Re: Trampoline jumping problems

[David Chisnall]

On 10 Mar 2014, at 19:10, Mathias Bauer <mathias_bauer@gmx.net> wrote:

> Hi dear list,
> 
> the code in libobjc2 that implements imp_ImplementationWithBlock does not 
> work on at least some ARM platforms.
> 
> At least on boards using an Exynos CPU I see random crashes when using 
> imp_ImplementationWithBlock for dynamically provided implementations for 
> property getters and setters.
> 
> The crash always happens at an address that is a page boundary - it's the 
> boundary of the current page for trampolines. So it seems that at the memory 
> of the IMP there is no trampoline code, instead of that this memory area 
> behaves like a playgound slide that finally lets the IP move to the page 
> boundary.
> 
> The trampoline and its two addresses are written to this memory through a 
> pointer memory-mapped to a file handle with PROT_WRITE, while another pointer 
> memory-mapped to the same file handle with PROT_READ|PROT_EXEC is used to 
> read and execute the data later.
> 
> It seems that on the architecture that experiences the crashes there is a 
> time lag between writing the data and the availability of the bytes as 
> executable code, because the crash goes away if I add some delay after 
> writing the data.
> 
> It seems that we somehow need to make sure that what was written can be 
> executed immediately after that.

I think on ARM we need an instruction memory barrier there.  We probably do on 
MIPS too.

Can you try adding this at the end of the imp_ImplementationWithBlock function 
and see if it fixes it on ARM?

volatile __asm__ ("imb")

David

-- Sent from my brain