powerpc/ftrace: Have arch_ftrace_get_regs() return NULL unless FL_SAVE_REGS is set

When FL_SAVE_REGS is not set we get here via ftrace_caller()
which doesn't save all registers.

ftrace_caller() explicitely clears regs.msr, so we can rely
on it to know where we come from. We don't expect MSR register
to be 0 at all when involving ftrace.

Fixes: 40b035efe2 ("powerpc/ftrace: Implement CONFIG_DYNAMIC_FTRACE_WITH_ARGS")
Reported-by: Naveen N. Rao <naveen.n.rao@linux.vnet.ibm.com>
Signed-off-by: Christophe Leroy <christophe.leroy@csgroup.eu>
Signed-off-by: Michael Ellerman <mpe@ellerman.id.au>
Link: https://lore.kernel.org/r/2f9a7e898c93cc7438ef5ccd47cb9c3a9c5b53ef.1644949750.git.christophe.leroy@csgroup.eu

arch/powerpc/include/asm/ftrace.h

@@ -30,7 +30,8 @@ struct ftrace_regs {
 
 static __always_inline struct pt_regs *arch_ftrace_get_regs(struct ftrace_regs *fregs)
 {
-	return &fregs->regs;
+	/* We clear regs.msr in ftrace_call */
+	return fregs->regs.msr ? &fregs->regs : NULL;
 }
 
 static __always_inline void ftrace_instruction_pointer_set(struct ftrace_regs *fregs,

arch/powerpc/kernel/trace/ftrace_mprofile.S

@@ -205,6 +205,10 @@ _GLOBAL(ftrace_caller)
 	PPC_STL	r0, _LINK(r1)
 	mr	r4, r0
 
+	/* Clear MSR to flag as ftrace_caller versus frace_regs_caller */
+	li	r8, 0
+	PPC_STL	r8, _MSR(r1)
+
 	/* Load &pt_regs in r6 for call below */
 	addi    r6, r1 ,STACK_FRAME_OVERHEAD