linux-kernel - [PATCH -next V6 1/7] riscv: ftrace: Fixup panic by disabling preemption

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Date:   Sat,  7 Jan 2023 08:35:43 -0500
From:   guoren@...nel.org
To:     anup@...infault.org, paul.walmsley@...ive.com, palmer@...belt.com,
        conor.dooley@...rochip.com, heiko@...ech.de, rostedt@...dmis.org,
        mhiramat@...nel.org, jolsa@...hat.com, bp@...e.de,
        jpoimboe@...nel.org, suagrfillet@...il.com, andy.chiu@...ive.com,
        e.shatokhin@...ro.com, guoren@...nel.org
Cc:     linux-riscv@...ts.infradead.org, linux-kernel@...r.kernel.org
Subject: [PATCH -next V6 1/7] riscv: ftrace: Fixup panic by disabling preemption

From: Andy Chiu <andy.chiu@...ive.com>

In RISCV, we must use an AUIPC + JALR pair to encode an immediate,
forming a jump that jumps to an address over 4K. This may cause errors
if we want to enable kernel preemption and remove dependency from
patching code with stop_machine(). For example, if a task was switched
out on auipc. And, if we changed the ftrace function before it was
switched back, then it would jump to an address that has updated 11:0
bits mixing with previous XLEN:12 part.

p: patched area performed by dynamic ftrace
ftrace_prologue:
p|      REG_S   ra, -SZREG(sp)
p|      auipc   ra, 0x? ------------> preempted
					...
				change ftrace function
					...
p|      jalr    -?(ra) <------------- switched back
p|      REG_L   ra, -SZREG(sp)
func:
	xxx
	ret

Fixes: afc76b8b8011 ("riscv: Using PATCHABLE_FUNCTION_ENTRY instead of MCOUNT")
Signed-off-by: Andy Chiu <andy.chiu@...ive.com>
Signed-off-by: Guo Ren <guoren@...nel.org>
---
 arch/riscv/Kconfig | 2 +-
 1 file changed, 1 insertion(+), 1 deletion(-)

diff --git a/arch/riscv/Kconfig b/arch/riscv/Kconfig
index e2b656043abf..ee0d39b26794 100644
--- a/arch/riscv/Kconfig
+++ b/arch/riscv/Kconfig
@@ -138,7 +138,7 @@ config RISCV
 	select HAVE_DYNAMIC_FTRACE_WITH_REGS if HAVE_DYNAMIC_FTRACE
 	select HAVE_FTRACE_MCOUNT_RECORD if !XIP_KERNEL
 	select HAVE_FUNCTION_GRAPH_TRACER
-	select HAVE_FUNCTION_TRACER if !XIP_KERNEL
+	select HAVE_FUNCTION_TRACER if !XIP_KERNEL && !PREEMPTION
 
 config ARCH_MMAP_RND_BITS_MIN
 	default 18 if 64BIT
-- 
2.36.1