Implement a readdir64() shim for Linux

[tavianator]

Partial fix for #1966.

[tavianator]

This doesn't quite work yet, but I think I need some help to fix it. I'm allocating the struct dirent as small as possible, in order to catch bugs like rust-lang/rust#93384 (comment). The pointer returned by readdir() may point to a region smaller than sizeof(struct dirent).

My libstd patch was supposed to handle this by using ptr::addr_of!(). And yet I get this from miri:

error: Undefined Behavior: dereferencing pointer failed: alloc1498 has size 24, so pointer to 280 bytes starting at offset 0 is out-of-bounds
   --> /home/tavianator/.rustup/toolchains/miri/lib/rustlib/src/rust/library/std/src/sys/unix/fs.rs:495:34
    |
495 |                 let entry_name = ptr::addr_of!((*entry_ptr).d_name) as *const u8;
    |                                  ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^ dereferencing pointer failed: alloc1498 has size 24, so pointer to 280 bytes starting at offset 0 is out-of-bounds
    |
    = help: this indicates a bug in the program: it performed an invalid operation, and caused Undefined Behavior
    = help: see https://doc.rust-lang.org/nightly/reference/behavior-considered-undefined.html for further information
            
    = note: inside `<std::sys::unix::fs::ReadDir as std::iter::Iterator>::next` at /home/tavianator/.rustup/toolchains/miri/lib/rustlib/src/rust/library/core/src/ptr/mod.rs:1523:5
    = note: inside `<std::fs::ReadDir as std::iter::Iterator>::next` at /home/tavianator/.rustup/toolchains/miri/lib/rustlib/src/rust/library/std/src/fs.rs:1411:9
note: inside `main` at dir.rs:2:18
   --> dir.rs:2:18
    |
2   |     for entry in std::fs::read_dir(".").unwrap() {
    |                  ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
    = note: this error originates in the macro `ptr::addr_of` (in Nightly builds, run with -Z macro-backtrace for more info)

[tavianator]

Hmm I guess this is just a libstd bug. addr_of!((*ptr).field) requires ptr to point to a properly sized/aligned region: https://play.rust-lang.org/?version=stable&mode=debug&edition=2021&gist=904126751c84698fc38c8dde790de358. I'll patch libstd again :P

[RalfJung]

addr_of!((*ptr).field) requires ptr to point to a properly sized/aligned region

Indeed, *ptr asserts that ptr is dereferencable (for the size and alignment determined by its type).

[RalfJung]

That's great, thanks a lot. :) I hadn't even thought of creating a new allocation for each call, but it makes a lot of sense.

[saethlin]

I'll patch libstd again :P

Another one for the trophy case? :P

[RalfJung]

I guess we could add it. ;)

[bors]

☔ The latest upstream changes (presumably #1970) made this pull request unmergeable. Please resolve the merge conflicts.

[bors]

☔ The latest upstream changes (presumably #2005) made this pull request unmergeable. Please resolve the merge conflicts.

[RalfJung]

(Looks at the force-push diff, confused) oh, you rebased! That makes force-push diffs harder to read so please only do it when needed to resolve conflicts. :)

[RalfJung]

Looking great, thanks a lot for staying with me through the many rounds of review. :)

@bors r+

[bors]

📌 Commit 0886419 has been approved by RalfJung

[bors]

⌛ Testing commit 0886419 with merge fc15f96...

[bors]

☀️ Test successful - checks-actions
Approved by: RalfJung
Pushing fc15f96 to master...