getargs.c skipitem() doesn't skip 'w*'

[larryhastings]

BPO	23927
Nosy	@larryhastings, @serhiy-storchaka, @miss-islington
PRs	bpo-23927: Make getargs.c skipitem() skipping 'w*'. #8192 [3.7] bpo-23927: Make getargs.c skipitem() skipping 'w*'. (GH-8192) #8251 [3.6] bpo-23927: Make getargs.c skipitem() skipping 'w*'. (GH-8192) #8252 [2.7] bpo-23927: Make getargs.c skipitem() skipping 'w*'. (GH-8192). #8255 gh-68114: Fix handling for removed PyArg_ParseTuple formatters #8204

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

Show more details

GitHub fields:

assignee = 'https://github.com/larryhastings'
closed_at = <Date 2018-07-23.19:38:55.308>
created_at = <Date 2015-04-13.02:44:28.359>
labels = ['interpreter-core', '3.7', '3.8', 'type-crash']
title = "getargs.c skipitem() doesn't skip 'w*'"
updated_at = <Date 2018-07-23.19:38:55.307>
user = 'https://github.com/larryhastings'

bugs.python.org fields:

activity = <Date 2018-07-23.19:38:55.307>
actor = 'serhiy.storchaka'
assignee = 'larry'
closed = True
closed_date = <Date 2018-07-23.19:38:55.308>
closer = 'serhiy.storchaka'
components = ['Interpreter Core']
creation = <Date 2015-04-13.02:44:28.359>
creator = 'larry'
dependencies = []
files = []
hgrepos = []
issue_num = 23927
keywords = ['patch']
message_count = 5.0
messages = ['240588', '321465', '321485', '321486', '321489']
nosy_count = 3.0
nosy_names = ['larry', 'serhiy.storchaka', 'miss-islington']
pr_nums = ['8192', '8251', '8252', '8255', '8204']
priority = 'low'
resolution = 'fixed'
stage = 'resolved'
status = 'closed'
superseder = None
type = 'crash'
url = 'https://bugs.python.org/issue23927'
versions = ['Python 2.7', 'Python 3.6', 'Python 3.7', 'Python 3.8']

[larryhastings]

tl;dr: skipitem() in Python/getsargs.c doesn't know about the 'w*' format unit, which could lead to a crash. It should know about it.

--

If:

• you have an extension that parses its arguments with
  PyArg_ParseTupleAndKeywords (or its cousins),
• you have optional positional parameters, and
• you run out of positional arguments

CPython will call skipitem() to skip over the remaining positional parameters so it can process keyword arguments. It does this by knowing all the format units, and iterating over them and throwing away the various varargs pointers until it hits the keyword arguments part of the format string.

Since nobody ever explained about the "w*" format unit to it, that means when it sees 'w*' it will misinterpret it.

In both 2.7 and 3.x, it'll interpret it as 'w', throw away one "char *" item from varargs, then the next thing that tries to parse a format unit (skipitem or convertsimple) will see the '*' and throw an 'impossible<bad format char>' exception.

(It's another bug that skipitem() supports 'w' and 'w#' in Python 3, but I filed that separately as bpo-23926 .)

[serhiy-storchaka]

New changeset 504373c by Serhiy Storchaka in branch 'master':
bpo-23927: Make getargs.c skipitem() skipping 'w*'. (GH-8192)
504373c

[serhiy-storchaka]

New changeset ef19fd2 by Serhiy Storchaka in branch '2.7':
[2.7] bpo-23927: Make getargs.c skipitem() skipping 'w*'. (GH-8192). (GH-8255)
ef19fd2

[miss-islington]

New changeset 48d2aeb by Miss Islington (bot) in branch '3.7':
bpo-23927: Make getargs.c skipitem() skipping 'w*'. (GH-8192)
48d2aeb

[miss-islington]

New changeset ec75620 by Miss Islington (bot) in branch '3.6':
bpo-23927: Make getargs.c skipitem() skipping 'w*'. (GH-8192)
ec75620