[Spice-devel] [PATCH spice-protocol v3] stats: Avoid holes in SpiceStat structure

[Christophe Fergeau]

On Fri, Jan 04, 2019 at 08:23:15AM +0000, Frediano Ziglio wrote:
> The SpiceStat structure can be 20 or 24 bytes depending on alignment.
> Being a memory mapped structure potentially used with lockless access,
> it is not good to have it unaligned.
> The current tool that reads this memory mapped file (reds_stats) is

This is more "The only tool that we know of", maybe some people built
other (potentially private/internal) tools, maybe not. So such changes
should be approached with care.. Since this is not a 'core' spice
structure, and I'd expect it's probably quickly quite limited when
someone starts using it, so this particular change is probably fine.
My understanding is that this change is mandatory for reds_stats to work
on Windows?


Christophe

> able to detect if the structure is either 20 or 24 bytes and act
> accordingly so changing this structure won't affect the behaviour
> (unless you have an old tool but as they are usually packaged together
> this is quite improbable).
> This will also help on Windows as in that system it is not possible for
> reds_stats to implement the same discovery trick implemented on Linux.
> On Windows it is not possible to read the size of the file mapping
> (on Windows to implement shared memory you can use a file mapping
> not associated to a file).
> The structure will change layout only on 32-bit architectures which is
> not recommended nowadays (the 64-bit platforms that we support align
> 64-bit integers to 64 bits).
> 
> Signed-off-by: Frediano Ziglio <fziglio at redhat.com>
> ---
>  spice/stats.h | 2 ++
>  1 file changed, 2 insertions(+)
> 
> Changes since v2:
> - typos and grammar.
> 
> Changes since v1:
> - update commit message and comment.
> 
> diff --git a/spice/stats.h b/spice/stats.h
> index d4c45ab..44c6482 100644
> --- a/spice/stats.h
> +++ b/spice/stats.h
> @@ -66,6 +66,8 @@ typedef struct SpiceStat {
>      uint32_t generation;
>      uint32_t num_of_nodes;
>      uint32_t root_index;
> +    /* to avoid holes in the structure on some platforms */
> +    uint32_t padding;
>      SpiceStatNode nodes[];
>  } SpiceStat;
>  
> -- 
> 2.20.1
> 
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