MSDOS Filesystem question related to "read-only" files

[Conrad Meyer]

On Mon, Nov 20, 2017 at 12:02 PM, Karl Denninger <karl at denninger.net> wrote:
> root at Test-MCP:/mnt # ls -al
> ...
> -rwxr-xr-x   1 hdmcp  wheel  127979 Nov 19 22:54
> cam2-2017-11-19-22-54-47.jpg
> ...
> root at Test-MCP:/mnt # chmod u-w *
> root at Test-MCP:/mnt # ls -al
> ...
> -rwxr-xr-x   1 hdmcp  wheel  127979 Nov 19 22:54
> cam2-2017-11-19-22-54-47.jpg
> ...
> Nope.  The "w" is still there.
>
> No error returned from the "chmod" command (or if I call it from a C
> program) but the file mode is NOT changed whether I'm doing it as the
> superuser or as the owner of the file (and directory)

Indeed, msdosfs does not reflect the READONLY attribute back to
userspace in the form of the mode.  That's a bug that could be fixed
pretty easily:

--- a/sys/fs/msdosfs/msdosfs_vnops.c
+++ b/sys/fs/msdosfs/msdosfs_vnops.c
@@ -287,6 +287,8 @@ msdosfs_getattr(struct vop_getattr_args *ap)
        vap->va_fileid = fileid;

        mode = S_IRWXU|S_IRWXG|S_IRWXO;
+       if ((dep->de_Attributes & ATTR_READONLY) != 0)
+               mode &= ~(S_IWUSR|S_IWGRP|S_IWOTH);
        vap->va_mode = mode &
            (ap->a_vp->v_type == VDIR ? pmp->pm_dirmask : pmp->pm_mask);
        vap->va_uid = pmp->pm_uid;

However, despite 'ls' showing 'w', it *does* set the READONLY
attribute on the file.  Try invoking ls(1) with '-o' and looking for
the "rdonly" or "readonly" flag.

Best,
Conrad