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gh-96735: Fix undefined behaviour in struct unpacking functions (#96739) 

This PR fixes undefined behaviour in the struct module unpacking support functions `bu_longlong`, `lu_longlong`, `bu_int` and `lu_int`; thanks to @kumaraditya303 for finding these.

The fix is to accumulate the bytes in an unsigned integer type instead of a signed integer type, then to convert to the appropriate signed type. In cases where the width matches, that conversion will typically be compiled away to a no-op.
(Evidence from Godbolt: https://godbolt.org/z/5zvxodj64 .)

To make the conversions efficient, I've specialised the relevant functions for their output size: for `bu_longlong` and `lu_longlong`, this only entails checking that the output size is indeed `8`. But `bu_int` and `lu_int` were used for format sizes `2` and `4` - I've split those into two separate functions each.

No tests, because all of the affected cases are already exercised by the test suite.
This commit is contained in:
Mark Dickinson 2022-09-25 10:55:33 +01:00 committed by GitHub
parent 817fa28f81
commit f5f047aa62
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GPG Key ID: 4AEE18F83AFDEB23
2 changed files with 73 additions and 28 deletions
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1
Misc/NEWS.d/next/Library/2022-09-10-16-46-16.gh-issue-96735.0YzJuG.rst Normal file
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@ -0,0 +1 @@
Fix undefined behaviour in :func:`struct.unpack`.

100
Modules/_struct.c
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@ -806,18 +806,37 @@ static const formatdef native_table[] = {
/* Big-endian routines. *****************************************************/
static PyObject *
bu_int(_structmodulestate *state, const char *p, const formatdef *f)
bu_short(_structmodulestate *state, const char *p, const formatdef *f)
{
long x = 0;
Py_ssize_t i = f->size;
unsigned long x = 0;
/* This function is only ever used in the case f->size == 2. */
assert(f->size == 2);
Py_ssize_t i = 2;
const unsigned char *bytes = (const unsigned char *)p;
do {
x = (x<<8) | *bytes++;
} while (--i > 0);
/* Extend the sign bit. */
if (SIZEOF_LONG > f->size)
x |= -(x & (1L << ((8 * f->size) - 1)));
return PyLong_FromLong(x);
/* Extend sign, avoiding implementation-defined or undefined behaviour. */
x = (x ^ 0x8000U) - 0x8000U;
return PyLong_FromLong(x & 0x8000U ? -1 - (long)(~x) : (long)x);
}
static PyObject *
bu_int(_structmodulestate *state, const char *p, const formatdef *f)
{
unsigned long x = 0;
/* This function is only ever used in the case f->size == 4. */
assert(f->size == 4);
Py_ssize_t i = 4;
const unsigned char *bytes = (const unsigned char *)p;
do {
x = (x<<8) | *bytes++;
} while (--i > 0);
/* Extend sign, avoiding implementation-defined or undefined behaviour. */
x = (x ^ 0x80000000U) - 0x80000000U;
return PyLong_FromLong(x & 0x80000000U ? -1 - (long)(~x) : (long)x);
}
static PyObject *
@ -835,16 +854,19 @@ bu_uint(_structmodulestate *state, const char *p, const formatdef *f)
static PyObject *
bu_longlong(_structmodulestate *state, const char *p, const formatdef *f)
{
long long x = 0;
Py_ssize_t i = f->size;
unsigned long long x = 0;
/* This function is only ever used in the case f->size == 8. */
assert(f->size == 8);
Py_ssize_t i = 8;
const unsigned char *bytes = (const unsigned char *)p;
do {
x = (x<<8) | *bytes++;
} while (--i > 0);
/* Extend the sign bit. */
if (SIZEOF_LONG_LONG > f->size)
x |= -(x & ((long long)1 << ((8 * f->size) - 1)));
return PyLong_FromLongLong(x);
/* Extend sign, avoiding implementation-defined or undefined behaviour. */
x = (x ^ 0x8000000000000000U) - 0x8000000000000000U;
return PyLong_FromLongLong(
x & 0x8000000000000000U ? -1 - (long long)(~x) : (long long)x);
}
static PyObject *
@ -1009,7 +1031,7 @@ static formatdef bigendian_table[] = {
{'c', 1, 0, nu_char, np_char},
{'s', 1, 0, NULL},
{'p', 1, 0, NULL},
{'h', 2, 0, bu_int, bp_int},
{'h', 2, 0, bu_short, bp_int},
{'H', 2, 0, bu_uint, bp_uint},
{'i', 4, 0, bu_int, bp_int},
{'I', 4, 0, bu_uint, bp_uint},
@ -1027,18 +1049,37 @@ static formatdef bigendian_table[] = {
/* Little-endian routines. *****************************************************/
static PyObject *
lu_int(_structmodulestate *state, const char *p, const formatdef *f)
lu_short(_structmodulestate *state, const char *p, const formatdef *f)
{
long x = 0;
Py_ssize_t i = f->size;
unsigned long x = 0;
/* This function is only ever used in the case f->size == 2. */
assert(f->size == 2);
Py_ssize_t i = 2;
const unsigned char *bytes = (const unsigned char *)p;
do {
x = (x<<8) | bytes[--i];
} while (i > 0);
/* Extend the sign bit. */
if (SIZEOF_LONG > f->size)
x |= -(x & (1L << ((8 * f->size) - 1)));
return PyLong_FromLong(x);
/* Extend sign, avoiding implementation-defined or undefined behaviour. */
x = (x ^ 0x8000U) - 0x8000U;
return PyLong_FromLong(x & 0x8000U ? -1 - (long)(~x) : (long)x);
}
static PyObject *
lu_int(_structmodulestate *state, const char *p, const formatdef *f)
{
unsigned long x = 0;
/* This function is only ever used in the case f->size == 4. */
assert(f->size == 4);
Py_ssize_t i = 4;
const unsigned char *bytes = (const unsigned char *)p;
do {
x = (x<<8) | bytes[--i];
} while (i > 0);
/* Extend sign, avoiding implementation-defined or undefined behaviour. */
x = (x ^ 0x80000000U) - 0x80000000U;
return PyLong_FromLong(x & 0x80000000U ? -1 - (long)(~x) : (long)x);
}
static PyObject *
@ -1056,16 +1097,19 @@ lu_uint(_structmodulestate *state, const char *p, const formatdef *f)
static PyObject *
lu_longlong(_structmodulestate *state, const char *p, const formatdef *f)
{
long long x = 0;
Py_ssize_t i = f->size;
unsigned long long x = 0;
/* This function is only ever used in the case f->size == 8. */
assert(f->size == 8);
Py_ssize_t i = 8;
const unsigned char *bytes = (const unsigned char *)p;
do {
x = (x<<8) | bytes[--i];
} while (i > 0);
/* Extend the sign bit. */
if (SIZEOF_LONG_LONG > f->size)
x |= -(x & ((long long)1 << ((8 * f->size) - 1)));
return PyLong_FromLongLong(x);
/* Extend sign, avoiding implementation-defined or undefined behaviour. */
x = (x ^ 0x8000000000000000U) - 0x8000000000000000U;
return PyLong_FromLongLong(
x & 0x8000000000000000U ? -1 - (long long)(~x) : (long long)x);
}
static PyObject *
@ -1213,7 +1257,7 @@ static formatdef lilendian_table[] = {
{'c', 1, 0, nu_char, np_char},
{'s', 1, 0, NULL},
{'p', 1, 0, NULL},
{'h', 2, 0, lu_int, lp_int},
{'h', 2, 0, lu_short, lp_int},
{'H', 2, 0, lu_uint, lp_uint},
{'i', 4, 0, lu_int, lp_int},
{'I', 4, 0, lu_uint, lp_uint},