gh-143050: add helper _PyLong_InitTag() by skirpichev · Pull Request #147956 · python/cpython

skirpichev · 2026-04-01T03:54:09Z

With this we can assume, that _PyLong_Set*() operate on non-immortal integers.

Issue: Remove redundant decref from _PyLong_Negate #143050

With this we can assume, that _PyLong_Set*() operate on non-immortal integers.

vstinner · 2026-04-01T11:34:50Z

 _PyLong_SetSignAndDigitCount(result, size != 0, size);
- /* The digit has to be initialized explicitly to avoid
- * use-of-uninitialized-value. */
- result->long_value.ob_digit[0] = 0;


While I'm not convinced that this initialization is useful, the comment says that it is. We should keep it, no?

I suggested to keep this initialization. You marked my comment as resolved, I don't understand. Do you consider that it's not needed?
An alternative would be to fill digits with a pattern to detect usage of non-initialized memory when Python is built in debug mode:
#ifdefPy_DEBUG// Fill digits with a known pattern to detect usage of uninitialized memorymemset(result->long_value.ob_digit, 0xff, sizeof(result->long_value.ob_digit[0]) *ndigits); #endif
We use this strategy for:
str: see unicode_fill_invalid() function.
PyBytesWriter: memset(byteswriter_data(writer), 0xff, byteswriter_allocated(writer)) in PyBytesWriter_Create()

Do you suggest to keep this just here? BTW, tests pass - I rerun one job again.

ob_digit[0] = 0; was added by commit fc130c4 of issue gh-102509 to fix the OSS Fuzz report https://issues.oss-fuzz.com/issues/42516166.
Looks like it's complaining about a use of unit values in _PyLong_FromByteArray
Allocation happens here as part of _PyLong_New
cpython/Objects/longobject.c
Line 911 in 41bc101
v=_PyLong_New(ndigits);

and the claimed use is at
cpython/Objects/longobject.c
Line 957 in 41bc101
return (PyObject*)maybe_small_long(long_normalize(v));
inside of maybe_small_long and IS_SMALL_INT
I understand that the maybe_small_long() at the end of _PyLong_FromByteArray() triggered the issue when Python was built with --with-memory-sanitizer (-fsanitize=memory). From what I understood, the sign was initialized (to zero) but not ob_digit[0].
Oh interesting, k_mul() already does what I proposed above:
#ifdefPy_DEBUG/* Fill with trash, to catch reference to uninitialized digits. */memset(ret->long_value.ob_digit, 0xDF, _PyLong_DigitCount(ret) *sizeof(digit)); #endif
k_lopsided_mul() sets explicitly all digits to zero:
memset(ret->long_value.ob_digit, 0, _PyLong_DigitCount(ret) *sizeof(digit));

vstinner · 2026-04-01T12:10:33Z

"Tests / Android (x86_64)" failed with: Error on ZipFile unknown archive. It's unrelated to this change.

 > Task :app:maxVersionSetup
"Install Android Emulator v.36.4.10" ready.
Installing Android Emulator in /usr/local/lib/android/sdk/emulator
"Install Android Emulator v.36.4.10" complete.
"Install Android Emulator v.36.4.10" finished.
Checking the license for package AOSP ATD Intel x86_64 Atom System Image in /usr/local/lib/android/sdk/licenses
License for package AOSP ATD Intel x86_64 Atom System Image accepted.
Preparing "Install AOSP ATD Intel x86_64 Atom System Image API 32 (revision 1)".
Warning: An error occurred while preparing SDK package AOSP ATD Intel x86_64 Atom System Image: Error on ZipFile unknown archive.:
java.io.IOException: Error on ZipFile unknown archive
at org.apache.commons.compress.archivers.zip.ZipFile.<init>(ZipFile.java:383)
at org.apache.commons.compress.archivers.zip.ZipFile.<init>(ZipFile.java:323)
at org.apache.commons.compress.archivers.zip.ZipFile.<init>(ZipFile.java:279)
at com.android.repository.util.InstallerUtil.unzip(InstallerUtil.java:101)
at com.android.repository.impl.installer.BasicInstaller.doPrepare(BasicInstaller.java:91)

skirpichev · 2026-04-01T12:12:49Z

This yes, but linked above fuzzer failure is relevant.

…

On Wed, Apr 1, 2026, 15:10 Victor Stinner ***@***.***> wrote: *vstinner* left a comment (python/cpython#147956) <#147956?email_source=notifications&email_token=AAQOKGA5T2QHBNZ3SVWPVWT4TUBM7A5CNFSNUABFM5UWIORPF5TWS5BNNB2WEL2JONZXKZKDN5WW2ZLOOQXTIMJWHE3DGNZXGIZKM4TFMFZW63VGMF2XI2DPOKSWK5TFNZ2LK4DSL5RW63LNMVXHIX3POBSW4X3DNRUWG2Y#issuecomment-4169637722> "Tests / Android (x86_64)" failed with: Error on ZipFile unknown archive. It's unrelated to this change. > Task :app:maxVersionSetup "Install Android Emulator v.36.4.10" ready. Installing Android Emulator in /usr/local/lib/android/sdk/emulator "Install Android Emulator v.36.4.10" complete. "Install Android Emulator v.36.4.10" finished. Checking the license for package AOSP ATD Intel x86_64 Atom System Image in /usr/local/lib/android/sdk/licenses License for package AOSP ATD Intel x86_64 Atom System Image accepted. Preparing "Install AOSP ATD Intel x86_64 Atom System Image API 32 (revision 1)". Warning: An error occurred while preparing SDK package AOSP ATD Intel x86_64 Atom System Image: Error on ZipFile unknown archive.: java.io.IOException: Error on ZipFile unknown archive at org.apache.commons.compress.archivers.zip.ZipFile.<init>(ZipFile.java:383) at org.apache.commons.compress.archivers.zip.ZipFile.<init>(ZipFile.java:323) at org.apache.commons.compress.archivers.zip.ZipFile.<init>(ZipFile.java:279) at com.android.repository.util.InstallerUtil.unzip(InstallerUtil.java:101) at com.android.repository.impl.installer.BasicInstaller.doPrepare(BasicInstaller.java:91) — Reply to this email directly, view it on GitHub <#147956?email_source=notifications&email_token=AAQOKGA5T2QHBNZ3SVWPVWT4TUBM7A5CNFSNUABFM5UWIORPF5TWS5BNNB2WEL2JONZXKZKDN5WW2ZLOOQXTIMJWHE3DGNZXGIZKM4TFMFZW63VGMF2XI2DPOKSWK5TFNZ2LK4DSL5RW63LNMVXHIX3POBSW4X3DNRUWG2Y#issuecomment-4169637722>, or unsubscribe <https://github.com/notifications/unsubscribe-auth/AAQOKGB4LCCKKOKT66AWXQD4TUBM7AVCNFSM6AAAAACXIS7L4WVHI2DSMVQWIX3LMV43OSLTON2WKQ3PNVWWK3TUHM2DCNRZGYZTONZSGI> . You are receiving this because you authored the thread.Message ID: ***@***.***>

vstinner · 2026-04-01T12:15:30Z

"Tests / CIFuzz / cpython3 (memory)" CI logs an use-of-uninitialized-value:

==45==WARNING: MemorySanitizer: use-of-uninitialized-value
#0 0x55af57984f54 in _PyLong_IsSmallInt /src/cpython3/Include/internal/pycore_long.h:243:5
#1 0x55af57984f54 in _PyLong_SetSignAndDigitCount /src/cpython3/Include/internal/pycore_long.h:305:5
#2 0x55af579844c6 in long_alloc /src/cpython3/Objects/longobject.c:190:5
#3 0x55af5798ef9a in PyLong_FromUnsignedLong /src/cpython3/Objects/longobject.c:442:5
#4 0x55af5798ef9a in PyLong_FromVoidPtr /src/cpython3/Objects/longobject.c:1505:12

pycore_long.h:243 is the following assertion in _PyLong_IsSmallInt():

assert((_PyLong_IsCompact(op)
&&_PY_IS_SMALL_INT(_PyLong_CompactValue(op)))
|| (!is_small_int));

Oh right, op->long_value.ob_digit[0] = 0; is needed before calling _PyLong_IsSmallInt() to have initialized memory (_PyLong_CompactValue()).

vstinner

What do you think of changing _PyLong_IsSmallInt() implementation like that?

staticinlinebool_PyLong_IsSmallInt(constPyLongObject*op)
{
assert(PyLong_Check(op));
boolis_small_int= (op->long_value.lv_tag&IMMORTALITY_BIT_MASK) !=0;
if (is_small_int) {
assert(PyLong_CheckExact(op));
assert(_Py_IsImmortal(op));
assert((_PyLong_IsCompact(op)
&&_PY_IS_SMALL_INT(_PyLong_CompactValue(op))));
}
returnis_small_int;
}

The current code runs many checks when is_small_int is false which are not needed.

skirpichev · 2026-04-01T12:30:14Z

What do you think of changing _PyLong_IsSmallInt() implementation like that?

Lets try, but I suspect it might fall in other place.

vstinner · 2026-04-01T12:57:26Z

 _PyLong_SetSignAndDigitCount(result, size != 0, size);
- /* The digit has to be initialized explicitly to avoid
- * use-of-uninitialized-value. */
- result->long_value.ob_digit[0] = 0;


I suggested to keep this initialization. You marked my comment as resolved, I don't understand. Do you consider that it's not needed?
An alternative would be to fill digits with a pattern to detect usage of non-initialized memory when Python is built in debug mode:
#ifdefPy_DEBUG// Fill digits with a known pattern to detect usage of uninitialized memorymemset(result->long_value.ob_digit, 0xff, sizeof(result->long_value.ob_digit[0]) *ndigits); #endif
We use this strategy for:
str: see unicode_fill_invalid() function.
PyBytesWriter: memset(byteswriter_data(writer), 0xff, byteswriter_allocated(writer)) in PyBytesWriter_Create()

vstinner · 2026-04-01T19:28:08Z

Ok, I managed to reproduce the MSAN (-fsanitize=memory) error on this PR.

Apply the patch:

diff --git a/Objects/longobject.c b/Objects/longobject.c
index b74a435b541..d46dbf0bd82 100644
--- a/Objects/longobject.c+++ b/Objects/longobject.c@@ -170,7 +170,7 @@ long_alloc(Py_ssize_t size)
Py_ssize_t ndigits = size ? size : 1;
if (ndigits == 1) {
- result = (PyLongObject *)_Py_FREELIST_POP(PyLongObject, ints);+ //result = (PyLongObject *)_Py_FREELIST_POP(PyLongObject, ints);
}
if (result == NULL) {
/* Number of bytes needed is: offsetof(PyLongObject, ob_digit) +

Build Python with:

./configure --with-memory-sanitizer --with-pydebug --without-pymalloc CC=clang LD=clang CFLAGS="-O0"
make clean
make

Then run:

$ PYTHONMALLOC=malloc ./python -c "import _testcapi; _testcapi.pylong_fromnativebytes(b'\x00', 1, 0, 0)"
==1285845==WARNING: MemorySanitizer: use-of-uninitialized-value
#0 0x00000117d5be in _PyLong_CompactValue /home/vstinner/python/main/./Include/cpython/longintrepr.h:136:5
#1 0x00000118fb5f in maybe_small_long /home/vstinner/python/main/Objects/longobject.c:71:27
#2 0x00000118f850 in _PyLong_FromByteArray /home/vstinner/python/main/Objects/longobject.c:1099:24
(...)
Uninitialized value was created by a heap allocation
#0 0x000000440526 in malloc (/home/vstinner/python/main/python+0x440526) (BuildId: 8ebc9d3744df4c174e3833d7dd40285db4f23ce2)
#1 0x0000014d53fb in _PyMem_RawMalloc /home/vstinner/python/main/Objects/obmalloc.c:65:12
#2 0x0000014e50d9 in PyObject_Malloc /home/vstinner/python/main/Objects/obmalloc.c:1649:12
#3 0x00000117b368 in long_alloc /home/vstinner/python/main/Objects/longobject.c:181:18
#4 0x00000118dfdc in _PyLong_FromByteArray /home/vstinner/python/main/Objects/longobject.c:1049:9
(...)
SUMMARY: MemorySanitizer: use-of-uninitialized-value /home/vstinner/python/main/./Include/cpython/longintrepr.h:136:5 in _PyLong_CompactValue
Exiting

vstinner · 2026-04-01T20:18:38Z

I created issue gh-147988 and PR gh-147989 to fill digits with a pattern in long_alloc() when Python is built in debug mode.

vstinner

LGTM

vstinner · 2026-04-01T23:47:08Z

Add helper _PyLong_InitTag()
With this we can assume, that _PyLong_SetSignAndDigitCount() and _PyLong_SetDigitCount() operate on non-immortal integers.

It's a good thing that these two functions now fail in debug mode if they are called on a small integer. It will catch bugs earlier. So I merged the change. Thanks for this enhancement.

With this we can assume, that _PyLong_SetSignAndDigitCount() and _PyLong_SetDigitCount() operate on non-immortal integers. Co-authored-by: Victor Stinner <vstinner@python.org>

pythongh-143050: add helper _PyLong_Init()
d7073e5
With this we can assume, that _PyLong_Set*() operate on non-immortal integers.

skirpichev added the skip news label Apr 1, 2026

bedevere-appBot mentioned this pull request Apr 1, 2026
Remove redundant decref from _PyLong_Negate #143050
Closed

bedevere-appBot added the awaiting review label Apr 1, 2026

vstinner reviewed Apr 1, 2026
View reviewed changes

Comment threadInclude/internal/pycore_long.h Outdated

skirpichev added 2 commits April 1, 2026 13:16

Merge branch 'master' into add-_PyLong_Init
095a186

address review: rename function to _PyLong_InitTag
ddd329f

skirpichev changed the title ~~gh-143050: add helper _PyLong_Init()~~gh-143050: add helper _PyLong_InitTag()Apr 1, 2026

address review: skip initialization of digit[0]
f6ab450

vstinner reviewed Apr 1, 2026
View reviewed changes

skirpichev added 2 commits April 1, 2026 15:28

address review: restore comment
f5df517

XXX redo _PyLong_IsSmallInt
21c59bb

vstinner reviewed Apr 1, 2026
View reviewed changes

skirpichev added 2 commits April 1, 2026 23:54

address review: fix comment
640b326

restore initialization of digit[0]
613d2a1

skirpichev requested a review from vstinner April 1, 2026 20:57

vstinner reviewed Apr 1, 2026
View reviewed changes

Comment threadInclude/internal/pycore_long.h Outdated

Update Include/internal/pycore_long.h
a2bd3c4

vstinner approved these changes Apr 1, 2026
View reviewed changes

bedevere-appBot added awaiting merge and removed awaiting review labels Apr 1, 2026

vstinner enabled auto-merge (squash) April 1, 2026 21:11

vstinner merged commit b456cb2 into python:mainApr 1, 2026
55 checks passed

bedevere-appBot removed the awaiting merge label Apr 1, 2026

skirpichev deleted the add-_PyLong_Init branch April 1, 2026 22:19

pablogsal mentioned this pull request Apr 4, 2026
gh-130273: Fix traceback color output with unicode characters #142529
Merged

Uh oh!

Uh oh!

Conversation

skirpichev commented Apr 1, 2026• edited by bedevere-app BotLoading Uh oh! There was an error while loading. Please reload this page.

Uh oh!

Uh oh!

Uh oh!

vstinnerApr 1, 2026

Choose a reason for hiding this comment

Uh oh!

vstinnerApr 1, 2026

Choose a reason for hiding this comment

Uh oh!

skirpichevApr 1, 2026

Choose a reason for hiding this comment

Uh oh!

vstinnerApr 1, 2026

Choose a reason for hiding this comment

Uh oh!

vstinner commented Apr 1, 2026

Uh oh!

skirpichev commented Apr 1, 2026 via email

Uh oh!

vstinner commented Apr 1, 2026

Uh oh!

vstinner left a comment

Choose a reason for hiding this comment

Uh oh!

skirpichev commented Apr 1, 2026

Uh oh!

Uh oh!

vstinnerApr 1, 2026

Choose a reason for hiding this comment

Uh oh!

vstinner commented Apr 1, 2026

Uh oh!

vstinner commented Apr 1, 2026

Uh oh!

Uh oh!

vstinner left a comment

Choose a reason for hiding this comment

Uh oh!

Uh oh!

vstinner commented Apr 1, 2026

Uh oh!

Reviewers

Assignees

Labels

Projects

Milestone

Development

Uh oh!

2 participants

skirpichev commented Apr 1, 2026•
edited by bedevere-app Bot
Loading