Draft: Implement ishidden for ZFS filesystems

[jakewilliami]

ZFS has some special considerations for hidden files, namely the mount points of ZFS,^{[1], [2]} or a ZFS-classified hidden file.^{[3], [4]}

Addresses #3, #12, #13.

Tasks:

• Implement iszfs function
• Implement _ishidden_zfs
  • Check if the file is a mount point for ZFS (i.e., is .zfs)
  • Check whether the file is hidden, from either z_pflags from znode,^[5] xat_hidden from xvattr (which uses z_pflags),^{[6], [7], [8]} or by obtaining the znode struct from a vnode^{[9], [10]} and then parsing z_pflags.

Unfortunately, I am very limited by what functions I have exported within the shared object files. Trying to keep it in pure Julia, but we will see. I've already spent far too long researching this, so I've made the decision that this feature will be implemented after this package is registered/released, in the interest of getting the package out there for use (as this feature is probably pretty niche).

[jakewilliami]

To relay some investigation from c95cb4f:

macOS:
$ gcc -o statfs statfs.c && ./statfs
offsetof(struct statfs, f_type): 0x3c
st.f_type: 0x1a

BSD:
$ gcc11 statfs.c -o statfs && ./statfs
offsetof(struct statfs, f_type): 0x4
st.f_type: 0xde

However, this is not always consistent with Julia:

macOS:
julia> UInt8(findfirst(==(0x1a), iszfs()))
0x45  # != 0x3c + 1 (Julia indexes from 1)

BSD:
julia> UInt8(findfirst(==(0xde), iszfs()))
0x05  # 0x04 + 1

[jakewilliami]

It appears that at offset 0x44 (or 0x45 in Julia) is the field f_fssubtype (which isn't even mentioned for statfs in the man page):

offsetof(struct statfs, f_fssubtype): 0x44

Hence, perhaps on macOS I should check both offsets.

[jakewilliami]

For future reference, this is how I determine the offsets of flags:

#include <stdio.h>
#include <stddef.h>
#include <sys/param.h>
#include <sys/mount.h>

int main() {
    struct statfs st;
    printf("offsetof(struct statfs, f_type): 0x%lx\n", offsetof(struct statfs, f_type));
    statfs("/", &st);
    printf("st.f_type: 0x%llx\n", (uint64_t) st.f_type);
    return 0;
}

[jakewilliami]

The equivalent of #20 (comment) for Linux:

#include <stdio.h>
#include <stddef.h>
#include <sys/vfs>    /* or <sys/statfs.h> */

int main() {
    printf("sizeof(struct statfs): %ld\n", sizeof(struct statfs));
    printf("offsetof(struct statfs, f_type): 0x%lx\n", offsetof(struct statfs, f_type));
    struct statfs st;
    statfs("/", &st);
    printf("st.f_type: 0x%lx\n", st.f_type);
    return 0;
}

This produced:

Linux:
$ gcc -o statfs statfs.c && ./statfs
sizeof(struct statfs): 120
offsetof(struct statfs, f_type): 0x0
st.f_type: 0xef53

Which appears to be consistent within Julia:

julia> statfs("/").ftype
0x000000000000ef53

julia> _iszfs("/")[0x01]
0x0000ef53