semihosting: Check for errors on SET_ARG()

The SET_ARG() macro returns an error indication; we check this in the
TARGET_SYS_GET_CMDLINE case but not when we use it in implementing
TARGET_SYS_ELAPSED.  Check for and handle the errors via the do_fault
codepath, and update the comment documenting the SET_ARG() and
GET_ARG() macros to note how they handle memory access errors.

Resolves: Coverity CID 1490287
Signed-off-by: Peter Maydell <peter.maydell@linaro.org>
Reviewed-by: Richard Henderson <richard.henderson@linaro.org>
Message-Id: <20220719121110.225657-4-peter.maydell@linaro.org>
Signed-off-by: Alex Bennée <alex.bennee@linaro.org>
Message-Id: <20220725140520.515340-9-alex.bennee@linaro.org>

semihosting/arm-compat-semi.c

@@ -171,6 +171,12 @@ static LayoutInfo common_semi_find_bases(CPUState *cs)
  * Read the input value from the argument block; fail the semihosting
  * call if the memory read fails. Eventually we could use a generic
  * CPUState helper function here.
+ * Note that GET_ARG() handles memory access errors by jumping to
+ * do_fault, so must be used as the first thing done in handling a
+ * semihosting call, to avoid accidentally leaking allocated resources.
+ * SET_ARG(), since it unavoidably happens late, instead returns an
+ * error indication (0 on success, non-0 for error) which the caller
+ * should check.
  */
 
 #define GET_ARG(n) do {                                 \
@@ -739,10 +745,14 @@ void do_common_semihosting(CPUState *cs)
     case TARGET_SYS_ELAPSED:
         elapsed = get_clock() - clock_start;
         if (sizeof(target_ulong) == 8) {
-            SET_ARG(0, elapsed);
+            if (SET_ARG(0, elapsed)) {
+                goto do_fault;
+            }
         } else {
-            SET_ARG(0, (uint32_t) elapsed);
-            SET_ARG(1, (uint32_t) (elapsed >> 32));
+            if (SET_ARG(0, (uint32_t) elapsed) ||
+                SET_ARG(1, (uint32_t) (elapsed >> 32))) {
+                goto do_fault;
+            }
         }
         common_semi_set_ret(cs, 0);
         break;