cutils: Improve qemu_strtod* error paths

Previous patches changed all integral qemu_strto*() error paths to
guarantee that *value is never left uninitialized.  Do likewise for
qemu_strtod.  Also, tighten qemu_strtod_finite() to never return a
non-finite value (prior to this patch, we were rejecting "inf" with
-EINVAL and unspecified result 0.0, but failing "9e999" with -ERANGE
and HUGE_VAL - which is infinite on IEEE machines - despite our
function claiming to recognize only finite values).

Auditing callers, we have no external callers of qemu_strtod, and
among the callers of qemu_strtod_finite:

- qapi/qobject-input-visitor.c:qobject_input_type_number_keyval() and
  qapi/string-input-visitor.c:parse_type_number() which reject all
  errors (does not matter what we store)

- utils/cutils.c:do_strtosz() incorrectly assumes that *endptr points
  to '.' on all failures (that is, it is not distinguishing between
  EINVAL and ERANGE; and therefore still does the WRONG THING for
  "9.9e999".  The change here does not entirely fix that (a later
  patch will tackle this more systematically), but at least it fixes
  the read-out-of-bounds first diagnosed in
  https://gitlab.com/qemu-project/qemu/-/issues/1629

- our testsuite, which we can update to match what we document

Signed-off-by: Eric Blake <eblake@redhat.com>
Reviewed-by: Hanna Czenczek <hreitz@redhat.com>
CC: qemu-stable@nongnu.org
Message-Id: <20230522190441.64278-19-eblake@redhat.com>

1 files changed, 19 insertions, 13 deletions

diff --git a/util/cutils.c b/util/cutils.c
index e3a49209a9..bde2da59bd 100644
--- a/util/cutils.c
+++ b/util/cutils.c
@@ -660,12 +660,13 @@ int qemu_strtou64(const char *nptr, const char **endptr, int base,
  *
  * @nptr may be null, and no conversion is performed then.
  *
- * If no conversion is performed, store @nptr in *@endptr and return
- * -EINVAL.
+ * If no conversion is performed, store @nptr in *@endptr, +0.0 in
+ * @result, and return -EINVAL.
  *
  * If @endptr is null, and the string isn't fully converted, return
- * -EINVAL. This is the case when the pointer that would be stored in
- * a non-null @endptr points to a character other than '\0'.
+ * -EINVAL with @result set to the parsed value.  This is the case
+ * when the pointer that would be stored in a non-null @endptr points
+ * to a character other than '\0'.
  *
  * If the conversion overflows, store +/-HUGE_VAL in @result, depending
  * on the sign, and return -ERANGE.
@@ -680,6 +681,7 @@ int qemu_strtod(const char *nptr, const char **endptr, double *result)
     char *ep;
 
     if (!nptr) {
+        *result = 0.0;
         if (endptr) {
             *endptr = nptr;
         }
@@ -694,24 +696,28 @@ int qemu_strtod(const char *nptr, const char **endptr, double *result)
 /**
  * Convert string @nptr to a finite double.
  *
- * Works like qemu_strtod(), except that "NaN" and "inf" are rejected
- * with -EINVAL and no conversion is performed.
+ * Works like qemu_strtod(), except that "NaN", "inf", and strings
+ * that cause ERANGE overflow errors are rejected with -EINVAL as if
+ * no conversion is performed, storing 0.0 into @result regardless of
+ * any sign.  -ERANGE failures for underflow still preserve the parsed
+ * sign.
  */
 int qemu_strtod_finite(const char *nptr, const char **endptr, double *result)
 {
-    double tmp;
+    const char *tmp;
     int ret;
 
-    ret = qemu_strtod(nptr, endptr, &tmp);
-    if (!ret && !isfinite(tmp)) {
+    ret = qemu_strtod(nptr, &tmp, result);
+    if (!isfinite(*result)) {
         if (endptr) {
             *endptr = nptr;
         }
+        *result = 0.0;
+        ret = -EINVAL;
+    } else if (endptr) {
+        *endptr = tmp;
+    } else if (*tmp) {
         ret = -EINVAL;
-    }
-
-    if (ret != -EINVAL) {
-        *result = tmp;
     }
     return ret;
 }