powerpc/uprobes: Implement arch_uretprobe_is_alive()

This helper is used to detect if a uprobe'd function has returned
through a setjmp/longjmp, rather than branching to the LR that was
updated previously by us. This fixes a SIGSEGV that gets generated when
programs use setjmp/longjmp with uretprobes.

We use the arm64 model (arch/arm64/kernel/probes/uprobes.c:
arch_uretprobe_is_alive()) for detecting when stack frames have been
removed from under us.

Reference:
https://marc.info/?l=linux-kernel&m=143748610330073
commit 7b868e48 ("uprobes/x86: Reimplement arch_uretprobe_is_alive()")
commit db087ef6 ("uprobes/x86: Make arch_uretprobe_is_alive(RP_CHECK_CALL) more
clever")

Tested with the test program from:
https://sourceware.org/git/gitweb.cgi?p=systemtap.git;a=blob;f=testsuite/systemtap.base/bz5274.c;hb=HEAD

And this script:
    $ cat test.sh
    #!/bin/bash

    perf probe -x ./bz5274 -a bz5274_main_return=main%return
    perf probe -x ./bz5274 -a bz5274_funca_return=funca%return
    perf probe -x ./bz5274 -a bz5274_funcb_return=funcb%return
    perf probe -x ./bz5274 -a bz5274_funcc_return=funcc%return
    perf probe -x ./bz5274 -a bz5274_funcd_return=funcd%return

    perf record -e 'probe_bz5274:*' -aR ./bz5274
Reported-by: Gustavo Luiz Duarte <gduarte@redhat.com>
Reported-by: zsun@redhat.com
Signed-off-by: Naveen N. Rao <naveen.n.rao@linux.vnet.ibm.com>
Acked-by: Srikar Dronamraju <srikar@linux.vnet.ibm.com>
Signed-off-by: Michael Ellerman <mpe@ellerman.id.au>

arch/powerpc/kernel/uprobes.c

@@ -205,3 +205,12 @@ arch_uretprobe_hijack_return_addr(unsigned long trampoline_vaddr, struct pt_regs
 
 	return orig_ret_vaddr;
 }
+
+bool arch_uretprobe_is_alive(struct return_instance *ret, enum rp_check ctx,
+				struct pt_regs *regs)
+{
+	if (ctx == RP_CHECK_CHAIN_CALL)
+		return regs->gpr[1] <= ret->stack;
+	else
+		return regs->gpr[1] < ret->stack;
+}