Re: Out of tree module using LSM

[Andi Kleen <andi@firstfloor.org>]

Alan Cox <alan@lxorguk.ukuu.org.uk> writes:

> If I want I can have 16 threads executing code in a shared object being
> written to by ten other programs at once and shared over a network while
> we are at it. Its probably not a good idea but I can do it if I have
> reason to.

Actually the kernel prevents writing to files which are executed currently
and you can also not execute what is currently open for writing.
That is what VM_DENYWRITE is good for.

Does not work for shared libraries though -- you cannot set VM_DENYWRITE
in user mmap aka ld.so Man page says

  MAP_DENYWRITE
              This flag is ignored.  (Long ago, it signalled that attempts to write to the underlying file should fail
              with ETXTBUSY.  But this was a source of denial-of-service attacks.)

-Andi