Andy Lutomirski [off-list ref] wrote:

quoted

 (1) You'd need at least one pre-loader binary image built into the kernel
     that you can map into userspace (you can't upcall to userspace to go get
     it for your core binfmt).  This could appear as, say, /proc/preloader,
     for the kernel to open and mmap.

No need for it to be visible at all.  I'm imagining the kernel making
a fresh mm_struct, directly mapping some text, running that text, and
then using the result as the mm_struct after execve.

What would you see in /proc/pid/maps?

quoted

 (2) Where would the kernel put the executable image?  It would have to
     parse the binary to find out where not to put it - otherwise the code
     might have to relocate itself.

In vmlinux.

You misunderstood the question.  I meant at what address would you map it into
userspace?  You would have to avoid anywhere the executable needs to place
something - though as long as you can manage to start the loader, you can
ditch the pre-loader, so that might not be a problem.

quoted

 (6) NOMMU could be particularly tricky.  For ELF-FDPIC at least, the stack
     size is set in the binary.  OTOH, you wouldn't have to relocate the
     pre-loader - you'd just mmap it MAP_PRIVATE and execute in place.

For nommu, forget about it.

Why?  If you do that, you have to have bimodal binfmts.  Note that the
ELF-FDPIC binfmt, at least, can be used for both MMU and NOMMU environments.
This may also be true of FLAT.

quoted

 (7) When the kernel finds it's dealing with a script, it goes back through
     the security calculation procedure again to deal with the interpreter.

The security calculation isn't what I'm worried about.  I'm worried
about the parser.

But you may have to redo the security calculation *after* doing the parsing.

Anyway, I didn't say this would be easy :)

True... :-)

David

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