Crash on mem::zeroed

[bshubenok-sigma]

I'm trying to run a stream capture example on arm64 device and getting this type of error:

The application panicked (crashed).
Message:  attempted to zero-initialize type `buffer::Metadata`, which is invalid
Location: library/core/src/panicking.rs:126

  ━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━ BACKTRACE ━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━
                                ⋮ 7 frames hidden ⋮
   8: core::panicking::panic_nounwind_fmt::hef625da4d1887edc
      at /rustc/cc66ad468955717ab92600c770da8c1601a4ff33/library/core/src/panicking.rs:96
   9: core::panicking::panic_nounwind::h39541cbd5a72f09a
      at /rustc/cc66ad468955717ab92600c770da8c1601a4ff33/library/core/src/panicking.rs:126
  10: core::mem::zeroed::hffae7c8a55c7cecb
      at /rustc/cc66ad468955717ab92600c770da8c1601a4ff33/library/core/src/mem/mod.rs:653
  11: v4l::io::mmap::stream::Stream::with_buffers::h57158218617a1fba
      at /home/noah/.cargo/git/checkouts/libv4l-rs-917e1f4ffd90b94a/9844291/src/io/mmap/stream.rs:55
  12: lib::mediaengine::MediaEngine::process_video::{{closure}}::h36fd47a510f39921

As if it is enclosed in unsafe this may be somehow expected, I guess, but how could I overcome this?
Or is this me lacking some compiler settings?

[MarijnS95]

Hmm that looks to have been a bad change in 384e7fb, this struct isn't marked as repr(C) either.

If you look at enum Memory for example 0 isn't a valid bit-pattern, only values 1-4 are valid (anything else is UB):

libv4l-rs/src/memory.rs

Lines 12 to 19 in 9844291

	#[repr(u32)]
	#[derive(Debug, Clone, Copy)]
	pub enum Memory {
	Mmap = 1,
	UserPtr = 2,
	Overlay = 3,
	DmaBuf = 4,
	}

@raymanfx is it okay if I replace these with default() once again?

[MarijnS95]

Hmm it looks like the conversion from v4l2_buffer -> Metadata -> v4l2_buffer is also lossy.

[MarijnS95]

For this specific case in mmap/stream.rs, I'm thinking of either wrapping it in MaybeUninit with // SAFETY docs about self.arena_index being valid, or wrapping it in an Option with an unwrap().

EDIT: Since there are user APIs passing arbitrary indices around, I'm leaning towards Option for additional validation.