posix.fstatat fails to propogate SYMLINK_NOFOLLOW when linking wasi-libc

[squeek502]

Zig Version

0.14.0-dev.653+91c17979f

Steps to Reproduce and Observed Behavior

const std = @import("std");

test "fstatat on a symlink with SYMLINK_NOFOLLOW" {
    var tmp = std.testing.tmpDir(.{});
    defer tmp.cleanup();

    const testfile = try tmp.dir.createFile("testfile", .{});
    testfile.close();

    try tmp.dir.symLink("testfile", "testsymlink", .{});

    const stat = try std.posix.fstatat(tmp.dir.fd, "testsymlink", std.posix.AT.SYMLINK_NOFOLLOW);

    std.testing.expect(stat.mode & std.posix.S.IFLNK == std.posix.S.IFLNK) catch |err| {
        std.debug.print("stat.mode={X}\n", .{stat.mode});
        return err;
    };
}

This test passes normally and when targeting wasm32-wasi and not linking libc:

$ zig test stat-symlink-test.zig -target wasm32-wasi -femit-bin=stat-symlink-test.wasm --test-no-exec
$ wasmtime --dir=. stat-symlink-test.wasm
All 1 tests passed.

But fails when linking libc (meaning wasi-libc):

$ wasmtime --dir=. stat-symlink-test-libc.wasm
stat.mode=8000
1/1 stat-symlink-test.test.fstatat on a symlink with SYMLINK_NOFOLLOW...FAIL (TestUnexpectedResult)
Unable to dump stack trace: not implemented for Wasm
0 passed; 0 skipped; 1 failed.

(that mode is IFREG aka a file)

This is not a problem with the parameters being passed to fstatat: I confirmed that the AT_SYMLINK_NOFOLLOW is being passed to the fstatat_sym call here:

zig/lib/std/posix.zig

Line 4382 in 059856a

switch (errno(fstatat_sym(dirfd, pathname, &stat, flags))) {

But strace shows that the NOFOLLOW flag is being dropped when going through wasmtime (the openat2 call should have O_NOFOLLOW, which it does when not linking wasi-libc):

openat2(13, "testsymlink", {flags=O_RDONLY|O_CLOEXEC|O_PATH, resolve=RESOLVE_NO_MAGICLINKS|RESOLVE_BENEATH}, 24) = 11
statx(11, "", AT_STATX_SYNC_AS_STAT|AT_EMPTY_PATH, STATX_ALL, {stx_mask=STATX_ALL|STATX_MNT_ID, stx_attributes=0, stx_mode=S_IFREG|0664, stx_size=0, ...}) = 0
close(11)                               = 0

---

My first thought was that this is a wasi-libc/wasmtime bug, similar to #20747, but I have been unable to find a reproduction when building an intended-to-be-equivalent C version via wasi-sdk. Here's the program I'm trying (I'm using wasi-sdk 23.0 and wasmtime 23.0.1):

#include <sys/stat.h>
#include <sys/types.h>
#include <fcntl.h>
#include <unistd.h>
#include <stdio.h>

int main() {
    mkdirat(AT_FDCWD, "testdir", 0777);
    int dirfd = openat(AT_FDCWD, "testdir", O_RDONLY|O_DIRECTORY);

    int fd = openat(dirfd, "testfile", O_RDONLY|O_CREAT);
    close(fd);

    symlinkat("testfile", dirfd, "testsymlink");

    struct stat st;
    fstatat(dirfd, "testsymlink", &st, AT_SYMLINK_NOFOLLOW);

    printf("%X\n", st.st_mode);

    if ((st.st_mode & S_IFLNK) == 0) return 1;
    return 0;
}

Built with:

$ WASI_SDK=/home/ryan/Downloads/wasi-sdk-23.0-x86_64-linux
$ $WASI_SDK/bin/clang --sysroot=$WASI_SDK/share/wasi-sysroot stat-symlink.c -o stat-symlink-sdk.wasm

And run with:

$ wasmtime --dir=. stat-symlink-sdk.wasm
A000

(0xA000 is S_IFLNK aka symlink)

Expected Behavior

The test to pass when linking wasi-libc

[rootbeer]

I've got a change with a bunch of WASI with-or-without-libc fixes coming soon, I'll see if I can fold this in.

[rootbeer]

I think the problem is that AT_SYMLINK_NOFOLLOW should be 0x1 on wasi-with-libc, but its 0x100 in Zig? I'm not entirely sure where wasi-libc defines these constants, but I see an 0x1 in here: https://github.com/WebAssembly/wasi-libc/blob/230d4be6c54bec93181050f9e25c87150506bdd0/expected/wasm32-wasip1/predefined-macros.txt#L52 (I see the 0x100 over here, but I think that's a vanilla musl header?)

My test show the 0x1 works, which bolsters my case, but I'm curious to know what the actual source of truth for these constants is. ..

[squeek502]

This might be relevant if Zig is calling into the "bottom half" of wasi-libc:

https://github.com/WebAssembly/wasi-libc/blob/main/libc-bottom-half/headers/public/__header_fcntl.h#L55
https://github.com/WebAssembly/wasi-libc/tree/main/libc-bottom-half

[rootbeer]

I believe I've found the header that defines the constants that wasi-libc expects its clients to use: lib/libc/include/wasm-wasi-musl/__header_fcntl.h. (This header is included by fcntl.h in the same directory and via a bunch of indirect pre-processing, overrides the default constants defined in fcntl.h, so someone who just glances at fcntl.h might see the wrong constants.)

The directory is included by wasm_libc.zig as the sysroot (the triple is wasm-wasi-musl):

        "-iwithsysroot",
        try comp.zig_lib_directory.join(arena, &[_][]const u8{ "libc", "include", triple }),

The "bottom-half" headers I linked to earlier, as I understand it, are for the wasi-libc implementation to talk to WASI APIs, so its not directly relevant here -- though it seems like many of the constants are defined to match across the two layers.

So, according to this header AT_SYMLINK_{FOLLOW,NOFOLLOW} should be 0x2 and 0x1 respectively.

I have fixes for this in #20991 (along with some testcases that exercise these values).

If you have a pure C compilation environment set up for WASI, you might trying printfing these various constants to see which values they map to?