Parse unix address from sockaddr without length

[pronebird]

What does this PR do

I noticed that feeding *const sockaddr_un into SockaddrStorage without length returns None. I had impression that it should actually work.

However extracting sa_len manually and feeding it into SockaddrStrorage works as expected and I am able to cast it to UnixAddr later on by calling SockaddrStorage::as_unix_addr().

 SockaddrStorage::from_raw(sa_ptr, Some((*sa_ptr).sa_len.into()))

Checklist:

• I have read CONTRIBUTING.md
• I have written necessary tests and rustdoc comments
• A change log has been added if this PR modifies nix's API

[asomers]

What new behavior is enabled, exactly? A test would be helpful to understand your aim. Also, you need a changelog entry.

[pronebird]

What new behavior is enabled, exactly? A test would be helpful to understand your aim. Also, you need a changelog entry.

That SockaddrStorage can actually parse a pointer to sockaddr_un when the length is not provided. Currently this doesn’t work:

SockaddrStorage::from_raw(sockaddr_un_ptr, None)

I think that should work since the length is actually held inside of the pointer and can be obtained automatically.

I can add a test and change entry.

[asomers]

There's a reason that it doesn't work. It's deliberate. The purpose of the length argument is to tell Nix how much of the struct is actually initialized. In C, pointers to sockaddrs are often not fully initialized, or even allocated. But reading uninitialized memory in Rust is UB. So that argument is important.

[pronebird]

The length argument is Option<socklen_t> so it's ok to pass None if I don't know the length when a syscall returns a non-null *sockaddr that has the AF_UNIX family set and a length inside of it.

The else branch is happily converting the whole range of types, i.e:
https://github.com/nix-rust/nix/blob/master/src/sys/socket/addr.rs#L1184-L1222

UnixAddr::from_raw() also happily accepts None as len and works on macOS (aka BSD):
https://github.com/nix-rust/nix/blob/master/src/sys/socket/addr.rs#L535-L569

So all this functionality is already there and it feels that there is just an omission in SockaddrStorage.

[pronebird]

So it seems that the following is not true for all platforms:

assert!(
  UnixAddr::size() <= mem::size_of::<libc::sockaddr_storage>() as libc::socklen_t
);

[pronebird]

I think this could be useful on apple platforms where sockaddr_un should fit into sockaddr_storage but I don't have time to pursue the completion of this PR.