Potentially Unsound Conversion of a shared reference into a mutable one

[gregovin]

While attempting to debug another issue, I ran miri on the sample test. It found this error

error: Undefined Behavior: trying to retag from <15569> for Unique permission at alloc5919[0x0], but that tag only grants SharedReadOnly permission for this location
--> /home/grego/RustTesting/ping-rs/src/linux_ping/icmp_header.rs:30:18
|
30 | unsafe { &mut *header }
| ^^^^^^^^^^^^
| |
| trying to retag from <15569> for Unique permission at alloc5919[0x0], but that tag only grants SharedReadOnly permission for this location
| this error occurs as part of retag at alloc5919[0x0..0x8]
|
= help: this indicates a potential bug in the program: it performed an invalid operation, but the Stacked Borrows rules it violated are still experimental
= help: see https://github.com/rust-lang/unsafe-code-guidelines/blob/master/wip/stacked-borrows.md for further information

I believe the fundamental problem here is that get_mut_ref takes a shared refence to some data and returns a mutable reference to it, which is could trigger undefined behavior. This isn't too bad because get_mut_ref is only accessible within the crate, but could cause difficult to deal with bugs if things are changed in the future. Based on some testing, this can be easily fixed by changing get_mut_ref to the following:

pub(crate) fn get_mut_ref(be_buffer: &mut [u8]) -> &mut IcmpEchoHeader {
let header = be_buffer.as_mut_ptr() as *mut IcmpEchoHeader;
unsafe { &mut *header }
}
And then changing the rest of the code to pass mutable references as needed