[Crash-utility] arm64: bug report: segfault

Hi Dave, When crash gets a prstatus for an AArch32 (compat) user mode stack frame, but assumes it can look at prstatus->sp for the stack pointer, and prstatus->sp has a stale AArch64 kernel address in it, then crash segfaults. This is because the stack pointer isn't a user stack address, and thus crash expects the stack to include at least two frames, which would mean fp is non-zero, but in this case it's not[1], and that leads to the calculation of a bad stack pointer (see arm64.c:1209), which then gets used as an offset into the stack buffer (see arm64.c:1001), overflowing it. The patch[2] I just sent resolves this issue for me, but only because it no longer tries to use prstatus->sp. We should probably still look into fixing this in another way, such as making sure fp is non-zero, as dumps can have all sorts of corruption breaking our assumptions. Thanks, drew [1] The dump was captured with QEMU, which doesn't require a real crash, i.e. panic() being called in the guest kernel, thus cpus can actually be in user mode, rather than in the 64-bit cpu-stop IPI handler, or other crashing kernel code. [2] https://www.redhat.com/archives/crash-utility/2015-December/msg00024.html