[PATCH] riscv/ftrace: fix ftrace_modify_call bug

[Song Shuai]

With this commit (riscv: ftrace: Reduce the detour code size to half)
patched, ftrace bug occurred When hosting kprobe and function tracer
at the same function.

Obviously, the variable caller in ftrace_modify_call was assigned by
rec->ip with 4 offset failing the code replacing at function entry.
And the caller should be assigned by rec->ip directly to indicate
the function entry.

The following is the ftrace bug log.

```
[  419.632855] 00000000f8776803: expected (ffe00297 1a4282e7) but got (1a8282e7 f0227179)
[  419.633390] ------------[ ftrace bug ]------------
[  419.633553] ftrace failed to modify
[  419.633569] [<ffffffff802091cc>] kernel_read+0x0/0x52
[  419.633863]  actual:   97:02:e0:ff:e7:82:82:1a
[  419.634087] Updating ftrace call site to call a different ftrace function
[  419.634279] ftrace record flags: e0000002
[  419.634487]  (2) R
[  419.634487]  expected tramp: ffffffff800093cc
[  419.634935] ------------[ cut here ]------------
```

Signed-off-by: Song Shuai <suagrfillet@xxxxxxxxx>
---
 arch/riscv/kernel/ftrace.c | 2 +-
 1 file changed, 1 insertion(+), 1 deletion(-)

diff --git a/arch/riscv/kernel/ftrace.c b/arch/riscv/kernel/ftrace.c
index 8c77f236fc71..61b24d767e2e 100644
--- a/arch/riscv/kernel/ftrace.c
+++ b/arch/riscv/kernel/ftrace.c
@@ -132,7 +132,7 @@ int ftrace_modify_call(struct dyn_ftrace *rec, unsigned long old_addr,
 		       unsigned long addr)
 {
 	unsigned int call[2];
-	unsigned long caller = rec->ip + 4;
+	unsigned long caller = rec->ip;
 	int ret;
 
 	make_call_t0(caller, old_addr, call);
-- 
2.20.1