linux-kernel - [PATCH v1 3/4] powerpc/ftrace: Have arch_ftrace_get_regs() return NULL unless FL_SAVE

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Message-Id: <2f9a7e898c93cc7438ef5ccd47cb9c3a9c5b53ef.1644949750.git.christophe.leroy@csgroup.eu>
Date:   Tue, 15 Feb 2022 19:31:24 +0100
From:   Christophe Leroy <christophe.leroy@...roup.eu>
To:     Steven Rostedt <rostedt@...dmis.org>,
        Ingo Molnar <mingo@...hat.com>,
        Benjamin Herrenschmidt <benh@...nel.crashing.org>,
        Paul Mackerras <paulus@...ba.org>,
        Michael Ellerman <mpe@...erman.id.au>
Cc:     Christophe Leroy <christophe.leroy@...roup.eu>,
        linux-kernel@...r.kernel.org, linuxppc-dev@...ts.ozlabs.org,
        "Naveen N . Rao" <naveen.n.rao@...ux.vnet.ibm.com>
Subject: [PATCH v1 3/4] 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.

Reported-by: Naveen N. Rao <naveen.n.rao@...ux.vnet.ibm.com>
Fixes: 40b035efe288 ("powerpc/ftrace: Implement CONFIG_DYNAMIC_FTRACE_WITH_ARGS")
Signed-off-by: Christophe Leroy <christophe.leroy@...roup.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