Make Sure Error Code Is Positive & Update Notes

Author: CuriousTommy

gen/bsdsyscalls/SYS.h

@@ -476,8 +476,13 @@ pseudo:									;\
  */
 
 #ifdef DARLING
-// In addition to changing the macro to use __darling_handle_svc instead of the
-// syscall (svc) instruction, We are also setting the carry flag as well.
+// Changes compared to Apple's implementation:
+//   * Replace `svc` instruction by calling `__darling_handle_svc` (as a 
+//     consequence, an additional `PUSH_FRAME` and `POP_FRAME` has been
+//     added)
+//   * Set the carry flag, by using the `cmn` instruction.
+//   * Convert the negative error code into a positive error code before
+//     calling `cerror` (errno must be a positive integer).
 #define DO_SYSCALL(num, cerror)                 \
 	mov   x16, #(num)                     %%\
 	PUSH_FRAME                            %%\
@@ -487,6 +492,7 @@ pseudo:									;\
 	b.cc  2f                              %%\
 	ARM64_STACK_PROLOG                    %%\
 	PUSH_FRAME                            %%\
+	neg x0, x0                            %%\
 	bl    _##cerror                       %%\
 	POP_FRAME                             %%\
 	ARM64_STACK_EPILOG                    %%\

libsyscall/custom/SYS.h

@@ -476,8 +476,13 @@ pseudo:									;\
  */
 
 #ifdef DARLING
-// In addition to changing the macro to use __darling_handle_svc instead of the
-// syscall (svc) instruction, We are also setting the carry flag as well.
+// Changes compared to Apple's implementation:
+//   * Replace `svc` instruction by calling `__darling_handle_svc` (as a 
+//     consequence, an additional `PUSH_FRAME` and `POP_FRAME` has been
+//     added)
+//   * Set the carry flag, by using the `cmn` instruction.
+//   * Convert the negative error code into a positive error code before
+//     calling `cerror` (errno must be a positive integer).
 #define DO_SYSCALL(num, cerror)                 \
 	mov   x16, #(num)                     %%\
 	PUSH_FRAME                            %%\
@@ -487,6 +492,7 @@ pseudo:									;\
 	b.cc  2f                              %%\
 	ARM64_STACK_PROLOG                    %%\
 	PUSH_FRAME                            %%\
+	neg x0, x0                            %%\
 	bl    _##cerror                       %%\
 	POP_FRAME                             %%\
 	ARM64_STACK_EPILOG                    %%\