[PATCH v1 3/4] powerpc/ftrace: Have arch_ftrace_get_regs() return NULL unless FL_SAVE_REGS is set

[Christophe Leroy]

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.

Reported-by: Naveen N. Rao <naveen.n.rao at linux.vnet.ibm.com>
Fixes: 40b035efe288 ("powerpc/ftrace: Implement CONFIG_DYNAMIC_FTRACE_WITH_ARGS")
Signed-off-by: Christophe Leroy <christophe.leroy at csgroup.eu>
---
 arch/powerpc/include/asm/ftrace.h           | 3 ++-
 arch/powerpc/kernel/trace/ftrace_mprofile.S | 4 ++++
 2 files changed, 6 insertions(+), 1 deletion(-)

diff --git a/arch/powerpc/include/asm/ftrace.h b/arch/powerpc/include/asm/ftrace.h
index 70b457097098..ff034ae4e472 100644
--- a/arch/powerpc/include/asm/ftrace.h
+++ b/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,
diff --git a/arch/powerpc/kernel/trace/ftrace_mprofile.S b/arch/powerpc/kernel/trace/ftrace_mprofile.S
index 8443902d5a05..eb077270ec2f 100644
--- a/arch/powerpc/kernel/trace/ftrace_mprofile.S
+++ b/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
 
-- 
2.34.1