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

[Dmitry V. Levin]

On Sat, Sep 02, 2017 at 02:26:26PM +0300, Stanislav Kinsburskiy wrote:
> 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).

Well, if sending a lengthy packet is not a problem, what is the problem
you were trying to solve by patching struct autofs_v5_packet?


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