GitLab

at 2021-04-13T22:16:26-07:00

Clean up comment and impl

In the description of the ud1 format, mention why we can't use
ext-reg-reg/mem even though it looks very much like what the ud1
format is.

In x86-arch.c, fix typo in arch_install_breakpoint that was setting
the result incorrectly (and caused a compiler warning).

@@ -2064,9 +2064,13 @@
  ;; The UD1 instruction.  The mod bits of the mod r/m byte MUST be #b11
  ;; so that the reg/mem field is actually a register.  This is a hack
 -;; to allow us to print out the reg/mem reg as a 32-bit reg.  Using
 -;; just reg/mem, the register sometimes printed out as a byte reg and
 -;; I (toy.raymond) don't know why.
 +;; to allow us to print out the reg/mem reg as a 32-bit reg.
 +;;
 +;; While the instruction looks like an ext-reg-reg/mem format with
 +;; fixed width value of 1, it isn't because we need to disassemble the
 +;; reg/mem field as a 32-bit reg. ext-reg-reg/mem needs a width prefix
 +;; byte to specify that, and we definitely don't want that.  Hence,
 +;; use a special instruction format for the UD1 instruction.
  (disassem:define-instruction-format
      (ud1 24 :default-printer '(:name :tab reg ", " reg/mem))
    (prefix    :field (byte 8 0) :value #b00001111)

@@ -220,13 +220,13 @@ arch_set_pseudo_atomic_interrupted(os_context_t * context)
  unsigned long
  arch_install_breakpoint(void *pc)
+ {
 -    unsigned long result = (unsigned char *) pc;
 +    unsigned long result = *(unsigned char *) pc;
 +    *(unsigned char *) pc = BREAKPOINT_INST;
      DPRINTF(debug_handlers,
              (stderr, "arch_install_breakpoint at %p, old code = 0x%lx\n",
               pc, result));
 -    *(unsigned char *) pc = BREAKPOINT_INST;
      return result;
+ }