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

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merged 10 commits into from
Sep 25, 2022
Merged

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

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22fdce6
Fix undefined behaviour in bu_ulonglong
mdickinson Sep 10, 2022
e12943b
Add regression test
mdickinson Sep 10, 2022
8efefd8
Also apply the fix to lu_longlong
mdickinson Sep 10, 2022
cba4b8b
Merge branch 'main' into fix-struct-unpack-undefined-behaviour
mdickinson Sep 10, 2022
0d49e73
Fix similar undefined behaviour issues in bu_int and lu_int
mdickinson Sep 10, 2022
bfd714f
Merge remote-tracking branch 'mdickinson/fix-struct-unpack-undefined-…
mdickinson Sep 10, 2022
5aeef09
Remove redundant test
mdickinson Sep 10, 2022
204e924
Fix accumulator type
mdickinson Sep 10, 2022
994703e
Branch-free sign-extension
mdickinson Sep 13, 2022
1294440
Fix unconventional spacing
mdickinson Sep 13, 2022
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1 change: 1 addition & 0 deletions Misc/NEWS.d/next/Library/2022-09-10-16-46-16.gh-issue-96735.0YzJuG.rst
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Original file line number Diff line number Diff line change
@@ -0,0 +1 @@
Fix undefined behaviour in :func:`struct.unpack`.
96 changes: 70 additions & 26 deletions Modules/_struct.c
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Original file line number Diff line number Diff line change
Expand Up @@ -805,19 +805,38 @@ static const formatdef native_table[] = {

/* Big-endian routines. *****************************************************/

static PyObject *
bu_short(_structmodulestate *state, const char *p, const formatdef *f)
{
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 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)
{
long x = 0;
Py_ssize_t i = f->size;
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 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 ^ 0x80000000U) - 0x80000000U;
return PyLong_FromLong(x & 0x80000000U ? -1 - (long)(~x) : (long)x);
}

static PyObject *
Expand All @@ -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 *
Expand Down Expand Up @@ -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},
Expand All @@ -1026,19 +1048,38 @@ static formatdef bigendian_table[] = {

/* Little-endian routines. *****************************************************/

static PyObject *
lu_short(_structmodulestate *state, const char *p, const formatdef *f)
{
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 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)
{
long x = 0;
Py_ssize_t i = f->size;
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 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 ^ 0x80000000U) - 0x80000000U;
return PyLong_FromLong(x & 0x80000000U ? -1 - (long)(~x) : (long)x);
}

static PyObject *
Expand All @@ -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 *
Expand Down Expand Up @@ -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},
Expand Down