Re: crashme on ARM - unkillable processes

[Linus Torvalds]

On Sun, 9 Nov 2003, Russell King wrote:
> 
> The code which crashme generated corrupted the user stack pointer.  We
> then tried to deliver a signal, found the user stack pointer invalid,
> and tried to deliver a SEGV to the process via force_sig().  Unfortunately,
> this signal never made it through for the reasons described previously.
> (We dequeued the ILL, found we couldn't setup the stack frame, force_sig,
> returned to userspace, generated another undefined instruction exception
> on the same instruction, etc.)

Ahh. I think I found why ARM has this problem, and others don't.

Your SA_NODEFER handling is broken.

The thing is, you only block a signal if its stack frame was successfully 
done _and_ SA_NODEFER is not set.

It should be the other way around. You should block a signal if it's stack 
frame was unsuccessful _or_ SA_NODEFER was not set.

(x86 gets this wrong too, in the sense that we don't even check to see if
the stack frame was successful - but since nobody sets SA_NODEFER anyway,
we don't really much care).

		Linus

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