riscv: ftrace: Fixup panic by disabling preemption

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@sifive.com>
Signed-off-by: Guo Ren <guoren@kernel.org>
Link: https://lore.kernel.org/r/20230112090603.1295340-2-guoren@kernel.org
Cc: stable@vger.kernel.org
Signed-off-by: Palmer Dabbelt <palmer@rivosinc.com>

diff --git a/arch/riscv/Kconfig b/arch/riscv/Kconfig
index e2b656043abf31128571abb8329327cc1044e96f..ee0d39b26794642a99608ce4f12b6eab08d5178e 100644 (file)
--- 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