semihosting: Fix handling of buffer in TARGET_SYS_TMPNAM

The TARGET_SYS_TMPNAM implementation has two bugs spotted by
Coverity:
 * confusion about whether 'len' has the length of the string
   including or excluding the terminating NUL means we
   lock_user() len bytes of memory but memcpy() len + 1 bytes
 * In the error-exit cases we forget to free() the buffer
   that asprintf() returned to us

Resolves: Coverity CID 1490285, 1490289
Signed-off-by: Peter Maydell <peter.maydell@linaro.org>
Reviewed-by: Richard Henderson <richard.henderson@linaro.org>
Message-Id: <20220719121110.225657-5-peter.maydell@linaro.org>
Signed-off-by: Alex Bennée <alex.bennee@linaro.org>
Message-Id: <20220725140520.515340-10-alex.bennee@linaro.org>

semihosting/arm-compat-semi.c

@@ -504,16 +504,25 @@ void do_common_semihosting(CPUState *cs)
         GET_ARG(1);
         GET_ARG(2);
         len = asprintf(&s, "/tmp/qemu-%x%02x", getpid(), (int)arg1 & 0xff);
+        if (len < 0) {
+            common_semi_set_ret(cs, -1);
+            break;
+        }
+
+        /* Allow for trailing NUL */
+        len++;
         /* Make sure there's enough space in the buffer */
-        if (len < 0 || len >= arg2) {
+        if (len > arg2) {
+            free(s);
             common_semi_set_ret(cs, -1);
             break;
         }
         p = lock_user(VERIFY_WRITE, arg0, len, 0);
         if (!p) {
+            free(s);
             goto do_fault;
         }
-        memcpy(p, s, len + 1);
+        memcpy(p, s, len);
         unlock_user(p, arg0, len);
         free(s);
         common_semi_set_ret(cs, 0);