[cc some more libc folks] I have a general question about this patch set: If I'm writing a user program, and I write a signal handler, there are two things I want to make sure I can still do: 1. I want to be able to unwind directly from the signal handler without involving sigreturn() -- that is, I want to make sure that siglongjmp() works. How does this work? Is INCSSP involved? How exactly does the user program know how much to increment SSP by? (And why on Earth does INCSSP only consider the low 8 bits of its argument? That sounds like a mistake. Can Intel still fix that? On the other hand, what happens if you INCSSP off the end of the shadow stack entirely? I assume the next access will fault as long as there's an appropriate guard page.) 2. I want to be able to modify the signal context from a signal handler such that, when the signal handler returns, it will return to a frame higher up on the call stack than where the signal started and to a different RIP value. How can I do this? I guess I can modify the shadow stack with WRSS if WR_SHSTK_EN=1, but how do I tell the kernel to kindly skip the frames I want to skip when I do sigreturn()? The reason I'm asking #2 is that I think it's time to resurrect my old vDSO syscall cancellation helper series here: https://lwn.net/Articles/679434/ and it's not at all clear to me that __vdso_abort_pending_syscall() can work without kernel assistance when CET is enabled. I want to make sure that it can be done, or I want to come up with some other way to allow a signal handler to abort a syscall while CET is on. I could probably change __vdso_abort_pending_syscall() to instead point RIP to __kernel_vsyscall's epilogue so that we con't change the depth of the call stack. But I could imagine that other user programs might engage in similar shenanigans and want to have some way to unwind a signal's return context without actually jumping there a la siglongjmp(). Also, what is the intended setting of WR_SHSTK_EN with this patch set applied? (I suppose we could just say that 32-bit processes should not use CET, but that seems a bit sad.)
