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Undefined behavior in Py_ABS() macro with LLONG_MIN #141370

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Undefined behavior in Py_ABS() macro with LLONG_MIN#141370
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docsDocumentation in the Doc dirtype-bugAn unexpected behavior, bug, or error

Description

@sethmlarson
sethmlarson
opened on Nov 10, 2025

Bug report

Bug description:

When the value passed to Py_ABS() macro is LLONG_MIN (or any min value of a signed integer), the macro suffers from undefined behavior. This was detected in a failure to build the OSS-Fuzz test suite.

CPython versions tested on:

CPython main branch

Operating systems tested on:

No response

Linked PRs

Activity

  1. sethmlarson
    added
    type-bugAn unexpected behavior, bug, or error
    on Nov 10, 2025
  2. StanFromIreland
    added
    interpreter-core(Objects, Python, Grammar, and Parser dirs)
    on Nov 10, 2025

  3. skirpichev commented on Nov 10, 2025

    @skirpichev
    skirpichev
    on Nov 10, 2025
    Member

    I don't think it's an issue. Same happens for abs/labs/llabs from the C stdlib, when the absolute value can't be represented in two’s complement:

    $ cat a.c
#include <stdlib.h>
#include <limits.h>
#include <stdio.h>
int main()
{
    long long lmin = LLONG_MIN, lmax = LLONG_MAX;

    printf("%lld %lld %lld\n", lmin, llabs(lmin), lmax);
    return 0;
}
$ cc a.c && ./a.out
-9223372036854775808 -9223372036854775808 9223372036854775807

    I suggest either close this or reclassify as a documentation issue. We could add remark like this: "If the result cannot be represented, the behavior is undefined."

  4. skirpichev
    added
    pendingThe issue will be closed if no feedback is provided
    on Nov 10, 2025

  5. serhiy-storchaka commented on Nov 11, 2025

    @serhiy-storchaka
    serhiy-storchaka
    on Nov 11, 2025
    Member

    This is partially a documentation issue, but we should also analyze every use of Py_ABS in the C code.

  6. skirpichev commented on Nov 11, 2025

    @skirpichev
    skirpichev
    on Nov 11, 2025
    Member

    I see few instances:

    1. float_as_integer_ratio_impl() in Objects/floatobject.c: here we are safe, because the exponent range will not hit INT_MIN (even for quadruple precision it fits into 16bit int).
    2. _PyLong_GCD() in Objects/longobject.c: here we just can't have LONG_MIN or LLONG_MIN value in PyLong with <= 2 digits.
    3. w_PyLong() in Python/marshal.c: here we cast result to unsigned type.
    4. r_PyLong() in Python/marshal.c: here we are safe, because -SIZE32_MAX <= n <= SIZE32_MAX
    5. _Py_strhex_impl() in Python/pystrhex.c: again cast to unsigned
    6. pytime_divide() in Python/pytime.c: here is just integer division with divisor k>=2.

    If you think all is ok with above, I'll reclassify this as a documentation issue.

    Details 
    $ git grep 'Py_ABS *('
Doc/c-api/intro.rst:.. c:macro:: Py_ABS(x)
Include/pymacro.h:#define Py_ABS(x) ((x) < 0 ? -(x) : (x))
Modules/_testcapimodule.c:    // Py_MIN(), Py_MAX(), Py_ABS()
Modules/_testcapimodule.c:    assert(Py_ABS(-5) == 5);
Objects/floatobject.c:    py_exponent = PyLong_FromLong(Py_ABS(exponent));
Objects/longobject.c:    x = Py_ABS(x);
Objects/longobject.c:    y = Py_ABS(y);
Python/marshal.c:        uint64_t abs_value = Py_ABS(long_export.value);
Python/marshal.c:    Py_ssize_t size = 1 + (Py_ABS(n) - 1) / marshal_ratio;
Python/marshal.c:    int shorts_in_top_digit = 1 + (Py_ABS(n) - 1) % marshal_ratio;
Python/pystrhex.c:    unsigned int abs_bytes_per_sep = Py_ABS(bytes_per_sep_group);
Python/pytime.c:        PyTime_t abs_r = Py_ABS(r);
Python/pytime.c:        if (abs_r > k / 2 || (abs_r == k / 2 && (Py_ABS(x) & 1))) {
  7. skirpichev
    self-assigned this
    on Nov 11, 2025
  8. skirpichev
    added a commit that references this issue on Nov 12, 2025

    pythongh-141370: document undefined behavior of Py_ABS()

    790ccfb

  9. serhiy-storchaka commented on Nov 12, 2025

    @serhiy-storchaka
    serhiy-storchaka
    on Nov 12, 2025
    Member

    Technically, negation of INT_MIN is an undefined behavior.

  10. serhiy-storchaka commented on Nov 12, 2025

    @serhiy-storchaka
    serhiy-storchaka
    on Nov 12, 2025
    Member

    To avoid an undefined behavior we should handle the special case separately using unsigned arithmetic.

  11. skirpichev commented on Nov 12, 2025

    @skirpichev
    skirpichev
    on Nov 12, 2025
    Member

    Technically, negation of INT_MIN is an undefined behavior.

    Yes, as an example (though, I think it's only corner case; ditto for other signed integer types).

    To avoid an undefined behavior we should handle the special case separately using unsigned arithmetic.

    Probably, this is possible, but it will be too costly just for one special case.

  12. skirpichev
    added
    docsDocumentation in the Doc dir
    and removed
    pendingThe issue will be closed if no feedback is provided
    on Nov 12, 2025
  13. github-project-automation
    added this to docs issueson Nov 12, 2025
  14. github-project-automation
    moved this to Todo in docs issueson Nov 12, 2025

  15. 7 remaining items

  16. skirpichev commented on Nov 14, 2025

    @skirpichev
    skirpichev
    on Nov 14, 2025
    Member

    @serhiy-storchaka, @sethmlarson, as documentation change was merged - I'm closing this.

    Please reopen if you think that any from few usage cases in the CPython misuse this macro.

  17. skirpichev
    closed this as completedon Nov 14, 2025
  18. serhiy-storchaka
    added a commit that references this issue on Nov 14, 2025

    pythongh-141370: Fix undefined behavior when using Py_ABS()

    1acabab

  19. serhiy-storchaka commented on Nov 14, 2025

    @serhiy-storchaka
    serhiy-storchaka
    on Nov 14, 2025
    Member

    There is an undefined behavior in two of these places.

  20. serhiy-storchaka
    reopened this on Nov 14, 2025
  21. serhiy-storchaka
    added a commit that references this issue on Dec 5, 2025

    gh-141370: Fix undefined behavior when using Py_ABS() (GH-141548)

    706fdda
  22. miss-islington
    added a commit that references this issue on Dec 5, 2025

    pythongh-141370: Fix undefined behavior when using Py_ABS() (pythonGH…

    71c61fe
  23. serhiy-storchaka
    added a commit that references this issue on Dec 5, 2025

    [3.13] pythongh-141370: Fix undefined behavior when using Py_ABS() (p…

    19b20d8
  24. Yhg1s
    added a commit that references this issue on Dec 5, 2025

    [3.13] gh-141370: Fix undefined behavior when using Py_ABS() (GH-141548…

    93d5c48
  25. StanFromIreland
    added 2 commits that reference this issue on Dec 6, 2025

    pythongh-141370: document undefined behavior of Py_ABS() (pythonGH-14…

    43237b6

    pythongh-141370: Fix undefined behavior when using Py_ABS() (pythonGH…

    c316ef0
  26. serhiy-storchaka
    added a commit that references this issue on Dec 12, 2025

    [3.14] gh-141370: Fix undefined behavior when using Py_ABS() (GH-141548…

    ebeb07f
  27. sethmlarson
    closed this as not plannedon Apr 24, 2026
  28. sethmlarson
    reopened this on Apr 24, 2026
  29. sethmlarson
    closed this as completedon Apr 24, 2026
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