Bug 223846 - msdosfs does not reflect READONLY to user
Summary: msdosfs does not reflect READONLY to user
Status: New
Alias: None
Product: Base System
Classification: Unclassified
Component: kern (show other bugs)
Version: 11.0-STABLE
Hardware: Any Any
: --- Affects Many People
Assignee: freebsd-fs mailing list
URL:
Keywords: patch
Depends on:
Blocks:
 
Reported: 2017-11-24 16:38 UTC by karl
Modified: 2018-04-28 06:34 UTC (History)
2 users (show)

See Also:


Attachments
deny writing to READONLY files in msdosfs (495 bytes, patch)
2018-04-28 06:34 UTC, Damjan Jovanovic
no flags Details | Diff

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Description karl 2017-11-24 16:38:57 UTC
As documented on freebsd-fs in response to a query I posted after observing the above behavior msdosfs does not reflect the READONLY attribute on files in an msdosfs-mounted filesystem back to userspace.

This means that while you can set the READONLY attribute on a file with chmod a subsequent "stat" of that file (from a program), or a display with "ls" will not reflect it in the file mode returned.

Conrad Meyer posted a short code snippet that would fix this; credit to him of course on the identification but IMHO this should be changed globally and MFC'd as appropriate.


--- 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;
Comment 1 Conrad Meyer freebsd_committer 2017-11-24 16:47:13 UTC
I committed it to CURRENT shortly after posting it:  r326031

Bruce raised some concerns, but in his typical long-winded fashion that I haven't had the energy to parse.  I think he would prefer READONLY not be reflected in mode flags, but I'm not sure why.

He does point out that showing a readonly mode flag does not prevent applications from writing to these files.  Maybe it should.
Comment 2 karl 2017-11-24 17:04:05 UTC
(In reply to Conrad Meyer from comment #1)

I would argue it should; what's the point of a read-only flag if it's ignored If you're going to have a permission system then IMHO it ought to be enforced, such as it is and within the constraints of the underlying file system (e.g. in the case of msdosfs the "owner" is effectively synthetic and there is no such thing as "group" or "other".)
Comment 3 Conrad Meyer freebsd_committer 2017-11-24 17:10:26 UTC
Yeah, I agree with you.
Comment 4 Damjan Jovanovic 2018-04-28 06:34:37 UTC
Created attachment 192866 [details]
deny writing to READONLY files in msdosfs

Hi. Here is my patch to deny writing to files with the READONLY attribute on msdosfs  filesystems. It just affects the file_mode variable in msdosfs_access(). My tests show writing and appending to READONLY files is successfully denied the same way on ufs, zfs, and msdosfs.