Add pread and pwrite shims

[newpavlov]

Requested in #2057.

[newpavlov]

Any updates on this? Lack of these shims is the main blocker for testing our code with MIRI.

[RalfJung]

This is in my review queue, but unfortunately I currently don't have a lot of time for reviewing -- sorry.

In terms of the general approach, as Ben said this is quite the departure from how all file system code works in Miri so far -- which is via a a portable implementation on top of std, using platform-specific functions only when absolutely needed. In that sense I think I would strongly prefer exploring an approach that only needs std APIs that are available everywhere. Miri is entirely sequential, so something like "record old position in file, seek, read, seek back to old position" should be a correct emulation I think?

[RalfJung]

Thanks for the PR! I still have some comments/questions.

[RalfJung]

Looks good, thanks. :)

@bors r+

[bors]

📌 Commit e472d33 has been approved by RalfJung

It is now in the queue for this repository.

[bors]

⌛ Testing commit e472d33 with merge aaff0eb...

[bors]

☀️ Test successful - checks-actions
Approved by: RalfJung
Pushing aaff0eb to master...

[newpavlov]

@RalfJung
Hi! Do you know when approximately we can expect for these changes to land on the Nightly toolchain? I tried to run tests on miri 0.1.0 (7c2012d 2024-07-26), but they still fail with:

unsupported operation: can't call foreign function pread64 on OS linux

[RalfJung]

I usually do a sync every weekend but this weekend I had other things to do.

I started a sync in rust-lang/rust#128333.