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gh-102701: Fix overflow in dictobject.c #102750

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Mar 17, 2023
Merged

gh-102701: Fix overflow in dictobject.c #102750

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0102cf0
gh-102701: Fix overflow in dictobject.c
methane Mar 16, 2023
cff813b
Add testcase
methane Mar 17, 2023
b35172f
Fix indent
methane Mar 17, 2023
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9 changes: 9 additions & 0 deletions Lib/test/test_bigmem.py
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Original file line number Diff line number Diff line change
Expand Up @@ -1248,6 +1248,15 @@ def test_sort(self, size):
self.assertEqual(l[-10:], [5] * 10)


class DictTest(unittest.TestCase):

@bigmemtest(size=357913941, memuse=160)
def test_dict(self, size):
# https://github.com/python/cpython/issues/102701
d = dict.fromkeys(range(size))
d[size] = 1


if __name__ == '__main__':
if len(sys.argv) > 1:
support.set_memlimit(sys.argv[1])
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1 change: 1 addition & 0 deletions Misc/NEWS.d/next/Core and Builtins/2023-03-16-17-24-44.gh-issue-102701.iNGVaS.rst
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@@ -0,0 +1 @@
Fix overflow when creating very large dict.
2 changes: 1 addition & 1 deletion Objects/dictobject.c
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Expand Up @@ -596,7 +596,7 @@ new_keys_object(PyInterpreterState *interp, uint8_t log2_size, bool unicode)

assert(log2_size >= PyDict_LOG_MINSIZE);

usable = USABLE_FRACTION(1<<log2_size);
usable = USABLE_FRACTION((size_t)1<<log2_size);
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I am asking for my curiosity(to become familiar with cpython); How does casting the result of 1<<log2_size to size_t fix the overflow in the dict?

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1 is int. And int is 32bit on most platforms. So 1 << 32 will overflow.
Here is example:

$ cat x.c
#include <stdio.h>
#include <stdlib.h>

#define USABLE_FRACTION(n) (((n) << 1)/3)

int main() {
    size_t x = USABLE_FRACTION(1 << 31);
    size_t y = USABLE_FRACTION((size_t)1 << 31);
    printf("x=%zd y=%zd\n", x, y);
}

$ gcc x.c && ./a.out
x.c:7:16: warning: shifting a negative signed value is undefined [-Wshift-negative-value]
    size_t x = USABLE_FRACTION(1 << 31);
               ^~~~~~~~~~~~~~~~~~~~~~~~
x.c:4:34: note: expanded from macro 'USABLE_FRACTION'
#define USABLE_FRACTION(n) (((n) << 1)/3)
                             ~~~ ^
1 warning generated.
x=0 y=1431655765

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Thanks!

if (log2_size < 8) {
log2_bytes = log2_size;
}
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