[PATCH] riscv: Fix early ftrace nop patching

[Alexandre Ghiti]

Commit c97bf629963e ("riscv: Fix text patching when IPI are used")
converted ftrace_make_nop() to use patch_insn_write() which does not
emit any icache flush relying entirely on __ftrace_modify_code() to do
that.

But we missed that ftrace_make_nop() was called very early directly when
converting mcount calls into nops (actually on riscv it converts 2B nops
emitted by the compiler into 4B nops).

This caused crashes on multiple HW as reported by Conor and Björn since
the booting core could have half-patched instructions in its icache
which would trigger an illegal instruction trap: fix this by emitting a
local flush icache when early patching nops.

Fixes: c97bf629963e ("riscv: Fix text patching when IPI are used")
Signed-off-by: Alexandre Ghiti <alexghiti@xxxxxxxxxxxx>
---
 arch/riscv/include/asm/cacheflush.h | 6 ++++++
 arch/riscv/kernel/ftrace.c          | 3 +++
 2 files changed, 9 insertions(+)

diff --git a/arch/riscv/include/asm/cacheflush.h b/arch/riscv/include/asm/cacheflush.h
index dd8d07146116..ce79c558a4c8 100644
--- a/arch/riscv/include/asm/cacheflush.h
+++ b/arch/riscv/include/asm/cacheflush.h
@@ -13,6 +13,12 @@ static inline void local_flush_icache_all(void)
 	asm volatile ("fence.i" ::: "memory");
 }
 
+static inline void local_flush_icache_range(unsigned long start,
+					    unsigned long end)
+{
+	local_flush_icache_all();
+}
+
 #define PG_dcache_clean PG_arch_1
 
 static inline void flush_dcache_folio(struct folio *folio)
diff --git a/arch/riscv/kernel/ftrace.c b/arch/riscv/kernel/ftrace.c
index 4f4987a6d83d..32e7c401dfb4 100644
--- a/arch/riscv/kernel/ftrace.c
+++ b/arch/riscv/kernel/ftrace.c
@@ -120,6 +120,9 @@ int ftrace_init_nop(struct module *mod, struct dyn_ftrace *rec)
 	out = ftrace_make_nop(mod, rec, MCOUNT_ADDR);
 	mutex_unlock(&text_mutex);
 
+	if (!mod)
+		local_flush_icache_range(rec->ip, rec->ip + MCOUNT_INSN_SIZE);
+
 	return out;
 }
 
-- 
2.39.2