add `access` FFI shim function for unix/posix

[programmerjake]

I'm trying to use which, but it errors:

error: unsupported operation: can't call foreign function `access` on OS `linux`
   --> /home/jacob/.cargo/registry/src/index.crates.io-1949cf8c6b5b557f/rustix-0.38.31/src/backend/libc/fs/syscalls.rs:723:18
    |
723 |     unsafe { ret(c::access(c_str(path), access.bits())) }
    |                  ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^ can't call foreign function `access` on OS `linux`
    |
    = help: if this is a basic API commonly used on this target, please report an issue with Miri
    = help: however, note that Miri does not aim to support every FFI function out there; for instance, we will not support APIs for things such as GUIs, scripting languages, or databases
    = note: BACKTRACE on thread `util::ready_val`:
    = note: inside `rustix::backend::fs::syscalls::access` at /home/jacob/.cargo/registry/src/index.crates.io-1949cf8c6b5b557f/rustix-0.38.31/src/backend/libc/fs/syscalls.rs:723:18: 723:55
    = note: inside closure at /home/jacob/.cargo/registry/src/index.crates.io-1949cf8c6b5b557f/rustix-0.38.31/src/fs/abs.rs:243:33: 243:76
    = note: inside `rustix::path::arg::with_c_str::<(), {closure@rustix::fs::abs::access<&std::path::Path>::{closure#0}}>` at /home/jacob/.cargo/registry/src/index.crates.io-1949cf8c6b5b557f/rustix-0.38.31/src/path/arg.rs:1046:18: 1046:22
    = note: inside `<&std::path::Path as rustix::path::arg::Arg>::into_with_c_str::<(), {closure@rustix::fs::abs::access<&std::path::Path>::{closure#0}}>` at /home/jacob/.cargo/registry/src/index.crates.io-1949cf8c6b5b557f/rustix-0.38.31/src/path/arg.rs:378:9: 378:51
    = note: inside `rustix::fs::abs::access::<&std::path::Path>` at /home/jacob/.cargo/registry/src/index.crates.io-1949cf8c6b5b557f/rustix-0.38.31/src/fs/abs.rs:243:5: 243:77
    = note: inside `<which::checker::ExecutableChecker as which::finder::Checker>::is_valid` at /home/jacob/.cargo/registry/src/index.crates.io-1949cf8c6b5b557f/which-6.0.1/src/checker.rs:17:9: 17:48
    = note: inside closure at /home/jacob/.cargo/registry/src/index.crates.io-1949cf8c6b5b557f/which-6.0.1/src/checker.rs:78:44: 78:66
    = note: inside `<std::slice::Iter<'_, std::boxed::Box<dyn which::finder::Checker>> as std::iter::Iterator>::all::<{closure@<which::checker::CompositeChecker as which::finder::Checker>::is_valid::{closure#0}}>` at /home/jacob/.rustup/toolchains/nightly-x86_64-unknown-linux-gnu/lib/rustlib/src/rust/library/core/src/slice/iter/macros.rs:268:25: 268:29
    = note: inside `<which::checker::CompositeChecker as which::finder::Checker>::is_valid` at /home/jacob/.cargo/registry/src/index.crates.io-1949cf8c6b5b557f/which-6.0.1/src/checker.rs:78:9: 78:67
    = note: inside closure at /home/jacob/.cargo/registry/src/index.crates.io-1949cf8c6b5b557f/which-6.0.1/src/finder.rs:98:30: 98:56
    = note: inside `std::ops::function::impls::<impl std::ops::FnMut<(&std::path::PathBuf,)> for &mut {closure@which::finder::Finder::find<std::ffi::OsString, std::ffi::OsString, std::path::PathBuf>::{closure#0}}>::call_mut` at /home/jacob/.rustup/toolchains/nightly-x86_64-unknown-linux-gnu/lib/rustlib/src/rust/library/core/src/ops/function.rs:294:13: 294:35
    = note: inside closure at /home/jacob/.rustup/toolchains/nightly-x86_64-unknown-linux-gnu/lib/rustlib/src/rust/library/core/src/iter/traits/iterator.rs:2829:20: 2829:33
    = note: inside closure at /home/jacob/.rustup/toolchains/nightly-x86_64-unknown-linux-gnu/lib/rustlib/src/rust/library/core/src/iter/adapters/map.rs:95:21: 95:35
    = note: inside `<std::vec::IntoIter<std::path::PathBuf> as std::iter::Iterator>::try_fold::<(), {closure@std::iter::adapters::map::map_try_fold<'_, std::path::PathBuf, std::path::PathBuf, (), std::ops::ControlFlow<std::path::PathBuf>, {closure@which::finder::Finder::path_search_candidates<std::vec::Vec<std::path::PathBuf>>::{closure#0}}, {closure@std::iter::Iterator::find::check<std::path::PathBuf, &mut {closure@which::finder::Finder::find<std::ffi::OsString, std::ffi::OsString, std::path::PathBuf>::{closure#0}}>::{closure#0}}>::{closure#0}}, std::ops::ControlFlow<std::path::PathBuf>>` at /home/jacob/.rustup/toolchains/nightly-x86_64-unknown-linux-gnu/lib/rustlib/src/rust/library/alloc/src/vec/into_iter.rs:346:25: 346:38
    = note: inside `<std::iter::Map<std::vec::IntoIter<std::path::PathBuf>, {closure@which::finder::Finder::path_search_candidates<std::vec::Vec<std::path::PathBuf>>::{closure#0}}> as std::iter::Iterator>::try_fold::<(), {closure@std::iter::Iterator::find::check<std::path::PathBuf, &mut {closure@which::finder::Finder::find<std::ffi::OsString, std::ffi::OsString, std::path::PathBuf>::{closure#0}}>::{closure#0}}, std::ops::ControlFlow<std::path::PathBuf>>` at /home/jacob/.rustup/toolchains/nightly-x86_64-unknown-linux-gnu/lib/rustlib/src/rust/library/core/src/iter/adapters/map.rs:121:9: 121:63
    = note: inside `<std::iter::Map<std::vec::IntoIter<std::path::PathBuf>, {closure@which::finder::Finder::path_search_candidates<std::vec::Vec<std::path::PathBuf>>::{closure#0}}> as std::iter::Iterator>::find::<&mut {closure@which::finder::Finder::find<std::ffi::OsString, std::ffi::OsString, std::path::PathBuf>::{closure#0}}>` at /home/jacob/.rustup/toolchains/nightly-x86_64-unknown-linux-gnu/lib/rustlib/src/rust/library/core/src/iter/traits/iterator.rs:2833:9: 2833:44
    = note: inside `either::iterator::<impl std::iter::Iterator for either::Either<std::iter::Once<std::path::PathBuf>, std::iter::Map<std::vec::IntoIter<std::path::PathBuf>, {closure@which::finder::Finder::path_search_candidates<std::vec::Vec<std::path::PathBuf>>::{closure#0}}>>>::find::<&mut {closure@which::finder::Finder::find<std::ffi::OsString, std::ffi::OsString, std::path::PathBuf>::{closure#0}}>` at /home/jacob/.cargo/registry/src/index.crates.io-1949cf8c6b5b557f/either-1.13.0/src/iterator.rs:117:43: 117:64
    = note: inside `<std::iter::Filter<either::Either<std::iter::Once<std::path::PathBuf>, std::iter::Map<std::vec::IntoIter<std::path::PathBuf>, {closure@which::finder::Finder::path_search_candidates<std::vec::Vec<std::path::PathBuf>>::{closure#0}}>>, {closure@which::finder::Finder::find<std::ffi::OsString, std::ffi::OsString, std::path::PathBuf>::{closure#0}}> as std::iter::Iterator>::next` at /home/jacob/.rustup/toolchains/nightly-x86_64-unknown-linux-gnu/lib/rustlib/src/rust/library/core/src/iter/adapters/filter.rs:98:9: 98:44
    = note: inside `<std::iter::Map<std::iter::Filter<either::Either<std::iter::Once<std::path::PathBuf>, std::iter::Map<std::vec::IntoIter<std::path::PathBuf>, {closure@which::finder::Finder::path_search_candidates<std::vec::Vec<std::path::PathBuf>>::{closure#0}}>>, {closure@which::finder::Finder::find<std::ffi::OsString, std::ffi::OsString, std::path::PathBuf>::{closure#0}}>, fn(std::path::PathBuf) -> std::path::PathBuf {which::finder::correct_casing}> as std::iter::Iterator>::next` at /home/jacob/.rustup/toolchains/nightly-x86_64-unknown-linux-gnu/lib/rustlib/src/rust/library/core/src/iter/adapters/map.rs:107:9: 107:25
    = note: inside closure at /home/jacob/.cargo/registry/src/index.crates.io-1949cf8c6b5b557f/which-6.0.1/src/lib.rs:59:45: 59:53
    = note: inside `std::result::Result::<std::iter::Map<std::iter::Filter<either::Either<std::iter::Once<std::path::PathBuf>, std::iter::Map<std::vec::IntoIter<std::path::PathBuf>, {closure@which::finder::Finder::path_search_candidates<std::vec::Vec<std::path::PathBuf>>::{closure#0}}>>, {closure@which::finder::Finder::find<std::ffi::OsString, std::ffi::OsString, std::path::PathBuf>::{closure#0}}>, fn(std::path::PathBuf) -> std::path::PathBuf {which::finder::correct_casing}>, which::Error>::and_then::<std::path::PathBuf, {closure@which::which<std::ffi::OsString>::{closure#0}}>` at /home/jacob/.rustup/toolchains/nightly-x86_64-unknown-linux-gnu/lib/rustlib/src/rust/library/core/src/result.rs:1353:22: 1353:27
    = note: inside `which::which::<std::ffi::OsString>` at /home/jacob/.cargo/registry/src/index.crates.io-1949cf8c6b5b557f/which-6.0.1/src/lib.rs:59:5: 59:89

[RalfJung]

Thanks for the report! Yeah we're missing many of the basic FS APIs. Do you know if access can be implemented on top of the standard library? If not, that greatly reduces the chance that Miri will support it -- it's a huge pain to have to maintain platform-specific implementations ourselves.

[programmerjake]

if you ignore the difference between real uid/gid and effective uid/gid (the distinction is mostly only useful for programs like sudo), then yes you should be able to implement it using std::fs::metadata, though if you want to support running unix code on win32 you may need some file extension parsing shenanigans to fake having an executable bit on win32. if not, the access function is available on all unix/posix systems so may not be worth trying to reimplement in terms of std::fs since you could just call it directly.

you can use the implementation from relibc for inspiration (note they got their unix mode rwx permission bit extraction logic wrong, bug report is in progress):
https://gitlab.redox-os.org/redox-os/relibc/-/blob/6d0f8d9bd7efb273a8beafdc046fde5cda33f075/src/platform/redox/mod.rs#L85

[oli-obk]

if you want to support running unix code on win32 you may need some file extension parsing shenanigans to fake having an executable bit on win32.

yes, all miri hosts must be able to run all targets (with very few exceptions that we try to avoid)