Re: [PATCH 1/7] libibverbs: Add link layer field port attribute

[Jason Gunthorpe <jgunthorpe-ePGOBjL8dl3ta4EC/59zMFaTQe2KTcn/@public.gmane.org>]

On Tue, Jul 19, 2011 at 09:20:25PM +0000, Hefty, Sean wrote:
> > More clearly: what initializes the memory you've redefined to be
> > link_layer in libibverbs 1.1.5 ? I see nothing.
> 
> Why does the value in an older library cause an issue?

ABI compatability means if dynamic linking succeeds then the result
works, so the behavior of a new app, using new functionality when it
links to the old library must work.

The commit comment states the goal is to have new apps see
IBV_LINK_LAYER_UNSPECIFIED for all possible combinations of old stuff
underneath.

> Or, how will this library version interact with an older kernel?

The kernel interaction is not what I'm talking about - AFAIK, the
kernel interaction is OK. Old kernels zero the entire ib_port_attr
structure before copy_to_user so the reserved bytes are guaranteed to
be 0.

The problem is with the userspace ABI out of libibverbs. Current
libibverbs does not zero its ib_port_attr structure before returning.

Build this with Or's patch applied, then run it against libibverbs 1.1.5

 struct ibv_port_attr attr;
 attr.link_layer = 100;
 ibv_query_port(..,&attr);
 assert(attr.link_layer == IBV_LINK_LAYER_UNSPECIFIED); // Fails!

That is an ABI breaking issue that is not dealt with by Or's patch.

Couple choices:
 1) Require and document callers using ibv_query_port to zero attr
    before hand when using link_layer
 2) Crank the symbol version on ibv_query_port so dynamic linking
    fails
 3) Inline the memset in ibverbs.h:

    static inline int fixed_ibv_query_port(...)
    {
       attr->link_layer = 0;
       return ibv_query_port(...);
    }
    #define ibv_query_port(...) fixed_ibv_query_port(...)

#2 might be the best?

Jason
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