bug#11245: 24.0.95; set-file-selinux-context fails when the filesystem doesn't support contexts

[Glenn Morris <rgm@gnu.org>]

Hi Karel,

We received this bug report about the SELinux support that you wrote for
Emacs. Could you take a look? Just hit reply-all to respond to the
report. Thanks.

http://debbugs.gnu.org/11245

goeran@uddeborg.se wrote:

> After upgrading to emacs to the one in Fedora 17 test release, numbered
> 24.0.95, I started to get messages about emacs failing to write backup
> files, and it writing a %backup%~ file instead.  Checking in the file
> system, emacs had successfully made a backup file.  After a bit of
> investigation, I believe it has to do with emacs trying to set SELinux
> context on a file system which doesn't support it.  In my case, my home
> directory is mounted via NFS.
>
> So to reproduce, I do the following, starting in a directory in an NFS
> file system:
>
> On the command line:
>     freddi$ touch apa
>     freddi$ ll -Z apa
>     -rw-rw-r--. göran göran system_u:object_r:nfs_t:SystemLow apa
>     freddi$ emacs -Q
>
> Then, in emacs:
>     M-x s e t - v a r <tab> <return> b a c k u <tab> b
>     y <tab> <return> t <return> C-x C-f a p a <return>
>     a p a C-x C-s
>
> Now I get an error message:
>     Cannot write backup file; backing up in ~/.emacs.d/%backup%~
>
> But there is an (empty) file "apa~", so the backup DID succeed.
>
> My understanding is that the root cause is the function
> set-file-selinux-context in fileio.c failing with a file error when the
> lsetfilecon() call fails.  But it is not really an error if it fails
> with errno set to ENOTSUP.  A system which does support SELinux may very
> well have some parts of the file tree where you can't modify the
> context.  Such as an NFS home directory.
>
> It seems to me that the case where lsetfilecon() fails with ENOTSUP
> should be handled in the same way as if is_selinux_enabled() returned
> false in the first place, i.e. just return without any error.  Do I miss
> something?
>
>
>
> In GNU Emacs 24.0.95.1 (x86_64-redhat-linux-gnu, GTK+ Version 2.24.10)
>  of 2012-04-06 on x86-13.phx2.fedoraproject.org
> Windowing system distributor `Fedora Project', version 11.0.11002000
> Configured using:
>  `configure '--build=x86_64-redhat-linux-gnu'
>  '--host=x86_64-redhat-linux-gnu' '--program-prefix='
>  '--disable-dependency-tracking' '--prefix=/usr' '--exec-prefix=/usr'
>  '--bindir=/usr/bin' '--sbindir=/usr/sbin' '--sysconfdir=/etc'
>  '--datadir=/usr/share' '--includedir=/usr/include'
>  '--libdir=/usr/lib64' '--libexecdir=/usr/libexec'
>  '--localstatedir=/var' '--sharedstatedir=/var/lib'
>  '--mandir=/usr/share/man' '--infodir=/usr/share/info' '--with-dbus'
>  '--with-gif' '--with-jpeg' '--with-png' '--with-rsvg' '--with-tiff'
>  '--with-xft' '--with-xpm' '--with-x-toolkit=gtk' '--with-gpm=no'
>  '--with-wide-int' 'build_alias=x86_64-redhat-linux-gnu'
>  'host_alias=x86_64-redhat-linux-gnu' 'CFLAGS=-DMAIL_USE_LOCKF -O2 -g
>  -pipe -Wall -Wp,-D_FORTIFY_SOURCE=2 -fexceptions -fstack-protector
>  --param=ssp-buffer-size=4 -m64 -mtune=generic' 'LDFLAGS=-Wl,-z,relro ''
>
> Important settings:
>   value of $LC_ALL: nil
>   value of $LC_COLLATE: nil
>   value of $LC_CTYPE: nil
>   value of $LC_MESSAGES: nil
>   value of $LC_MONETARY: nil
>   value of $LC_NUMERIC: nil
>   value of $LC_TIME: nil
>   value of $LANG: sv_SE.utf8
>   value of $XMODIFIERS: nil
>   locale-coding-system: utf-8-unix
>   default enable-multibyte-characters: t
>
> Major mode: Fundamental
>
> Minor modes in effect:
>   tooltip-mode: t
>   mouse-wheel-mode: t
>   tool-bar-mode: t
>   menu-bar-mode: t
>   file-name-shadow-mode: t
>   global-font-lock-mode: t
>   font-lock-mode: t
>   blink-cursor-mode: t
>   auto-composition-mode: t
>   auto-encryption-mode: t
>   auto-compression-mode: t
>   line-number-mode: t
>   transient-mark-mode: (only . t)
>
> Recent input:
> M-x s e t - v a r <tab> <return> b a c k u <tab> b 
> <tab> <return> t <return> C-x C-f a p a <return> a 
> p a C-x C-s M-x r e p o r t - e m a <tab> <return>
>
> Recent messages:
> For information about GNU Emacs and the GNU system, type C-h C-a.
> Saving file /home/göran/apa...
> Cannot write backup file; backing up in ~/.emacs.d/%backup%~
> Wrote /home/göran/apa
>
> Load-path shadows:
> None found.
>
> Features:
> (shadow sort gnus-util mail-extr emacsbug message format-spec rfc822 mml
> mml-sec mm-decode mm-bodies mm-encode mail-parse rfc2231 mailabbrev
> gmm-utils mailheader sendmail regexp-opt rfc2047 rfc2045 ietf-drums
> mm-util mail-prsvr mail-utils cus-edit easymenu cus-start cus-load
> wid-edit help-fns time-date tooltip ediff-hook vc-hooks lisp-float-type
> mwheel x-win x-dnd tool-bar dnd fontset image fringe lisp-mode register
> page menu-bar rfn-eshadow timer select scroll-bar mouse jit-lock
> font-lock syntax facemenu font-core frame cham georgian utf-8-lang
> misc-lang vietnamese tibetan thai tai-viet lao korean japanese hebrew
> greek romanian slovak czech european ethiopic indian cyrillic chinese
> case-table epa-hook jka-cmpr-hook help simple abbrev minibuffer loaddefs
> button faces cus-face files text-properties overlay sha1 md5 base64
> format env code-pages mule custom widget hashtable-print-readable
> backquote make-network-process dbusbind dynamic-setting
> system-font-setting font-render-setting move-toolbar gtk x-toolkit x
> multi-tty emacs)