Add svm_flags field to sockaddr_vm

[rtzoeller]

Testing libc on Fedora 34 emits the following error:

bad field offset svm_zero of sockaddr_vm: rust: 12 (0xc) != c 13 (0xd)
bad field size svm_zero of sockaddr_vm: rust: 4 (0x4) != c 3 (0x3)
bad field type svm_zero of sockaddr_vm: rust: 0x7ffe56b1165c != c 0x7ffe56b1165d

Kernel version:

$ uname -r
5.14.9-200.fc34.x86_64

[rust-highfive]

r? @JohnTitor

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[rtzoeller]

I'm guessing the Linux kernel on the CI instances is older than the change this corresponds to, otherwise it would have been detected previously. I'm not sure this can be submitted quite yet as a result.

[JohnTitor]

This would be a breaking change, should follow this: https://github.com/rust-lang/libc/blob/master/CONTRIBUTING.md#breaking-change-policy

[rtzoeller]

This would be a breaking change, should follow this: https://github.com/rust-lang/libc/blob/master/CONTRIBUTING.md#breaking-change-policy

Can you provide an example of what you're expecting for this type of change?

The example in that document makes sense for me if libc were to remove an item, but it's less clear what the expectation is for an in-place breaking change.

[JohnTitor]

Adding a new public feild to a struct is a breaking change and we should tell users that we're going to do it, otherwise user code would be broken suddenly.

[james-callahan]

Has there been any followup to this?
Without the svm_flags field you can no longer use e.g. nitro enclaves on AWS.

See also rust-vsock/vsock-rs#29