Re: [PATCH] memory: Directly dispatch alias accesses on origin memory region

[Paolo Bonzini <pbonzini@redhat.com>]

On 16/08/20 19:30, Philippe Mathieu-Daudé wrote:
> There is an issue when accessing an alias memory region via the
> memory_region_dispatch_read() / memory_region_dispatch_write()
> calls:
> 
> The memory_region_init_alias() flow is:
> 
>   memory_region_init_alias()
>   -> memory_region_init()
>      -> object_initialize(TYPE_MEMORY_REGION)
>         -> memory_region_initfn()
>            -> mr->ops = &unassigned_mem_ops;
> 
> Later when accessing the alias, the memory_region_dispatch_read()
> flow is:
> 
>   memory_region_dispatch_read()
>   -> memory_region_access_valid(mr)
>      -> mr->ops->valid.accepts()
>         -> unassigned_mem_accepts()
>         <- false
>      <- false
>    <- MEMTX_DECODE_ERROR
> 
> The caller gets a MEMTX_DECODE_ERROR while the access is OK.

What is the path that leads to this call?

> Fix by directly dispatching aliases accesses to its origin region.
> 
> Fixes: 2cdfcf272d ("memory: assign MemoryRegionOps to all regions")

I don't think the "Fixes" is okay because you'd have gotten a different
bug before.

> +    if (mr->alias) {
> +        addr += mr->alias_offset;
> +        mr = mr->alias;
> +    }

Also, I think this would have to be a while loop.

Paolo