Re: [PATCH] hostmem-file: add offset option

[David Hildenbrand <david@redhat.com>]

On 02.02.23 23:56, Alexander Graf wrote:
> Add an option for hostmem-file to start the memory object at an offset
> into the target file. This is useful if multiple memory objects reside
> inside the same target file, such as a device node.
> 
> In particular, it's useful to map guest memory directly into /dev/mem
> for experimentation.
> 
> Signed-off-by: Alexander Graf <graf@amazon.com>
> ---

[...]

>   
> -    block = qemu_ram_alloc_from_fd(size, mr, ram_flags, fd, 0, readonly, errp);
> +    block = qemu_ram_alloc_from_fd(size, mr, ram_flags, fd, offset, readonly,
> +                                   errp);

IIUC, the existing offset implementation is a bit broken:


(1) qemu_ram_alloc_from_fd() does not handle the offset parameter 
correctly: The "file_size > 0 && file_size < size" calculation does not 
seem to consider the offset when checking for file sizes.

(2) file_ram_alloc() most probably does not handle ftruncate() 
correctly. Especially on a yet empty file (where we set truncate=true), 
that might be insufficient.

(3) file_ram_alloc() and/or qemu_ram_alloc_from_fd() do not verify that 
the offset is properly aligned to block->page_size.


-- 
Thanks,

David / dhildenb