mirror of
https://github.com/python/cpython.git
synced 2025-12-08 06:10:17 +00:00
cec1e9dfd7
* Correctly pre-check for int-to-str conversion (#96537) Converting a large enough `int` to a decimal string raises `ValueError` as expected. However, the raise comes _after_ the quadratic-time base-conversion algorithm has run to completion. For effective DOS prevention, we need some kind of check before entering the quadratic-time loop. Oops! =) The quick fix: essentially we catch _most_ values that exceed the threshold up front. Those that slip through will still be on the small side (read: sufficiently fast), and will get caught by the existing check so that the limit remains exact. The justification for the current check. The C code check is: ```c max_str_digits / (3 * PyLong_SHIFT) <= (size_a - 11) / 10 ``` In GitHub markdown math-speak, writing $M$ for `max_str_digits`, $L$ for `PyLong_SHIFT` and $s$ for `size_a`, that check is: $$\left\lfloor\frac{M}{3L}\right\rfloor \le \left\lfloor\frac{s - 11}{10}\right\rfloor$$ From this it follows that $$\frac{M}{3L} < \frac{s-1}{10}$$ hence that $$\frac{L(s-1)}{M} > \frac{10}{3} > \log_2(10).$$ So $$2^{L(s-1)} > 10^M.$$ But our input integer $a$ satisfies $|a| \ge 2^{L(s-1)}$, so $|a|$ is larger than $10^M$. This shows that we don't accidentally capture anything _below_ the intended limit in the check. <!-- gh-issue-number: gh-95778 --> * Issue: gh-95778 <!-- /gh-issue-number --> Co-authored-by: Gregory P. Smith [Google LLC] <greg@krypto.org> Co-authored-by: Christian Heimes <christian@python.org> Co-authored-by: Mark Dickinson <dickinsm@gmail.com> |
||
|---|---|---|
| .. | ||
| 2.0.rst | ||
| 2.1.rst | ||
| 2.2.rst | ||
| 2.3.rst | ||
| 2.4.rst | ||
| 2.5.rst | ||
| 2.6.rst | ||
| 2.7.rst | ||
| 3.0.rst | ||
| 3.1.rst | ||
| 3.2.rst | ||
| 3.3.rst | ||
| 3.4.rst | ||
| 3.5.rst | ||
| 3.6.rst | ||
| 3.7.rst | ||
| 3.8.rst | ||
| 3.9.rst | ||
| changelog.rst | ||
| index.rst | ||