Thread: Kernel bug? (attn matsp)

Originally Posted by brewbuck

Yep! I finally realized this. I was also having some trouble with position-independence -- using -fPIC doesn't work, because it causes inter-module function dispatch to go through the PLT. But the PLT is useless because I'm the dynamic linker and I have nobody to set it up for me

So I ended up using -fpie instead, linked with -shared, and now all is well.

I still think the call to exec() should fail if the program interpretter would be mapped over the top of the ELF image. That's just dumb.

Neither kernel nor user space never prevent you from shouting yourself.
rm -rf / is sure worse then your case, but still kernel doesn't prevent it.

And even if it did - I don't think you was better off with execve failed with EINVAL than you are now.

mmap with MMAP_FIXED flag set also will happily overwrite already mapped memory.

By the way, why you need your own dynamic linker ?
What did you miss in the standard one ?

Originally Posted by brewbuck

A person could conceivably want to do "rm -rf /". But it makes no sense to EVER map the interpreter on top of the program image. Without the image, there is nothing for the interpreter to do. It's a scenario that makes no sense.

I still can't see how "rm -rf /" make sense.
But the point is - kernel usually not held your hand. It provides services and
left to user space to make sense of it.

And I think usually it's a good thing.

That's a design flaw. You should be required to deliberately unmap the region first before mapping something else there (or at least specify a flag indicating your intention, i.e. MAP_REPLACE). Again, it leaves the door open for massive confusion when things can silently map on top of other things when that is not the intention.

Agree. But I think this behavior required by POSIX, so no hope to that change :(

If I can implement a dynamic linker, then I know I understand how ELF dynamic linking works in the minutest detail.

I want someday to do it too. But that day is still a long way away :)