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Re: [ARM/FDPIC v6 13/24] [ARM] FDPIC: Force LSB bit for PC in Cortex-M architecture
On 17/09/2019 13:38, Wilco Dijkstra wrote:
Hi Christophe,
Can you explain this in more detail - it doesn't make sense to me to force the
Thumb bit during unwinding since it should already be correct, even on a
Thumb-only CPU. Perhaps the kernel code that pushes an incorrect address on
the stack could be fixed instead?
Without this, when we are unwinding across a signal frame we can jump
to an even address which leads to an exception.
This is needed in __gnu_persnality_sigframe_fdpic() when restoring the
PC from the signal frame since the PC saved by the kernel has the LSB
bit set to zero.
Wilco
.
Indeed, I've noticed the problem mentioned by Matthew since I committed that patch.
I was about to propose a fix, replacing #if (__thumb__) with #if (!__ARM_ARCH_ISA_ARM), but you are right: maybe the kernel code should be fixed instead.
So far I haven't managed to reproduce a failure in FDPIC mode without this patch though...
Thanks and sorry for the breakage.
Christophe