py/formatfloat: Fix further cases of buffer overflow in formatting.

Includes extensive test cases to catch hopefully all cases where
buffer might overflow.

1 files changed, 24 insertions, 0 deletions

diff --git a/tests/float/string_format_modulo2.py b/tests/float/string_format_modulo2.py
new file mode 100644
index 0000000000..d35f2a2c02
--- /dev/null
+++ b/tests/float/string_format_modulo2.py
@@ -0,0 +1,24 @@
+# test formatting floats with large precision, that it doesn't overflow the buffer
+
+def test(num, num_str):
+    if num == float('inf') or num == 0.0 and num_str != '0.0':
+        # skip numbers that overflow or underflow the FP precision
+        return
+    for kind in ('e', 'f', 'g'):
+        # check precision either side of the size of the buffer (32 bytes)
+        for prec in range(23, 36, 2):
+            fmt = '%.' + '%d' % prec + kind
+            s = fmt % num
+            check = abs(float(s) - num)
+            if num > 1:
+                check /= num
+            if check > 1e-6:
+                print('FAIL', num_str, fmt, s, len(s), check)
+
+# check pure zero
+test(0.0, '0.0')
+
+# check most powers of 10, making sure to include exponents with 3 digits
+for e in range(-101, 102):
+    num = pow(10, e)
+    test(num, '1e%d' % e)