Trampoline jumping problems

[Mathias Bauer]

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.

Regards,
Mathias