Note that (a) the issue was with clearing the direction flag on signal handler entry, not saving it; and (b) it's since been fixed in the kernel to conform to t

Note that (a) the issue was with clearing the direction flag on signal handler entry, not saving it; and (b) it's since been fixed in the kernel to conform to the ABI (which GCC blindly trusted) [1]. And after reading that thread, I'm not as convinced as I was a few minutes ago that this was an obvious kernel issue. Yes, the kernel mismatched the published ABI, but callee-vs-caller save and setup is not always consistent, and for good reason. E.g. registers are usually callee-save, since there are many registers and small functions use few of them. (This is what the kernel assumed.) But rarely-set/often-used flags (such as the flag in question) may make more sense as caller-save and even caller-setup, since this reduces save/setup overhead to only the cases where the flag is actually set. (This is what the ABI dictated and GCC assumed.) [1] https://lkml.org/lkml/2008/3/5/306 https://lkml.org/lkml/2008/3/5/306