target/s390x: Fix translation exception on illegal instruction

Hitting an uretprobe in a s390x TCG guest causes a SIGSEGV. What
happens is:

* uretprobe maps a userspace page containing an invalid instruction.
* uretprobe replaces the target function's return address with the
  address of that page.
* When tb_gen_code() is called on that page, tb->size ends up being 0
  (because the page starts with the invalid instruction), which causes
  virt_page2 to point to the previous page.
* The previous page is not mapped, so this causes a spurious
  translation exception.

tb->size must never be 0: even if there is an illegal instruction, the
instruction bytes that have been looked at must count towards tb->size.
So adjust s390x's translate_one() to act this way for both illegal
instructions and instructions that are known to generate exceptions.

Signed-off-by: Ilya Leoshkevich <iii@linux.ibm.com>
Reviewed-by: David Hildenbrand <david@redhat.com>
Message-Id: <20210416154939.32404-2-iii@linux.ibm.com>
Signed-off-by: Cornelia Huck <cohuck@redhat.com>

target/s390x/translate.c

@@ -6412,7 +6412,8 @@ static DisasJumpType translate_one(CPUS390XState *env, DisasContext *s)
         qemu_log_mask(LOG_UNIMP, "unimplemented opcode 0x%02x%02x\n",
                       s->fields.op, s->fields.op2);
         gen_illegal_opcode(s);
-        return DISAS_NORETURN;
+        ret = DISAS_NORETURN;
+        goto out;
     }
 
 #ifndef CONFIG_USER_ONLY
@@ -6428,7 +6429,8 @@ static DisasJumpType translate_one(CPUS390XState *env, DisasContext *s)
         /* privileged instruction */
         if ((s->base.tb->flags & FLAG_MASK_PSTATE) && (insn->flags & IF_PRIV)) {
             gen_program_exception(s, PGM_PRIVILEGED);
-            return DISAS_NORETURN;
+            ret = DISAS_NORETURN;
+            goto out;
         }
 
         /* if AFP is not enabled, instructions and registers are forbidden */
@@ -6455,15 +6457,17 @@ static DisasJumpType translate_one(CPUS390XState *env, DisasContext *s)
             }
             if (dxc) {
                 gen_data_exception(dxc);
-                return DISAS_NORETURN;
+                ret = DISAS_NORETURN;
+                goto out;
             }
         }
 
         /* if vector instructions not enabled, executing them is forbidden */
         if (insn->flags & IF_VEC) {
             if (!((s->base.tb->flags & FLAG_MASK_VECTOR))) {
                 gen_data_exception(0xfe);
-                return DISAS_NORETURN;
+                ret = DISAS_NORETURN;
+                goto out;
             }
         }
 
@@ -6484,7 +6488,8 @@ static DisasJumpType translate_one(CPUS390XState *env, DisasContext *s)
             (insn->spec & SPEC_r1_f128 && !is_fp_pair(get_field(s, r1))) ||
             (insn->spec & SPEC_r2_f128 && !is_fp_pair(get_field(s, r2)))) {
             gen_program_exception(s, PGM_SPECIFICATION);
-            return DISAS_NORETURN;
+            ret = DISAS_NORETURN;
+            goto out;
         }
     }
 
@@ -6544,6 +6549,7 @@ static DisasJumpType translate_one(CPUS390XState *env, DisasContext *s)
     }
 #endif
 
+out:
     /* Advance to the next instruction.  */
     s->base.pc_next = s->pc_tmp;
     return ret;