSET
 If defined, an integer-valued C expression for which rtl will be generated
 to add it to the exception handler address before it is searched in the

gcc/except.cc

@@ -2281,6 +2281,10 @@ expand_eh_return (void)
   emit_move_insn (EH_RETURN_STACKADJ_RTX, const0_rtx);
 #endif
 
+#ifdef EH_RETURN_TAKEN_RTX
+  emit_move_insn (EH_RETURN_TAKEN_RTX, const0_rtx);
+#endif
+
   around_label = gen_label_rtx ();
   emit_jump (around_label);
 
@@ -2291,6 +2295,10 @@ expand_eh_return (void)
   emit_move_insn (EH_RETURN_STACKADJ_RTX, crtl->eh.ehr_stackadj);
 #endif
 
+#ifdef EH_RETURN_TAKEN_RTX
+  emit_move_insn (EH_RETURN_TAKEN_RTX, const1_rtx);
+#endif
+
   if (targetm.have_eh_return ())
     emit_insn (targetm.gen_eh_return (crtl->eh.ehr_handler));
   else
@@ -2301,7 +2309,19 @@ expand_eh_return (void)
 	error ("%<__builtin_eh_return%> not supported on this target");
     }
 
+#ifdef EH_RETURN_TAKEN_RTX
+  rtx_code_label *eh_done_label = gen_label_rtx ();
+  emit_jump (eh_done_label);
+#endif
+
   emit_label (around_label);
+
+#ifdef EH_RETURN_TAKEN_RTX
+  for (rtx tmp : { EH_RETURN_STACKADJ_RTX, EH_RETURN_HANDLER_RTX })
+    if (tmp && REG_P (tmp))
+      emit_clobber (tmp);
+  emit_label (eh_done_label);
+#endif
 }
 
 /* Convert a ptr_mode address ADDR_TREE to a Pmode address controlled by

gcc/testsuite/gcc.target/aarch64/return_address_sign_1.c

@@ -37,16 +37,5 @@ func3 (int a, int b, int c)
   /* autiasp */
 }
 
-/* eh_return.  */
-void __attribute__ ((target ("arch=armv8.3-a")))
-func4 (long offset, void *handler, int *ptr, int imm1, int imm2)
-{
-  /* no paciasp */
-  *ptr = imm1 + foo (imm1) + imm2;
-  __builtin_eh_return (offset, handler);
-  /* no autiasp */
-  return;
-}
-
-/* { dg-final { scan-assembler-times "autiasp" 3 } } */
 /* { dg-final { scan-assembler-times "paciasp" 3 } } */
+/* { dg-final { scan-assembler-times "autiasp" 3 } } */

gcc/testsuite/gcc.target/aarch64/return_address_sign_2.c

@@ -14,5 +14,18 @@ func1 (int a, int b, int c)
   /* retaa */
 }
 
-/* { dg-final { scan-assembler-times "paciasp" 1 } } */
-/* { dg-final { scan-assembler-times "retaa" 1 } } */
+/* eh_return.  */
+void __attribute__ ((target ("arch=armv8.3-a")))
+func4 (long offset, void *handler, int *ptr, int imm1, int imm2)
+{
+  /* paciasp */
+  *ptr = imm1 + foo (imm1) + imm2;
+  if (handler)
+    /* br */
+    __builtin_eh_return (offset, handler);
+  /* retaa */
+  return;
+}
+
+/* { dg-final { scan-assembler-times "paciasp" 2 } } */
+/* { dg-final { scan-assembler-times "retaa" 2 } } */

gcc/testsuite/gcc.target/aarch64/return_address_sign_b_1.c

@@ -37,16 +37,5 @@ func3 (int a, int b, int c)
   /* autibsp */
 }
 
-/* eh_return.  */
-void __attribute__ ((target ("arch=armv8.3-a")))
-func4 (long offset, void *handler, int *ptr, int imm1, int imm2)
-{
-  /* no pacibsp */
-  *ptr = imm1 + foo (imm1) + imm2;
-  __builtin_eh_return (offset, handler);
-  /* no autibsp */
-  return;
-}
-
 /* { dg-final { scan-assembler-times "pacibsp" 3 } } */
 /* { dg-final { scan-assembler-times "autibsp" 3 } } */

gcc/testsuite/gcc.target/aarch64/return_address_sign_b_2.c

@@ -14,5 +14,18 @@ func1 (int a, int b, int c)
   /* retab */
 }
 
-/* { dg-final { scan-assembler-times "pacibsp" 1 } } */
-/* { dg-final { scan-assembler-times "retab" 1 } } */
+/* eh_return.  */
+void __attribute__ ((target ("arch=armv8.3-a")))
+func4 (long offset, void *handler, int *ptr, int imm1, int imm2)
+{
+  /* pacibsp */
+  *ptr = imm1 + foo (imm1) + imm2;
+  if (handler)
+    /* br */
+    __builtin_eh_return (offset, handler);
+  /* retab */
+  return;
+}
+
+/* { dg-final { scan-assembler-times "pacibsp" 2 } } */
+/* { dg-final { scan-assembler-times "retab" 2 } } */