Re: memop struct packing, 32/64 bits

[Keir Fraser <keir@xen.org>]

On 19/01/2012 20:49, "Konrad Rzeszutek Wilk" <konrad@darnok.org> wrote:

> On Thu, Jan 19, 2012 at 12:30:21PM -0800, Andres Lagar-Cavilla wrote:
>> Hi,
>> I had the following painful experience. I declared
>> 
>> struct xen_mem_event_op {
>>     uint8_t       op;           /* XENMEM_*_op_* */
>>     domid_t       domain;
>>     uint64_t buffer;
>>     uint64_t gfn;          /* IN:  gfn of page being operated on */
>> };
>> typedef struct xen_mem_event_op xen_mem_event_op_t;
>> 
>> to be passed as the argument of a memory op called form the toolstack. The
>> hypervisor is 64 bits and the toolstack is 32 bits. My toolstack code
>> simply:
>> 
>>     xen_mem_event_op_t meo;
>> ... set fields ...
>>     return do_memory_op(xch, mode, &meo, sizeof(meo));
>> 
>> No joy because 32 bits was packing the struct differently than 64 bits.
>> Namely, both were adding a 1 byte pad between 'op' and 'domain', but when
>> compiled in 64 bits mode for the hypervisor, an additional 4 byte pad was
>> thrown between 'domain' and 'buffer'.
>> 
> 
>> The first question is, what is the preferred way around this. Declare pads
>> inside the struct?
> 
> Yes. And also use __attribute(__packed__);

Not used in any public header files.

>> Exploring the include/public/memory.h declarations and toolstack code, I
>> see that no current declare includes __attribute__((aligned)) or
>> __attribute__((packed)), or explicit pads.
> 
> Sometimes they have #pragma(4), which will make the alignment be
> 32-bit (4-bytes) for 64-bit builds as well.

Not in any public header files. It's used in the auto-generated 32-on-64
compat headers, for the hypervisor's private use.

>> So how come things don't break more often for 32 bit toolstacks? pure
>> luck? Am I missing something?
> 
> If you do not use 'memcpy' you can escape some of these misues.

Mainly you just have to be careful.

 -- Keir

>> 
>> Thanks!
>> Andres
>> 
>> 
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