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

[Dmitry V. Levin]

On Thu, Aug 31, 2017 at 02:40:23PM +0300, Dmitry V. Levin wrote:
> 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.

Alternatively, a much more simple fix would be to change 64-bit kernels
not to send the trailing 4 padding bytes of 64-bit aligned
struct autofs_v5_packet.  That is, just send
offsetofend(struct autofs_v5_packet, name) bytes instead of
sizeof(struct autofs_v5_packet) regardless of architecture.


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