Commits on Dec 4, 2012

1. Update mtime when mmap'ed pages become writable …

   GNU Tar is sensitive to situations where mtimes on open files are
   updated to times in the past.
   
   When doing write operations on mmap'ed files, the modification time update
   presently occurs during a flush. Observant programs that open such files
   shortly after they are closed will observe that modification times
   appear to occur in the past. This caused a failure that was observed
   during binary package generation on Gentoo where PaX-marking is done on
   certain files via mmap almost immediately prior to package generation,
   which relies on GNU tar. It can be replicated with a Hello World program
   by doing:
   
   gcc hello.c && scanelf -Xxz -r a.out && tar -cf hello.tgz a.out
   
   Brian Behlendorf suggested that we could prevent this by doing mtime
   updates in the kernel's page_mkwrite() callback function, which is
   invoked whenever a page becomes writeable. We implement that by
   replacing the page_mkwrite function with a wrapper function that invokes
   zfs_inode_update() after invoking the kernel's generic page_mkwrite. It
   would be better to do an mtime update and flush on each close()
   operation done from userland on a dirty file, but unfortunately, the
   Linux kernel provides us with no way to do that.
   
   Signed-off-by: Richard Yao <ryao@cs.stonybrook.edu>

   

   ryao committed Dec 4, 2012
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