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")