Add struct_util module from linux-loader

[alxiord]

See also rust-vmm/linux-loader#20, rust-vmm/linux-loader#23 (comment)

struct_util is a generic utility module that reads structs from bytes. It should be included in this utility crate and consumed by linux-loader from here.

[bonzini]

There is some overlap between this and vm-memory's Bytes and ByteValued traits. Can you try instead implementing ByteValued on the structs and using the read_obj/write_obj methods from the Bytes trait? The main limitation is that you need a mutable reference to slice but it shouldn't be a problem for linux-loader.

[alxiord]

Indeed we can impl ByteValued on the headers currently read with struct_util, and that gets us elegantly rid of struct_util; later on, the GuestMemory reads the whole kernel image, not just the header, as a generic F: Read (for elf format: link).

We'd be replacing

let mut ehdr: elf::Elf64_Ehdr = Default::default();
unsafe { struct_util::read_struct(kernel_image, &mut ehdr).map_err(|_| Error::ReadElfHeader)?; }

with

let mut ehdr_bytes = vec![0u8; mem::size_of::<elf::Elf64_Ehdr>()];
kernel_image.read_exact(ehdr_bytes.as_mut_slice()).map_err(|_| Error::ReadElfHeader)?;
let ehdr = elf::Elf64_Ehdr::from_mut_slice(ehdr_bytes.as_mut_slice()).ok_or(Error::ReadElfHeader)?;

which I don't necessarily find more elegant, but personally prefer rather than move struct_util around and make it pub.
It might be more elegant if kernel_image were a Bytes and not just a Read + Seek, so we could read_obj from it directly, but I think that would be too restrictive on linux-loader.

[bonzini]

Yes, it's not super nice but at least it doesn't have unsafe. You can also put ehdr_bytes.as_mut_slice() into a local variable which is a bit nicer. Or perhaps use an array like

let mut ehdr_bytes = [0u8; mem::size_of::<elf::Elf64_Ehdr>()];
kernel_image.read_exact(&mut ehdr_bytes[..]).map_err(|_| Error::ReadElfHeader)?;
let ehdr = elf::Elf64_Ehdr::from_mut_slice(&mut ehdr_bytes[..]).ok_or(Error::ReadElfHeader)?;

?

[bonzini]

Also:

let mut ehdr_bytes = [0u8; mem::size_of::<elf::Elf64_Ehdr>()];
ehdr_bytes.as_volatile_slice().read_from(0, kernel_image, ehdr_bytes.len()).map_err(|_| Error::ReadElfHeader)?;
let ehdr = elf::Elf64_Ehdr::from_mut_slice(&mut ehdr_bytes[..]).ok_or(Error::ReadElfHeader)?;

... and finally, we could implement VolatileMemory on ByteValued (#83) and do

let mut ehdr: elf::Elf64_Ehdr = Default::default();
ehdr.as_volatile_slice().read_from(0, kernel_image, ehdr.len()).map_err(|_| Error::ReadElfHeader)?;

[alxiord]

The last solution looks great and I find it to be the most elegant alternative to struct_util. Until #83 is merged & released, I'll go ahead with the other solution.

[andreeaflorescu]

Transferred this issue from vmm-sys-util to vm-memory as the implementation is in vm-memory now.

Fix introduced in: #83