`fopen`/`openat` does not seem to preserve access flags

Tested on Linux with wasmtime v9.0.1 and wasi-sdk-20.0.

The following code (saved as open-rw.c) tries to open a file (that exists) for reading and writing:

#include <stdio.h>
#include <stdlib.h>

int main() {
    // Note: using an `openat` call directly here with O_WRONLY or O_RDWR would have the same result
    FILE* file = fopen("testfile", "w");
    if (!file) return 1;
    fclose(file);
    return 0;
}

EDIT: The above code originally used the invalid flags "rw", it has been corrected to use "w"

However, when using wasi-libc, the actual syscall (for me it's openat2) is being passed O_RDONLY instead of the expected O_WRONLY. Note: same applies to O_RDWR, O_RDONLY is still passed instead.

Compiled with wasi-sdk-20.0:

$ ls testfile
testfile
$ $WASI_SDK/bin/clang --sysroot=$WASI_SDK/share/wasi-sysroot open-rw.c -o open-rw-sdk.wasm
$ strace -e trace=openat2 wasmtime --dir=. open-rw-sdk.wasm
openat2(3, "testfile", {flags=O_RDONLY|O_CLOEXEC, resolve=RESOLVE_NO_MAGICLINKS|RESOLVE_BENEATH}, 24) = 5
+++ exited with 0 +++

Note that access flags are preserved in the equivalent Zig code (unless -lc to link libc is used in which case wasi-libc is built/linked and the O_RDONLY behavior occurs)

const std = @import("std");

pub fn main() !void {
    var file = try std.fs.cwd().openFile("testfile", .{ .mode = .read_write });
    defer file.close();
}

$ zig build-exe -target wasm32-wasi open-rw.zig -femit-bin=open-rw-zig.wasm
$ strace -e trace=openat2 wasmtime --dir=. open-rw-zig.wasm
openat2(3, "testfile", {flags=O_RDWR|O_APPEND|O_NOFOLLOW|O_CLOEXEC, resolve=RESOLVE_NO_MAGICLINKS|RESOLVE_BENEATH}, 24) = 5
+++ exited with 0 +++

And are also preserved in the equivalent Rust code:

use std::fs::OpenOptions;

fn main() {
  let mut _file = OpenOptions::new().read(true).write(true).open("testfile").unwrap();
}

$ strace -e trace=openat2 wasmtime --dir=. open-rw-rust.wasm
openat2(3, "testfile", {flags=O_RDWR|O_CLOEXEC, resolve=RESOLVE_NO_MAGICLINKS|RESOLVE_BENEATH}, 24) = 5
+++ exited with 0 +++

Start by running the open-rw.c reproducer with wasi-libc and tracing the fopen/openat path to the openat2 call. Follow how O_WRONLY and O_RDWR are translated, then verify that the resulting syscall preserves the requested access flags without regressing the existing read-only case.