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

[Dmitry V. Levin]

On Fri, Sep 01, 2017 at 12:15:17PM +0300, Stanislav Kinsburskiy wrote:
> 31.08.2017 20:22, Dmitry V. Levin пишет:
> > 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.


-- 
ldv
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