[PATCH v6 01/14] riscv: prevent null-pointer dereference with sbi_remote_fence_i

[Heiko Stuebner <heiko@sntech.de>]

The callback used inside sbi_remote_fence_i is set at sbi probe time
to the needed variant. Before that it is a NULL pointer.

Some users like the flush_icache_*() functions suggest a generic
functionality, that doesn't depend on a specific boot-stage but
uses sbi_remote_fence_i as one option to flush other cpu cores.

So they definitly shouldn't run into null-pointer dereference
issues when called "too early" during boot.

So introduce an empty function to be the standard for the __sbi_rfence
function pointer until sbi_init has run.

Users of sbi_remote_fence_i will have separate code for the local
cpu and sbi_init() is called before other cpus are brought up.
So there are no other cpus present at the time when the issue
might happen.

Signed-off-by: Heiko Stuebner <heiko@sntech.de>
---
 arch/riscv/kernel/sbi.c | 10 +++++++++-
 1 file changed, 9 insertions(+), 1 deletion(-)

diff --git a/arch/riscv/kernel/sbi.c b/arch/riscv/kernel/sbi.c
index f72527fcb347..c839acd668d3 100644
--- a/arch/riscv/kernel/sbi.c
+++ b/arch/riscv/kernel/sbi.c
@@ -15,11 +15,19 @@
 unsigned long sbi_spec_version __ro_after_init = SBI_SPEC_VERSION_DEFAULT;
 EXPORT_SYMBOL(sbi_spec_version);
 
+static int __sbi_rfence_none(int fid, const struct cpumask *cpu_mask,
+			     unsigned long start, unsigned long size,
+			     unsigned long arg4, unsigned long arg5)
+{
+	return -EOPNOTSUPP;
+}
+
 static void (*__sbi_set_timer)(uint64_t stime) __ro_after_init;
 static int (*__sbi_send_ipi)(const struct cpumask *cpu_mask) __ro_after_init;
 static int (*__sbi_rfence)(int fid, const struct cpumask *cpu_mask,
 			   unsigned long start, unsigned long size,
-			   unsigned long arg4, unsigned long arg5) __ro_after_init;
+			   unsigned long arg4, unsigned long arg5)
+			   __ro_after_init = __sbi_rfence_none;
 
 struct sbiret sbi_ecall(int ext, int fid, unsigned long arg0,
 			unsigned long arg1, unsigned long arg2,
-- 
2.30.2