Support fstat on non-file-backed FDs

[RalfJung]

fstat is supposed to work on all FDs, but we currently only support it on file-backed FDs.

I am not sure what exactly it should do for other FDs; figuring that out would be the first step in working on this issue. So to work on this you should be willing to do some research and explain the results so that we can then together figure out how to best implement this in Miri. However, I don't expect this to be complicated from the Miri side, so prior experience with Miri is not required.

[RalfJung]

Seems like it does indeed not error for non-file-backed FDs, but returns something.

[Chirag-Janbandhu]

Hello!

I am interested in working on this issue. I am looking to start contributing to the Rust project and this seems like a great entry point for me to learn the codebase and contribution process.

Could I please be assigned this issue?

Thank you!

[RalfJung]

That's great to hear. :) You can assign yourself with @rustbot claim.

Note that you should have good experience using Rust already before tackling issues in this repository -- working on Miri is not a great way to learn Rust. But if you already know Rust, we can figure out the rest. :D

Also, please have a look at https://github.com/rust-lang/miri/blob/master/CONTRIBUTING.md.

[Chirag-Janbandhu]

@rustbot claim

[enthropy7]

@Chirag-Janbandhu hi, are you still working on this?

[enthropy7]

@rustbot claim

[enthropy7]

I researched what fstat returns for non-file-backed FDs. Socketpairs return S_IFSOCK with size 0, pipes return S_IFIFO with size 0, and eventfd/epoll return S_IFREG with size 0. Stdin/stdout/stderr return S_IFCHR or S_IFREG depending on whether they're connected to a TTY. I implemented support by updating FileMetadata::from_fd_num to catch errors from fd.metadata() and create synthetic metadata based on the FD type (determined by the name() method). The implementation maps FD names to appropriate file types, sets size to 0, and uses appropriate permissions (0666 for sockets/pipes, 0600 for others). I've added comprehensive tests in tests/pass-dep/libc/libc-fstat-non-file.rs that verify fstat works correctly on all supported non-file-backed FD types. The implementation is straightforward and follows existing Miri patterns. Btw i thought about realization of this error even when i saw the issue in first time. and i got it done in a few days. just testing now and pulling a solution. :3

upd: fixing a problem that i skipped during my first approach to fight this issue. therefore, i will solve it ioday.