bhyveload(8): Implement loader_callbacks::diskwrite

[cem]

Summary

The method was optional prior to r365938, which made it mandatory but did
not implement it for bhyveload. The code path might not be hit unless the
user's loader was configured to write to a file on disk, such as
nextboot(8)'s nextboot.conf.

[grehan]

Looks fine.

[tsoome]

I assume, bhyveload does open disk images RW.

[cem]

Thanks all!

In D26710#595262, @tsoome wrote:

I assume, bhyveload does open disk images RW.

I was worried about possible privdrop / sandboxing issues as well, but I think this is correct regardless. Without a diskwrite method, we segfault and crash. If the disk is opened RO, worst case the write operation errors, at the guest loader level, with EACCES or whatever. I did not check how the disk is opened by loader, but in practice: before this change, bhyveload(8) reliably cored after guest nextboot(8); afterwards, it does not.

[grehan]

I did not check how the disk is opened by loader

The open is currently read-only.

fd = open(path, O_RDONLY);

[jhb]

We should perhaps try to open O_RDWR and fallback to O_RDONLY if that fails. I know I use nextboot quite frequently during development (which is one my main use cases for bhyve)