linux-kernel - Re: [PATCH 2/2] um: Use race-free temporary file creation

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Message-ID: <565A31B6.5050802@nod.at>
Date:	Sat, 28 Nov 2015 23:59:02 +0100
From:	Richard Weinberger <richard@....at>
To:	Mickaël Salaün <mic@...ikod.net>,
	linux-kernel@...r.kernel.org
Cc:	Jeff Dike <jdike@...toit.com>,
	Tristan Schmelcher <tschmelcher@...gle.com>,
	Greg Kroah-Hartman <gregkh@...uxfoundation.org>,
	user-mode-linux-devel@...ts.sourceforge.net,
	user-mode-linux-user@...ts.sourceforge.net,
	Al Viro <viro@...iv.linux.org.uk>
Subject: Re: [PATCH 2/2] um: Use race-free temporary file creation

Am 28.11.2015 um 23:56 schrieb Mickaël Salaün:
> 
> On 28/11/2015 23:07, Richard Weinberger wrote:
>> Am 28.11.2015 um 22:32 schrieb Mickaël Salaün:
>>> Open the memory mapped file with the O_TMPFILE flag when available.
>>>
>>> Signed-off-by: Mickaël Salaün <mic@...ikod.net>
>>> ---
>>>  arch/um/os-Linux/mem.c | 12 ++++++++++++
>>>  1 file changed, 12 insertions(+)
>>>
>>> diff --git a/arch/um/os-Linux/mem.c b/arch/um/os-Linux/mem.c
>>> index 798aeb4..fe52e2d 100644
>>> --- a/arch/um/os-Linux/mem.c
>>> +++ b/arch/um/os-Linux/mem.c
>>> @@ -106,6 +106,18 @@ static int __init make_tempfile(const char *template)
>>>  		}
>>>  	}
>>>  
>>> +#ifdef O_TMPFILE
>>> +	fd = open(tempdir, O_CLOEXEC | O_RDWR | O_EXCL | O_TMPFILE, 0700);
>>> +	/*
>>> +	 * If the running system does not support O_TMPFILE flag then retry
>>> +	 * without it.
>>> +	 */
>>> +	if (fd != -1 || (errno != EINVAL && errno != EISDIR &&
>>
>> Why are you handling EISDIR?
> 
> I follow the man page for open [1], I think it was a workaround needed for some kernel versions just after the O_TMPFILE was added but before the support for EOPNOTSUPP.
> We may need to add the EACCES too for some version of glibc [2, 3]?

Makes sense! :)

> 1. http://man7.org/linux/man-pages/man2/openat.2.html#BUGS
> 2. Commit 69a91c237ab0ebe4e9fdeaf6d0090c85275594ec and https://sourceware.org/bugzilla/show_bug.cgi?id=17523
> 3. https://bugs.gentoo.org/529044
> 
>>
>>> +			errno != EOPNOTSUPP))
>>> +		return fd;
>>> +	errno = 0;
>>
>> Why are you resetting errno?
> 
> It's to ignore/reset the error code from open, but it may not be needed because of the next call to malloc?

But then you'd have to reset errno after every syscall. :-)

Thanks,
//richard
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