fuse: fix incorrect len field in command headers

[cagatay-y]

FUSE seems to expect in the length field of the header the total length of the command without the padding. This PR fixes #1111.

[mkroening]

Thanks for the swift fix!

I am confused about how this works. When I extract the expression with slice_from_raw_parts_mut into a variable, rust-analyzer reports the type as *mut [u8]. len is not the length of the entire struct, rather the payload, so the slice we create here does not cover the whole struct. I validated this by checking the length of the pointed slice.

The reason I originally used 0 as the slice length was because that was how it was in the previous version of the code, but I think reverting 93fc6cb and using ptr_metadata here may be worth the unstable feature. I included a commit with the reversion in my PR (#1113) for #1111, but I can remove it if you prefer.

len never refers to the length of the entire struct but to the number of elements. For structs with DSTs as the last field, the metadata of the last field is used. So when whe create a pointer with the correct metadata via ptr::slice_from_raw_parts_mut everything should be fine, although it might be confusing at first. References:

• Dynamically Sized Types - The Rust Reference
• Slice DST pointer to pointer cast - Operator expressions - The Rust Reference

I still think ptr_metadata does not improve the situation here, aside from the fact that it will probably take ages to stabilize still. Please remove that commit and then I can merge this. :)

[mkroening]

Awesome, thanks again! :)

[cagatay-y]

No problem, sorry for breaking the code earlier 😅

[mkroening]

No problem, sorry for breaking the code earlier 😅

No worries, it happens to everybody. :)