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Date:	Fri, 25 Mar 2016 08:50:10 +0100
From:	Nicolai Stange <nicstange@...il.com>
To:	Jan Kara <jack@...e.cz>
Cc:	Nicolai Stange <nicstange@...il.com>,
	Andrew Morton <akpm@...ux-foundation.org>,
	Al Viro <viro@...iv.linux.org.uk>, Jan Kara <jack@...e.com>,
	Johannes Weiner <hannes@...xchg.org>,
	Michal Hocko <mhocko@...e.com>,
	Ross Zwisler <ross.zwisler@...ux.intel.com>,
	Mel Gorman <mgorman@...hsingularity.net>,
	Junichi Nomura <j-nomura@...jp.nec.com>,
	Hugh Dickins <hughd@...gle.com>,
	Matthew Wilcox <willy@...ux.intel.com>, linux-mm@...ck.org,
	linux-kernel@...r.kernel.org
Subject: Re: [PATCH] mm/filemap: generic_file_read_iter(): check for zero reads unconditionally

Jan Kara <jack@...e.cz> writes:

> On Thu 24-03-16 12:08:58, Nicolai Stange wrote:
>> If
>> - generic_file_read_iter() gets called with a zero read length,
>> - the read offset is at a page boundary,
>> - IOCB_DIRECT is not set
>> - and the page in question hasn't made it into the page cache yet,
>> then do_generic_file_read() will trigger a readahead with a req_size hint
>> of zero.
>> 
>> Since roundup_pow_of_two(0) is undefined, UBSAN reports
>> 
>>   UBSAN: Undefined behaviour in include/linux/log2.h:63:13
>>   shift exponent 64 is too large for 64-bit type 'long unsigned int'
>>   CPU: 3 PID: 1017 Comm: sa1 Tainted: G L 4.5.0-next-20160318+ #14
>>   [...]
>>   Call Trace:
>>    [...]
>>    [<ffffffff813ef61a>] ondemand_readahead+0x3aa/0x3d0
>>    [<ffffffff813ef61a>] ? ondemand_readahead+0x3aa/0x3d0
>>    [<ffffffff813c73bd>] ? find_get_entry+0x2d/0x210
>>    [<ffffffff813ef9c3>] page_cache_sync_readahead+0x63/0xa0
>>    [<ffffffff813cc04d>] do_generic_file_read+0x80d/0xf90
>>    [<ffffffff813cc955>] generic_file_read_iter+0x185/0x420
>>    [...]
>>    [<ffffffff81510b06>] __vfs_read+0x256/0x3d0
>>    [...]
>> 
>> when get_init_ra_size() gets called from ondemand_readahead().
>> 
>> The net effect is that the initial readahead size is arch dependent for
>> requested read lengths of zero: for example, since
>> 
>>   1UL << (sizeof(unsigned long) * 8)
>> 
>> evaluates to 1 on x86 while its result is 0 on ARMv7, the initial readahead
>> size becomes 4 on the former and 0 on the latter.
>> 
>> What's more, whether or not the file access timestamp is updated for zero
>> length reads is decided differently for the two cases of IOCB_DIRECT
>> being set or cleared: in the first case, generic_file_read_iter()
>> explicitly skips updating that timestamp while in the latter case, it is
>> always updated through the call to do_generic_file_read().
>> 
>> According to POSIX, zero length reads "do not modify the last data access
>> timestamp" and thus, the IOCB_DIRECT behaviour is POSIXly correct.
>> 
>> Let generic_file_read_iter() unconditionally check the requested read
>> length at its entry and return immediately with success if it is zero.
>> 
>> Signed-off-by: Nicolai Stange <nicstange@...il.com>
>
> Makes sense to me. You can add:
>
> Reviewed-by: Jan Kara <jack@...e.cz>

Thank you very much for reviewing this!

Nicolai


>
> 								Honza
>
>> diff --git a/mm/filemap.c b/mm/filemap.c
>> index 7c00f10..a8c69c8 100644
>> --- a/mm/filemap.c
>> +++ b/mm/filemap.c
>> @@ -1840,15 +1840,16 @@ generic_file_read_iter(struct kiocb *iocb, struct iov_iter *iter)
>>  	ssize_t retval = 0;
>>  	loff_t *ppos = &iocb->ki_pos;
>>  	loff_t pos = *ppos;
>> +	size_t count = iov_iter_count(iter);
>> +
>> +	if (!count)
>> +		goto out; /* skip atime */
>>  
>>  	if (iocb->ki_flags & IOCB_DIRECT) {
>>  		struct address_space *mapping = file->f_mapping;
>>  		struct inode *inode = mapping->host;
>> -		size_t count = iov_iter_count(iter);
>>  		loff_t size;
>>  
>> -		if (!count)
>> -			goto out; /* skip atime */
>>  		size = i_size_read(inode);
>>  		retval = filemap_write_and_wait_range(mapping, pos,
>>  					pos + count - 1);
>> -- 
>> 2.7.4
>> 
>> 

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