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b126196838
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>
116 lines
3.5 KiB
C
116 lines
3.5 KiB
C
#ifndef Py_INTERNAL_LONG_H
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#define Py_INTERNAL_LONG_H
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#ifdef __cplusplus
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extern "C" {
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#endif
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#ifndef Py_BUILD_CORE
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# error "this header requires Py_BUILD_CORE define"
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#endif
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#include "pycore_global_objects.h" // _PY_NSMALLNEGINTS
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#include "pycore_runtime.h" // _PyRuntime
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/*
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* Default int base conversion size limitation: Denial of Service prevention.
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*
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* Chosen such that this isn't wildly slow on modern hardware and so that
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* everyone's existing deployed numpy test suite passes before
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* https://github.com/numpy/numpy/issues/22098 is widely available.
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*
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* $ python -m timeit -s 's = "1"*4300' 'int(s)'
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* 2000 loops, best of 5: 125 usec per loop
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* $ python -m timeit -s 's = "1"*4300; v = int(s)' 'str(v)'
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* 1000 loops, best of 5: 311 usec per loop
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* (zen2 cloud VM)
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*
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* 4300 decimal digits fits a ~14284 bit number.
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*/
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#define _PY_LONG_DEFAULT_MAX_STR_DIGITS 4300
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/*
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* Threshold for max digits check. For performance reasons int() and
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* int.__str__() don't checks values that are smaller than this
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* threshold. Acts as a guaranteed minimum size limit for bignums that
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* applications can expect from CPython.
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*
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* % python -m timeit -s 's = "1"*640; v = int(s)' 'str(int(s))'
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* 20000 loops, best of 5: 12 usec per loop
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*
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* "640 digits should be enough for anyone." - gps
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* fits a ~2126 bit decimal number.
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*/
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#define _PY_LONG_MAX_STR_DIGITS_THRESHOLD 640
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#if ((_PY_LONG_DEFAULT_MAX_STR_DIGITS != 0) && \
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(_PY_LONG_DEFAULT_MAX_STR_DIGITS < _PY_LONG_MAX_STR_DIGITS_THRESHOLD))
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# error "_PY_LONG_DEFAULT_MAX_STR_DIGITS smaller than threshold."
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#endif
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/* runtime lifecycle */
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extern PyStatus _PyLong_InitTypes(PyInterpreterState *);
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extern void _PyLong_FiniTypes(PyInterpreterState *interp);
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/* other API */
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#define _PyLong_SMALL_INTS _Py_SINGLETON(small_ints)
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// _PyLong_GetZero() and _PyLong_GetOne() must always be available
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// _PyLong_FromUnsignedChar must always be available
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#if _PY_NSMALLPOSINTS < 257
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# error "_PY_NSMALLPOSINTS must be greater than or equal to 257"
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#endif
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// Return a borrowed reference to the zero singleton.
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// The function cannot return NULL.
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static inline PyObject* _PyLong_GetZero(void)
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{ return (PyObject *)&_PyLong_SMALL_INTS[_PY_NSMALLNEGINTS]; }
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// Return a borrowed reference to the one singleton.
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// The function cannot return NULL.
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static inline PyObject* _PyLong_GetOne(void)
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{ return (PyObject *)&_PyLong_SMALL_INTS[_PY_NSMALLNEGINTS+1]; }
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static inline PyObject* _PyLong_FromUnsignedChar(unsigned char i)
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{
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return Py_NewRef((PyObject *)&_PyLong_SMALL_INTS[_PY_NSMALLNEGINTS+i]);
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}
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PyObject *_PyLong_Add(PyLongObject *left, PyLongObject *right);
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PyObject *_PyLong_Multiply(PyLongObject *left, PyLongObject *right);
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PyObject *_PyLong_Subtract(PyLongObject *left, PyLongObject *right);
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int _PyLong_AssignValue(PyObject **target, Py_ssize_t value);
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/* Used by Python/mystrtoul.c, _PyBytes_FromHex(),
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_PyBytes_DecodeEscape(), etc. */
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PyAPI_DATA(unsigned char) _PyLong_DigitValue[256];
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/* Format the object based on the format_spec, as defined in PEP 3101
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(Advanced String Formatting). */
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PyAPI_FUNC(int) _PyLong_FormatAdvancedWriter(
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_PyUnicodeWriter *writer,
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PyObject *obj,
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PyObject *format_spec,
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Py_ssize_t start,
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Py_ssize_t end);
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PyAPI_FUNC(int) _PyLong_FormatWriter(
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_PyUnicodeWriter *writer,
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PyObject *obj,
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int base,
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int alternate);
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PyAPI_FUNC(char*) _PyLong_FormatBytesWriter(
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_PyBytesWriter *writer,
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char *str,
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PyObject *obj,
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int base,
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int alternate);
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#ifdef __cplusplus
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}
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#endif
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#endif /* !Py_INTERNAL_LONG_H */
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