[Devel] [PATCH] autofs: fix autofs_v5_packet structure for compat mode

[Dmitry V. Levin]

On Thu, Aug 31, 2017 at 01:48:27PM +0300, Stanislav Kinsburskiy wrote:
> 
> 
> 31.08.2017 13:38, Dmitry V. Levin пишет:
> > On Thu, Aug 31, 2017 at 02:11:34PM +0400, Stanislav Kinsburskiy wrote:
> >> Due to integer variables alignment size of struct autofs_v5_packet in 300
> >> bytes in 32-bit architectures (instead of 304 bytes in 64-bits architectures).
> >>
> >> This may lead to memory corruption (64 bits kernel always send 304 bytes,
> >> while 32-bit userspace application expects for 300).
> >>
> >> https://jira.sw.ru/browse/PSBM-71078
> >>
> >> Signed-off-by: Stanislav Kinsburskiy <skinsbursky at virtuozzo.com>
> >> ---
> >>  include/uapi/linux/auto_fs4.h |    2 ++
> >>  1 file changed, 2 insertions(+)
> >>
> >> diff --git a/include/uapi/linux/auto_fs4.h b/include/uapi/linux/auto_fs4.h
> >> index e02982f..8729a47 100644
> >> --- a/include/uapi/linux/auto_fs4.h
> >> +++ b/include/uapi/linux/auto_fs4.h
> >> @@ -137,6 +137,8 @@ struct autofs_v5_packet {
> >>  	__u32 pid;
> >>  	__u32 tgid;
> >>  	__u32 len;
> >> +	__u32 blob;		/* This is needed to align structure up to 8
> >> +				   bytes for ALL archs including 32-bit */
> >>  	char name[NAME_MAX+1];
> >>  };
> > 
> > This change breaks ABI because it changes offsetof(struct autofs_v5_packet, name).
> > If you need to fix the alignment, use  __attribute__((aligned(8))).
> > 
> 
> Nice to know you're watching.
> Yes, attribute is better.
> But how ABI is broken? On x86_64 this alignment is implied, so nothing is changed.

Your change increases offsetof(struct autofs_v5_packet, name) by 4 on all
architectures.  On architectures where the structure is 32-bit aligned
this also leads to increase of its size by 4.

> > An alignment change would also be an ABI breakage on 32-bit architectures,
> > though.
> > 
> 
> True.
> But from my POW better have it working on 64bit archs for 32bit apps.
> But anyway, upstream guys will device, whether they want 32-bit autofs applications properly work on 64 or 32 bits.

Let's fix old bugs without introducing new bugs.
The right fix here seems to be a compat structure, that is, both 64-bit
and 32-bit kernels should send the same 32-bit aligned structure, and
it has to be the same structure sent by traditional 32-bit kernels.


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