MSDOS Filesystem question related to "read-only" files

[Karl Denninger]

root at Test-MCP:/home/karl/HD-MCP # cd /mnt
root at Test-MCP:/mnt # mount
/dev/ufs/rootfs on / (ufs, local, noatime, soft-updates, nfsv4acls)
devfs on /dev (devfs, local)
tmpfs on /tmp (tmpfs, local)
/dev/da0s1 on /mnt (msdosfs, local)
root at Test-MCP:/mnt # ls -al
total 408
drwxr-xr-x   1 hdmcp  wheel    4096 Jan  1  1980 .
drwxr-xr-x  20 root   wheel     512 Nov 20 16:04 ..
drwxr-xr-x   1 hdmcp  wheel    4096 Nov 19 11:20 System Volume Information
-rwxr-xr-x   1 hdmcp  wheel  127979 Nov 19 22:54
cam2-2017-11-19-22-54-47.jpg
-rwxr-xr-x   1 hdmcp  wheel  271295 Nov 19 22:57
cam2-2017-11-19-22-57-20.jpg
root at Test-MCP:/mnt # chmod u-w *
root at Test-MCP:/mnt # ls -al
total 408
drwxr-xr-x   1 hdmcp  wheel    4096 Jan  1  1980 .
drwxr-xr-x  20 root   wheel     512 Nov 20 16:04 ..
drwxr-xr-x   1 hdmcp  wheel    4096 Nov 19 11:20 System Volume Information
-rwxr-xr-x   1 hdmcp  wheel  127979 Nov 19 22:54
cam2-2017-11-19-22-54-47.jpg
-rwxr-xr-x   1 hdmcp  wheel  271295 Nov 19 22:57
cam2-2017-11-19-22-57-20.jpg
root at Test-MCP:/mnt #

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)

On 11/20/2017 12:30, Conrad Meyer wrote:
> Hi Karl,
>
> In fact, msdosfs in FreeBSD should set the FAT READONLY attribute
> under two conditions:
>
> 1. The owner chmod's the file non-writeable (chmod u-w) (what you've
> described, I think).  Or,
> 2. The super user or otherwise privileged user sets the "readonly"
> flag on the file via chflags(1).
>
> How have you determined that chmod u-w does nothing?  What version of
> FreeBSD are you using?
>
> Best,
> Conrad
>
>
> On Mon, Nov 20, 2017 at 7:36 AM, Karl Denninger <karl at denninger.net> wrote:
>> I'm running into an interesting issue here and wondering if there's a
>> way to do this under FreeBSD.
>>
>> MSDOS filesystems have a "primitive" permission capability;
>> specifically, they can have a "Read-only" attribute on a file.  It looks
>> like OpenBSD supports this from reading their man pages.
>>
>> FreeBSD doesn't appear to.  When you mount a msdos filesystem (e.g. a
>> USB stick) whoever owns the parent directory where you mount it gives
>> you the permissions and "ownership" of files on said filesystem.  All
>> good so far.  But attempting to chmod a file to remove write permission
>> "succeeds" (returns success) but does nothing.
>>
>> Is this capability simply not present on FreeBSD?  I'm interested in
>> using it as a means of "flagging" files on a USB stick in an application
>> that I do not want to remove if the stick fills (basically, to "protect"
>> them from being aged off) and it appears there's no way to do it, other
>> than to use something unique in the filename that I would then have to
>> pay attention to.
>>
>> --
>> Karl Denninger
>> karl at denninger.net <mailto:karl at denninger.net>
>> /The Market Ticker/
>> /[S/MIME encrypted email preferred]/

-- 
Karl Denninger
karl at denninger.net <mailto:karl at denninger.net>
/The Market Ticker/
/[S/MIME encrypted email preferred]/
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