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

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Message-ID: <565A3128.8010605@digikod.net>
Date:	Sat, 28 Nov 2015 23:56:40 +0100
From:	Mickaël Salaün <mic@...ikod.net>
To:	Richard Weinberger <richard@....at>, 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


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]?

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?

 Mickaël


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