Re: [Qemu-devel] [PATCH v7 1/8] linker-loader: Add new 'write pointer' command

[Eric Blake]

On 02/16/2017 03:43 AM, Igor Mammedov wrote:

>> +++ b/hw/acpi/bios-linker-loader.c
>> @@ -78,6 +78,21 @@ struct BiosLinkerLoaderEntry {
>>              uint32_t length;
>>          } cksum;
>>  
>> +        /*
>> +         * COMMAND_WRITE_POINTER - write the fw_cfg file (originating from
>> +         * @dest_file) at @wr_pointer.offset, by adding a pointer to
>> +         * @src_offset within the table originating from @src_file.
>> +         * 1,2,4 or 8 byte unsigned addition is used depending on
>> +         * @wr_pointer.size.
>> +         */
>> +        struct {
>> +            char dest_file[BIOS_LINKER_LOADER_FILESZ];
>> +            char src_file[BIOS_LINKER_LOADER_FILESZ];
>> +            uint32_t dst_offset;
>> +            uint32_t src_offset;
>> +            uint8_t size;
>> +        } wr_pointer;
>> +
>>          /* padding */
>>          char pad[124];
> Shouldn't padding be reduced by 4 bytes to keep 
> sizeof(BiosLinkerLoaderEntry) the same as before patch,
> so that old bios would be able to skip this unknown command
> and read the next at the right offset?

No, because you are in the middle of a union rather than a struct (the
outer BiosLinkerLoaderEntry struct size is determined by the largest
member of the union, which is 'char pad[124]'; the new wr_pointer
addition to the union does not change the size of the union).

-- 
Eric Blake   eblake redhat com    +1-919-301-3266
Libvirt virtualization library http://libvirt.org

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