bio printf: Avoid using rounding errors in range check

There is a problem casting ULONG_MAX to double which clang-10 is warning about.
ULONG_MAX typically cannot be exactly represented as a double.  ULONG_MAX + 1
can be and this fix uses the latter, however since ULONG_MAX cannot be
represented exactly as a double number we subtract 65535 from this number,
and the result has at most 48 leading one bits, and can therefore be
represented as a double integer without rounding error.  By adding
65536.0 to this number we achive the correct result, which should avoid the
warning.

The addresses a symptom of the underlying problem: we print doubles via an
unsigned long integer.  Doubles have a far greater range and should be printed
better.

Reviewed-by: Paul Dale <paul.dale@oracle.com>
(Merged from https://github.com/openssl/openssl/pull/11955)

1 files changed, 7 insertions, 1 deletions

diff --git a/crypto/bio/b_print.c b/crypto/bio/b_print.c
index 0d6fafcc2d..6b995f8233 100644
--- a/crypto/bio/b_print.c
+++ b/crypto/bio/b_print.c
@@ -635,7 +635,13 @@ fmtfp(char **sbuffer,
             fvalue = tmpvalue;
     }
     ufvalue = abs_val(fvalue);
-    if (ufvalue > ULONG_MAX) {
+    /*
+     * By subtracting 65535 (2^16-1) we cancel the low order 15 bits
+     * of ULONG_MAX to avoid using imprecise floating point values.
+     * The second condition is necessary to catch NaN values.
+     */
+    if (ufvalue >= (double)(ULONG_MAX - 65535) + 65536.0
+            || !(ufvalue == ufvalue) /* NaN */) {
         /* Number too big */
         return 0;
     }