pass big values for arg to fcntl.ioctl

[ukl]

BPO	30665
Nosy	@Yhg1s, @mdickinson, @vstinner, @serhiy-storchaka
PRs	bpo-30665: fcntl.ioctl() now accepts larger range of integers #2286
Files	unsigned-long-arg-for-ioctl.patch

^{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 = None
created_at = <Date 2017-06-14.14:27:52.134>
labels = ['type-bug', '3.9', '3.10']
title = 'pass big values for arg to fcntl.ioctl'
updated_at = <Date 2020-05-28.01:38:07.976>
user = 'https://bugs.python.org/ukl'

bugs.python.org fields:

activity = <Date 2020-05-28.01:38:07.976>
actor = 'ned.deily'
assignee = 'none'
closed = False
closed_date = None
closer = None
components = []
creation = <Date 2017-06-14.14:27:52.134>
creator = 'ukl'
dependencies = []
files = ['46951']
hgrepos = []
issue_num = 30665
keywords = ['patch']
message_count = 4.0
messages = ['296008', '296242', '296307', '296405']
nosy_count = 6.0
nosy_names = ['twouters', 'mark.dickinson', 'vstinner', 'serhiy.storchaka', 'ukl', 'cfi']
pr_nums = ['2286']
priority = 'normal'
resolution = None
stage = 'patch review'
status = 'open'
superseder = None
type = 'behavior'
url = 'https://bugs.python.org/issue30665'
versions = ['Python 3.9', 'Python 3.10']

[ukl]

When passing a big integer value to fcntl.ioctl this might result in

 OverflowError: signed integer is greater than maximum

. Traditionally ioctl(3) takes a pointer as third argument. The fcntl module however uses an int (format specifier 'i') which is too small to hold a pointer on 64 bit architectures.

The fix is to use 'k' (and an unsigned long) instead of 'i' (and an int). An unsigned long is suitable to hold a pointer value on AFAIK all platforms that matter today. (And it works on all platforms where int is already good enough, so the suggested change doesn't do any harm.)

A patch is attached.

[serhiy-storchaka]

The FreeBSD [1], OpenBSD [2], and Solaris 10 [3] documentations specify the third argument of ioctl() as either an int or a pointer.

Passing a 64-bit long instead of a 32-bit int on big-endian platform can cause incorrect interpretation of an argument.

[1] https://www.freebsd.org/cgi/man.cgi?query=ioctl&sektion=2
[2] http://man.openbsd.org/ioctl.2
[3] https://docs.oracle.com/cd/E26505_01/html/816-5167/ioctl-2.html

[ukl]

So the only option to fix this is to determine the type of arg from the request parameter? I failed to find the implementation of ioctl for linux in glibc, the best I found is one that only seems to be used on powerpc[1] which seems to assume that the third argument is a pointer. The Linux kernel prototype takes an unsigned long[2] and encodes the size of the data pointed to in the request using _IOC[3]. On hurd there are different macros to determine the type. I'm not fluent in *BSD and Solaris (and other operating systems), so I cannot add specifics for them. I cannot even say which OS that runs Python can run in 64 bit BE mode.

[1] https://sourceware.org/git/?p=glibc.git;a=blob;f=sysdeps/unix/sysv/linux/powerpc/ioctl.c;h=e2e3d3357f9f5ee9ffe5e9f0588ebae9976c0dfd;hb=HEAD
[2] https://git.kernel.org/pub/scm/linux/kernel/git/torvalds/linux.git/tree/fs/ioctl.c?h=v4.12-rc5#n691
[3] https://git.kernel.org/pub/scm/linux/kernel/git/torvalds/linux.git/tree/include/uapi/asm-generic/ioctl.h

[serhiy-storchaka]

PR 2286 uses for the third argument char* on Linux and int or unsigned int on other platforms. It keeps some degree of integer overflow control.

[serhiy-storchaka]

My PR also is not correct, because even on Linux some calls require an argument of type int (e.g. file descriptor).

It is not possible safely pass integer arguments in wider range without introducing an explicit parameter for this. But what is the use case? What calls require an integer argument in wider range? If it needs a pointer, it is usually a pointer to some request-specific structure, and you can pass a buffer object that represent such structure. The address of the buffer will be passed to the ioctl() system call. It is currently not possible to pass an arbitrary pointer.

For now, int seems working well in existing code. If there are concrete examples for integer arguments in the wider range or arbitrary pointers, we will try to find different solution.