[Devel] [RFC PATCH 2/2] autofs: sent 32-bit sized packet for 32-bit process

[Stanislav Kinsburskiy]

01.09.2017 21:23, Dmitry V. Levin P?P8QP5Q:
> On Fri, Sep 01, 2017 at 05:02:45PM +0300, Stanislav Kinsburskiy wrote:
>>
>>
>> 01.09.2017 16:53, Dmitry V. Levin P?P8QP5Q:
>>> On Fri, Sep 01, 2017 at 12:15:17PM +0300, Stanislav Kinsburskiy wrote:
>>>> 31.08.2017 20:22, Dmitry V. Levin P?P8QP5Q:
>>>>> On Thu, Aug 31, 2017 at 05:57:11PM +0400, Stanislav Kinsburskiy wrote:
>>>>>> The structure autofs_v5_packet (except name) is not aligned by 8 bytes, which
>>>>>> lead to different sizes in 32 and 64-bit architectures.
>>>>>> Let's form 32-bit compatible packet when daemon has 32-bit addressation.
>>>>>>
>>>>>> Signed-off-by: Stanislav Kinsburskiy <skinsbursky at virtuozzo.com>
>>>>>> ---
>>>>>>  fs/autofs4/waitq.c |   11 +++++++++--
>>>>>>  1 file changed, 9 insertions(+), 2 deletions(-)
>>>>>>
>>>>>> diff --git a/fs/autofs4/waitq.c b/fs/autofs4/waitq.c
>>>>>> index 309ca6b..484cf2e 100644
>>>>>> --- a/fs/autofs4/waitq.c
>>>>>> +++ b/fs/autofs4/waitq.c
>>>>>> @@ -153,12 +153,19 @@ static void autofs4_notify_daemon(struct autofs_sb_info *sbi,
>>>>>>  	{
>>>>>>  		struct autofs_v5_packet *packet = &pkt.v5_pkt.v5_packet;
>>>>>>  		struct user_namespace *user_ns = sbi->pipe->f_cred->user_ns;
>>>>>> +		size_t name_offset;
>>>>>>  
>>>>>> -		pktsz = sizeof(*packet);
>>>>>> +		if (sbi->is32bit)
>>>>>> +			name_offset = offsetof(struct autofs_v5_packet, len) +
>>>>>> +				      sizeof(packet->len);
>>>>>> +		else
>>>>>> +			name_offset = offsetof(struct autofs_v5_packet, name);
>>>>>
>>>>> This doesn't help at all because the offset of struct autofs_v5_packet.name
>>>>> does not change.
>>>>>
>>>>>> +		pktsz = name_offset + sizeof(packet->name);
>>>>>
>>>>> What changes is pktsz: it's either sizeof(struct autofs_v5_packet)
>>>>> or 4 bytes less, depending on the architecture.
>>>>
>>>> Indeed. Thanks!
>>>>
>>>>> For example,
>>>>>
>>>>> #ifdef CONFIG_COMPAT
>>>>> 	if (__alignof__(compat_u64) < __alignof__(u64) && sbi->is32bit)
>>>>
>>>> Unfortunately I don't get this "alignof" checks.
>>>> The intention of "is32bit" was to define this compat case completely.
>>>> Why are they required?
>>>
>>> On some 32-bit architectures like arm, u64 is 64-bit aligned, on others
>>> like x86 it is not.  This alignof check ensures that compat 32-bit
>>> architectures with 64-bit alignment are not going to be broken
>>> by the change.
>>
>> Thanks for the explanation!
>> But looks like the issue is hidden so deep (thanks to O_DIRECT), that becomes unimportant.
> 
> I'm not quite sure how O_DIRECT makes this unimportant.
> For example, automount_dispatch_io from systemd tries to read exactly
> sizeof(union autofs_v5_packet_union) bytes and fails in case of short read.
> 

That's weird. Can you reproduce it?
The problem is that kernel always send arch packet (i.e. 300 bytes for x86, or 304 for x86_64).
So, how can systemd get less? It can get more, but only if it'll try to read more.
Otherwise, as "man pipe(2)" says:

*  If a read(2) specifies a buffer size that is smaller than the next packet, then the requested number of bytes are read, and the excess bytes in the packet are discarded.

> 
> 
> 
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