[issue35561] Valgrind reports Syscall param epoll_ctl(event) points to uninitialised byte(s)

[Benjamin Peterson]

Benjamin Peterson  added the comment:

The valgrind bug was also fixed https://bugs.kde.org/show_bug.cgi?id=422623.

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[issue35561] Valgrind reports Syscall param epoll_ctl(event) points to uninitialised byte(s)

[miss-islington]

miss-islington  added the comment:


New changeset 296383b6d05f9617283aeb5b601106f84b016198 by Miss Islington (bot) 
in branch '3.7':
bpo-35561: Supress valgrind false alarm on epoll_ctl(event) (GH-18060)
https://github.com/python/cpython/commit/296383b6d05f9617283aeb5b601106f84b016198


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[issue35561] Valgrind reports Syscall param epoll_ctl(event) points to uninitialised byte(s)

[miss-islington]

miss-islington  added the comment:


New changeset 23793edf0d61aa98478541d0ff8f2b900ff1813d by Miss Islington (bot) 
in branch '3.8':
bpo-35561: Supress valgrind false alarm on epoll_ctl(event) (GH-18060)
https://github.com/python/cpython/commit/23793edf0d61aa98478541d0ff8f2b900ff1813d


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[issue35561] Valgrind reports Syscall param epoll_ctl(event) points to uninitialised byte(s)

[STINNER Victor]

STINNER Victor  added the comment:

Thanks Nikolaus Rath for the bug report, and thnaks Zackery Spytz for the fix!

The fix will be backported automatically to 3.7 and 3.8 as soon as CI tests 
pass.

--
resolution:  -> fixed
stage: patch review -> resolved
status: open -> closed
versions: +Python 3.7

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[issue35561] Valgrind reports Syscall param epoll_ctl(event) points to uninitialised byte(s)

[STINNER Victor]

STINNER Victor  added the comment:


New changeset d8ef64422a75f40cecdb1a7ee43492607d3daaf6 by Victor Stinner 
(Zackery Spytz) in branch 'master':
bpo-35561: Supress valgrind false alarm on epoll_ctl(event) (GH-18060)
https://github.com/python/cpython/commit/d8ef64422a75f40cecdb1a7ee43492607d3daaf6


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[issue35561] Valgrind reports Syscall param epoll_ctl(event) points to uninitialised byte(s)

[miss-islington]

Change by miss-islington :


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pull_requests: +17464
pull_request: https://github.com/python/cpython/pull/18071

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[issue35561] Valgrind reports Syscall param epoll_ctl(event) points to uninitialised byte(s)

[miss-islington]

Change by miss-islington :


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pull_request: https://github.com/python/cpython/pull/18070

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[issue35561] Valgrind reports Syscall param epoll_ctl(event) points to uninitialised byte(s)

[Zackery Spytz]

Change by Zackery Spytz :


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pull_request: https://github.com/python/cpython/pull/18060

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[issue35561] Valgrind reports Syscall param epoll_ctl(event) points to uninitialised byte(s)

[STINNER Victor]

STINNER Victor  added the comment:

> Thank you, Victor, for the comments.  I will update PR 17782.

Since it's a very different approach (modify C vs suppress the false alarm), I 
suggest instead to open a new different PR.

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[issue35561] Valgrind reports Syscall param epoll_ctl(event) points to uninitialised byte(s)

[Zackery Spytz]

Zackery Spytz  added the comment:

Thank you, Victor, for the comments.  I will update PR 17782.

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[issue35561] Valgrind reports Syscall param epoll_ctl(event) points to uninitialised byte(s)

[STINNER Victor]

STINNER Victor  added the comment:

> Initialize ev.data to 0

I dislike this option, since the code is legit: Valgrind produces a false alarm.

> Use Misc/valgrind-python.supp to ignore this *false alarm*

I prefer this option.

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[issue35561] Valgrind reports Syscall param epoll_ctl(event) points to uninitialised byte(s)

[STINNER Victor]

STINNER Victor  added the comment:

On my Fedora 31, epoll_event structure is defined in sys/epoll.h as:

typedef union epoll_data
{
  void *ptr;
  int fd;
  uint32_t u32;
  uint64_t u64;
} epoll_data_t;

struct epoll_event
{
  uint32_t events;  /* Epoll events */
  epoll_data_t data;/* User data variable */
} __EPOLL_PACKED;


I can reproduce the issue using this Python script:

import select
p = select.epoll()
p.register(1, select.EPOLLIN)
p.poll(0)

The Linux syscall is defined as:

asmlinkage long sys_epoll_ctl(int epfd, int op, int fd,
struct epoll_event __user *event);

It's implemented in fs/eventpoll.c:

https://github.com/torvalds/linux/blob/c79f46a282390e0f5b306007bf7b11a46d529538/fs/eventpoll.c#L2077-L2083

Valgrind complains that ev.data is only partially initialized: it's a 64-bit 
structure, but we only set ev.data.fd. I guess that the glibc implements 
epoll_ctl() as a raw system call: Valgrind is unable to know if it's ok to only 
partially initialize the epoll_data union.


Benjamin: "I suspect Valgrind is being too conservative. union epoll_data is 64 
bits wide but the file descriptor only occupies the first 32 bits. The last 32 
bits don't need to be initialized by the application."

Right. But Valgrind is not smart enough.

I see 2 options:

* Initialize ev.data to 0
* Use Misc/valgrind-python.supp to ignore this *false alarm*

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[issue35561] Valgrind reports Syscall param epoll_ctl(event) points to uninitialised byte(s)

[Zackery Spytz]

Change by Zackery Spytz :


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nosy: +ZackerySpytz
versions: +Python 3.9

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[issue35561] Valgrind reports Syscall param epoll_ctl(event) points to uninitialised byte(s)

[Zackery Spytz]

Change by Zackery Spytz :


--
keywords: +patch
pull_requests: +17215
stage:  -> patch review
pull_request: https://github.com/python/cpython/pull/17782

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[issue35561] Valgrind reports Syscall param epoll_ctl(event) points to uninitialised byte(s)

[Nikolaus Rath]

Nikolaus Rath  added the comment:

Is there a good reason to not explicitly initialize them nevertheless? Valgrind 
is a common tool, and this false positive will affect everyone attempting to 
run it on a Python extension module.

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[issue35561] Valgrind reports Syscall param epoll_ctl(event) points to uninitialised byte(s)

[Benjamin Peterson]

Benjamin Peterson  added the comment:

I suspect Valgrind is being too conservative. union epoll_data is 64 bits wide 
but the file descriptor only occupies the first 32 bits. The last 32 bits don't 
need to be initialized by the application.

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nosy: +benjamin.peterson

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[issue35561] Valgrind reports Syscall param epoll_ctl(event) points to uninitialised byte(s)

[Nikolaus Rath]

Nikolaus Rath  added the comment:

$ ./configure CFLAGS="-O0 -g" --with-valgrind && make -j8

still gives

==13281== Memcheck, a memory error detector
==13281== Copyright (C) 2002-2015, and GNU GPL'd, by Julian Seward et al.
==13281== Using Valgrind-3.12.0.SVN and LibVEX; rerun with -h for copyright info
==13281== Command: /home/nikratio/clones/cpython/python 
/home/nikratio/in-progress/pyfuse3/test/../examples/hello.py 
/tmp/pytest-of-nikratio/pytest-15/test_hello_hello_py_0
==13281== 
==13281== Syscall param epoll_ctl(event) points to uninitialised byte(s)
==13281==at 0x584906A: epoll_ctl (syscall-template.S:84)
==13281==by 0xB5C07FA: pyepoll_internal_ctl (selectmodule.c:1392)
==13281==by 0xB5C08CB: select_epoll_register_impl (selectmodule.c:1438)
==13281==by 0xB5C112A: select_epoll_register (selectmodule.c.h:599)
==13281==by 0x173031: _PyMethodDef_RawFastCallKeywords (call.c:658)
==13281==by 0x2FEFCD: _PyMethodDescr_FastCallKeywords (descrobject.c:290)
==13281==by 0x21ED25: call_function (ceval.c:4611)
==13281==by 0x21AB4E: _PyEval_EvalFrameDefault (ceval.c:3183)
==13281==by 0x21007C: PyEval_EvalFrameEx (ceval.c:533)
==13281==by 0x17245F: function_code_fastcall (call.c:285)
==13281==by 0x1728B5: _PyFunction_FastCallKeywords (call.c:410)
==13281==by 0x21EDF4: call_function (ceval.c:4634)
==13281==  Address 0xffeff2d94 is on thread 1's stack
==13281==  in frame #1, created by pyepoll_internal_ctl (selectmodule.c:1379)

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[issue35561] Valgrind reports Syscall param epoll_ctl(event) points to uninitialised byte(s)

[Stefan Krah]

Stefan Krah  added the comment:

"--with-valgrind --with-pydebug" looks suspicious, it essentially
mixes two different memory checkers.

1) CFLAGS="-O0 -g" --without-pymalloc

2) CFLAGS="-O0 -g" --with-valgrind

should both work.


Can you try if this fixes the error?

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nosy: +skrah

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[issue35561] Valgrind reports Syscall param epoll_ctl(event) points to uninitialised byte(s)

[Nikolaus Rath]

Nikolaus Rath  added the comment:

Same error with 3.7.1 and 3.6.7 (though line numbers differ slightly).

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[issue35561] Valgrind reports Syscall param epoll_ctl(event) points to uninitialised byte(s)

[Nikolaus Rath]

New submission from Nikolaus Rath :

With current git master, configured --with-valgrind --with-pydebug, I get:

==31074== Command: /home/nikratio/clones/cpython/python 
/home/nikratio/in-progress/pyfuse3/test/../examples/hello.py 
/tmp/pytest-of-nikratio/pytest-11/test_hello_hello_py_0
==31074== 
==31074== Syscall param epoll_ctl(event) points to uninitialised byte(s)
==31074==at 0x584906A: epoll_ctl (syscall-template.S:84)
==31074==by 0xBDAA493: pyepoll_internal_ctl (selectmodule.c:1392)
==31074==by 0xBDAA59F: select_epoll_register_impl (selectmodule.c:1438)
==31074==by 0xBDAA5F8: select_epoll_register (selectmodule.c.h:599)
==31074==by 0x174E15: _PyMethodDef_RawFastCallKeywords (call.c:658)
==31074==by 0x300BCA: _PyMethodDescr_FastCallKeywords (descrobject.c:290)
==31074==by 0x21FC05: call_function (ceval.c:4611)
==31074==by 0x22B5E7: _PyEval_EvalFrameDefault (ceval.c:3183)
==31074==by 0x2206FF: PyEval_EvalFrameEx (ceval.c:533)
==31074==by 0x173B61: function_code_fastcall (call.c:285)
==31074==by 0x174737: _PyFunction_FastCallKeywords (call.c:410)
==31074==by 0x21FDF4: call_function (ceval.c:4634)
==31074==  Address 0xffeffeb4c is on thread 1's stack
==31074==  in frame #1, created by pyepoll_internal_ctl (selectmodule.c:1379)

To reproduce:

$ python-dev -m pip install --user pyfuse3  # for dependencies
$ git clone https://github.com/libfuse/pyfuse3.git
$ valgrind --trace-children=yes "--trace-children-skip=*mount*" python-dev -m 
pytest test/


pyfuse3 provides a C extension module, but I believe the problem is in the 
interpreter core as the stacktrace does not include anything from the extension.

--
components: Interpreter Core
messages: 332348
nosy: nikratio
priority: normal
severity: normal
status: open
title: Valgrind reports Syscall param epoll_ctl(event) points to uninitialised 
byte(s)
type: compile error
versions: Python 3.8

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