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

[Stanislav Kinsburskiy]

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.

> 
> 
> 
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