Valgrind warning in PyUnicode_Decode: false alarm with GCC builtin strcmp()

[vstinner]

BPO	38118
Nosy	@vstinner, @miss-islington
PRs	bpo-38118: Ignore Valgrind false alarm in PyUnicode_Decode() #16651 [3.8] bpo-38118: Ignore Valgrind false alarm in PyUnicode_Decode() (GH-16651) #16652 [3.7] bpo-38118: Ignore Valgrind false alarm in PyUnicode_Decode() (GH-16651) #16653 [3.7] bpo-38118: Ignore Valgrind false alarm in PyUnicode_Decode() (GH-16651) #16654
Files	valgrind_strcmp_warn.c

^{Note: these values reflect the state of the issue at the time it was migrated and might not reflect the current state.}

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GitHub fields:

assignee = None
closed_at = <Date 2019-10-08.13:58:13.065>
created_at = <Date 2019-09-11.15:28:57.117>
labels = ['interpreter-core', '3.7', '3.8', '3.9']
title = 'Valgrind warning in PyUnicode_Decode: false alarm with GCC builtin strcmp()'
updated_at = <Date 2019-10-08.13:58:13.064>
user = 'https://github.com/vstinner'

bugs.python.org fields:

activity = <Date 2019-10-08.13:58:13.064>
actor = 'vstinner'
assignee = 'none'
closed = True
closed_date = <Date 2019-10-08.13:58:13.065>
closer = 'vstinner'
components = ['Interpreter Core']
creation = <Date 2019-09-11.15:28:57.117>
creator = 'vstinner'
dependencies = []
files = ['48647']
hgrepos = []
issue_num = 38118
keywords = ['patch']
message_count = 10.0
messages = ['351939', '352170', '352245', '354183', '354197', '354202', '354205', '354206', '354207', '354208']
nosy_count = 2.0
nosy_names = ['vstinner', 'miss-islington']
pr_nums = ['16651', '16652', '16653', '16654']
priority = 'normal'
resolution = 'fixed'
stage = 'resolved'
status = 'closed'
superseder = None
type = None
url = 'https://bugs.python.org/issue38118'
versions = ['Python 3.7', 'Python 3.8', 'Python 3.9']

[vstinner]

Running tokenize.py in Valgrind emits 2 warnings:

$ ./configure --with-valgrind
$ make
$ echo|PYTHONMALLOC=malloc valgrind ./python Lib/tokenize.py

(A)

==30746== Conditional jump or move depends on uninitialised value(s)
==30746== at 0x4A01F7: PyUnicode_Decode (unicodeobject.c:3395)
==30746== by 0x4A01F7: PyUnicode_Decode (unicodeobject.c:3350)
==30746== by 0x4A062E: PyUnicode_FromEncodedObject (unicodeobject.c:3268)
==30746== by 0x4A090D: unicode_new (unicodeobject.c:15125)
==30746== by 0x46F515: type_call (typeobject.c:969)
==30746== by 0x435F82: _PyObject_MakeTpCall (call.c:167)
==30746== by 0x422B3A: _PyObject_Vectorcall (abstract.h:104)
==30746== by 0x422B3A: _PyObject_Vectorcall (abstract.h:96)
==30746== by 0x422B3A: call_function (ceval.c:4984)
==30746== by 0x423C26: _PyEval_EvalFrameDefault (ceval.c:3496)
==30746== by 0x4217D2: function_code_fastcall (call.c:292)
==30746== by 0x4360FC: _PyObject_FastCallDict (call.c:109)
==30746== by 0x436309: _PyObject_Call_Prepend (call.c:447)
==30746== by 0x4731F0: slot_tp_init (typeobject.c:6767)
==30746== by 0x46F562: type_call (typeobject.c:989)

(B)

==30746== Conditional jump or move depends on uninitialised value(s)
==30746== at 0x4DC8FF: PyState_AddModule (pystate.c:707)
==30746== by 0x485A5E1: PyInit_zlib (zlibmodule.c:1472)
==30746== by 0x4CA49C: _PyImport_LoadDynamicModuleWithSpec (importdl.c:164)
==30746== by 0x4C8462: _imp_create_dynamic_impl (import.c:2292)
==30746== by 0x4C8462: _imp_create_dynamic (import.c.h:330)
==30746== by 0x570783: cfunction_vectorcall_FASTCALL (methodobject.c:382)
==30746== by 0x435ACD: PyVectorcall_Call (call.c:203)
==30746== by 0x42424F: do_call_core (ceval.c:5031)
==30746== by 0x42424F: _PyEval_EvalFrameDefault (ceval.c:3557)
==30746== by 0x4AF4CF: _PyEval_EvalCodeWithName (ceval.c:4296)
==30746== by 0x435D3F: _PyFunction_Vectorcall (call.c:355)
==30746== by 0x422AA8: _PyObject_Vectorcall (abstract.h:106)
==30746== by 0x422AA8: call_function (ceval.c:4984)
==30746== by 0x428E51: _PyEval_EvalFrameDefault (ceval.c:3465)
==30746== by 0x4217D2: function_code_fastcall (call.c:292)

Warning (A) goes away when Python is compiled with gcc -O1.

Warning (B) is still present when Python is compiled with gcc -O0.

It's unclear to me if it's a false alarm or a real issue.

On Fedora, the issue can be seen with:

echo|valgrind python3 /usr/lib64/python3.7/tokenize.py

On Fedora 30, python3 is Python 3.7.4.

Downstream Fedora issue: https://bugzilla.redhat.com/show_bug.cgi?id=1751208

[vstinner]

==30746== Conditional jump or move depends on uninitialised value(s)
==30746== at 0x4DC8FF: PyState_AddModule (pystate.c:707)

Oh, this issue looks like a duplicate of bpo-38124 which has just been fixed by:

New changeset 39de95b by Benjamin Peterson in branch 'master':
closes bpo-38124: Fix bounds check in PyState_AddModule. (GH-16007)
39de95b

[vstinner]

I reopen the issue, the PyUnicode_Decode warning is not fixed yet.

vstinner@apu$ echo|PYTHONMALLOC=malloc valgrind ./python Lib/tokenize.py
==6832== Memcheck, a memory error detector
==6832== Copyright (C) 2002-2017, and GNU GPL'd, by Julian Seward et al.
==6832== Using Valgrind-3.15.0 and LibVEX; rerun with -h for copyright info
==6832== Command: ./python Lib/tokenize.py
==6832==
==6832== Conditional jump or move depends on uninitialised value(s)
==6832== at 0x4D87E9: PyUnicode_Decode (unicodeobject.c:3395)
==6832== by 0x4D8E6E: PyUnicode_FromEncodedObject (unicodeobject.c:3268)
==6832== by 0x4D9283: unicode_new (unicodeobject.c:15125)
==6832== by 0x49271B: type_call (typeobject.c:969)
==6832== by 0x440E97: _PyObject_MakeTpCall (call.c:167)
==6832== by 0x42D6D3: _PyObject_Vectorcall (abstract.h:104)
==6832== by 0x42D6D3: call_function (ceval.c:4984)
==6832== by 0x42D6D3: _PyEval_EvalFrameDefault (ceval.c:3496)
==6832== by 0x4216CA: function_code_fastcall (call.c:292)
==6832== by 0x4411B5: _PyObject_FastCallDict (call.c:109)
==6832== by 0x441413: _PyObject_Call_Prepend (call.c:447)
==6832== by 0x497320: slot_tp_init (typeobject.c:6772)
==6832== by 0x492767: type_call (typeobject.c:989)
==6832== by 0x440E97: _PyObject_MakeTpCall (call.c:167)
==6832==
1,0-1,1: NL '\n'
2,0-2,0: ENDMARKER ''
==6832==
==6832== HEAP SUMMARY:
==6832== in use at exit: 1,179,135 bytes in 8,467 blocks
==6832== total heap usage: 93,677 allocs, 85,210 frees, 12,109,969 bytes allocated
==6832==
==6832== LEAK SUMMARY:
==6832== definitely lost: 0 bytes in 0 blocks
==6832== indirectly lost: 0 bytes in 0 blocks
==6832== possibly lost: 454,934 bytes in 1,972 blocks
==6832== still reachable: 724,201 bytes in 6,495 blocks
==6832== suppressed: 0 bytes in 0 blocks
==6832== Rerun with --leak-check=full to see details of leaked memory
==6832==
==6832== Use --track-origins=yes to see where uninitialised values come from
==6832== For lists of detected and suppressed errors, rerun with: -s
==6832== ERROR SUMMARY: 1 errors from 1 contexts (suppressed: 0 from 0)

[vstinner]

==6832== at 0x4D87E9: PyUnicode_Decode (unicodeobject.c:3395)

This warning is a false alarm: the GCC builtin strcmp() function seems to read past the NUL byte. When recompiling Python with -fno-builtin GCC option, the warning is gone.

The valgrind warning can be reproduced using attached valgrind_strcmp_warn.c:

On Fedora 30 with:

• gcc (GCC) 9.2.1 20190827 (Red Hat 9.2.1-1)
• valgrind-3.15.0

GCC builtin strcmp() emits a false alarm in valgrind: it reads past the NUL
byte.

$ gcc -O3 valgrind_strcmp_warn.c -o valgrind_strcmp_warn -D NDEBUG=1 -g && valgrind ./valgrind_strcmp_warn
(...)
==29173== Conditional jump or move depends on uninitialised value(s)
==29173==    at 0x4011CB: PyUnicode_Decode.part.0 (valgrind_strcmp_warn.c:276)
==29173==    by 0x4898F42: (below main) (in /usr/lib64/libc-2.29.so)
==29173==
==29173== Conditional jump or move depends on uninitialised value(s)
==29173==    at 0x4011EE: PyUnicode_Decode.part.0 (valgrind_strcmp_warn.c:280)
==29173==    by 0x4898F42: (below main) (in /usr/lib64/libc-2.29.so)
==29173==
==29173== Conditional jump or move depends on uninitialised value(s)
==29173==    at 0x401221: PyUnicode_Decode.part.0 (valgrind_strcmp_warn.c:282)
==29173==    by 0x4898F42: (below main) (in /usr/lib64/libc-2.29.so)
(...)
==29173== ERROR SUMMARY: 3 errors from 3 contexts (suppressed: 0 from 0)

versus

$ gcc -O3 valgrind_strcmp_warn.c -o valgrind_strcmp_warn -D NDEBUG=1 -fno-builtin -g && valgrind ./valgrind_strcmp_warn
==29217== Memcheck, a memory error detector
(...)
==29217== ERROR SUMMARY: 0 errors from 0 contexts (suppressed: 0 from 0)

[vstinner]

The valgrind false alarm on GCC builtin strcmp() has been reported to the Valgrind bug tracker in 2011, with an update in 2012 and... nothing :-(
https://bugs.kde.org/show_bug.cgi?id=264936

curl worked around the issue by disabling valgrind on the impacted test:
https://lists.fedoraproject.org/pipermail/scm-commits/2011-February/567665.html

Passing "--partial-loads-ok=yes" to Valgrind has no effect... It became the default:
http://valgrind.org/docs/manual/mc-manual.html#opt.partial-loads-ok

[vstinner]

It seems like Python 2.7 is not affected.

[vstinner]

New changeset 03ab6b4 by Victor Stinner in branch 'master':
bpo-38118: Ignore Valgrind false alarm in PyUnicode_Decode() (GH-16651)
03ab6b4

[miss-islington]

New changeset 364532f by Miss Islington (bot) in branch '3.8':
bpo-38118: Ignore Valgrind false alarm in PyUnicode_Decode() (GH-16651)
364532f