Subject: bug#9002: file-error Doing chown operation not permitted

From: Paul Eggert <[email-masked]>
Date: Mon, 18 Jul 2011 10:24:06 -0700

In yet more testing I found a couple more issues.
First, many systems allow you to change gid on files
that you can't set the uid of, if you're a member of
the group and own the file.  Second, if we can't set
the uid and/or gid we shouldn't enable the setuid and/or
gid bits, as the result would be setuid and/or setgid
to the wrong user.  I pushed the following further
patch into the trunk.

* fileio.c (Fcopy_file): Adjust mode if fchown fails.  (Bug#9002)
If fchown fails to set both uid and gid, try to set just gid,
as that is sometimes allowed.  Adjust the file's mode to eliminate
setuid or setgid bits that are inappropriate if fchown fails.
=== modified file 'src/fileio.c'
--- src/fileio.c	2011-07-17 01:18:51 +0000
+++ src/fileio.c	2011-07-18 17:15:29 +0000
@@ -38,8 +38,6 @@
 #include <selinux/context.h>
 #endif

-#include <ignore-value.h>
-
 #include "lisp.h"
 #include "intervals.h"
 #include "buffer.h"
@@ -1961,9 +1959,21 @@
      owner and group.  */
   if (input_file_statable_p)
     {
+      int mode_mask = 07777;
       if (!NILP (preserve_uid_gid))
-	ignore_value (fchown (ofd, st.st_uid, st.st_gid));
-      if (fchmod (ofd, st.st_mode & 07777) != 0)
+	{
+	  /* Attempt to change owner and group.  If that doesn't work
+	     attempt to change just the group, as that is sometimes allowed.
+	     Adjust the mode mask to eliminate setuid or setgid bits
+	     that are inappropriate if the owner and group are wrong.  */
+	  if (fchown (ofd, st.st_uid, st.st_gid) != 0)
+	    {
+	      mode_mask &= ~06000;
+	      if (fchown (ofd, -1, st.st_gid) == 0)
+		mode_mask |= 02000;
+	    }
+	}
+      if (fchmod (ofd, st.st_mode & mode_mask) != 0)
 	report_file_error ("Doing chmod", Fcons (newname, Qnil));
     }
 #endif	/* not MSDOS */