[PATCH] riscv: ftrace: Fix ftrace_modify_call failure on kprobed functions

From: Pu Lehui <[email protected]>

We are frequently hitting the following splat during the riscv bpf
selftests:

00000000026dc75a: expected (7c3ff297) but got (00100073)
------------[ ftrace bug ]------------
ftrace failed to modify
[<ffffffff03c44c1c>] bpf_kfunc_common_test+0x4/0x20 [bpf_testmod]
 actual:   e7:82:c2:ce
Updating ftrace call site to call a different ftrace function
ftrace record flags: 80100002
 (2)
 expected tramp: ffffffff80043904
------------[ cut here ]------------
WARNING: kernel/trace/ftrace.c:2278 at ftrace_bug+0x46e/0x4b0, CPU#1: test_progs/98
...
[<ffffffff80008f4e>] ftrace_bug+0x46e/0x4b0
[<ffffffff803d3e86>] ftrace_replace_code+0x16e/0x170
[<ffffffff803d42b6>] ftrace_modify_all_code+0x12e/0x1b8
[<ffffffff800430f4>] arch_ftrace_update_code+0x14/0x28
[<ffffffff803e0324>] ftrace_startup+0x14c/0x2a0
[<ffffffff803e133c>] ftrace_startup_subops+0x584/0x1050
[<ffffffff804500e6>] register_ftrace_graph+0x4e6/0x1018
[<ffffffff804cf9f6>] register_fprobe_ips+0xc66/0x12f8
[<ffffffff8049abe8>] bpf_kprobe_multi_link_attach+0x5d8/0xe68
[<ffffffff8050fcaa>] __sys_bpf+0x3d5a/0x47f0
[<ffffffff805107ee>] __riscv_sys_bpf+0xae/0x168
[<ffffffff80034d78>] syscall_handler+0x60/0x100
[<ffffffff8228b4f4>] do_trap_ecall_u+0x174/0x208
[<ffffffff822b69c4>] handle_exception+0x16c/0x178

After debugging, it can be triggered by similar commands below:
```
echo do_nanosleep > set_ftrace_filter
echo function > current_tracer
echo 'p do_nanosleep' > kprobe_events
echo 1 > events/kprobes/enable
echo 'f do_nanosleep' > dynamic_events
echo 1 > events/fprobes/enable
```

The reason is that attaching a kprobe to an ftrace-traced function entry
replaces its initial auipc insn with ebreak. When ftrace_modify_call
later runs, it expects auipc insn, so verification fails and triggers
ftrace_bug.

The expected auipc logic remains conceptually unchanged, and kprobe
single-stepping ensures normal execution. Therefore, if the first insn
is ebreak, bypassing the check to continue patching the jalr insn is
safe and avoids ftrace failures.

Fixes: b2137c3b6d7a ("riscv: ftrace: prepare ftrace for atomic code patching")
Signed-off-by: Pu Lehui <[email protected]>
---
 arch/riscv/kernel/ftrace.c | 5 ++++-
 1 file changed, 4 insertions(+), 1 deletion(-)

diff --git a/arch/riscv/kernel/ftrace.c b/arch/riscv/kernel/ftrace.c
index b430edfb83f4..be8b68514417 100644
--- a/arch/riscv/kernel/ftrace.c
+++ b/arch/riscv/kernel/ftrace.c
@@ -12,6 +12,7 @@
 #include <linux/stop_machine.h>
 #include <asm/cacheflush.h>
 #include <asm/text-patching.h>
+#include <asm/insn.h>
 
 #ifdef CONFIG_DYNAMIC_FTRACE
 void ftrace_arch_code_modify_prepare(void)
@@ -63,7 +64,9 @@ static int __ftrace_modify_call(unsigned long source, unsigned long target, bool
 		if (copy_from_kernel_nofault(replaced, (void *)source, 2 * MCOUNT_INSN_SIZE))
 			return -EFAULT;
 
-		if (replaced[0] != call[0]) {
+		/* Bypass the check if the auipc insn is a kprobe breakpoint */
+		if (replaced[0] != call[0] &&
+		    !(riscv_insn_is_ebreak(replaced[0]) || riscv_insn_is_c_ebreak(replaced[0]))) {
 			pr_err("%p: expected (%08x) but got (%08x)\n",
 			       (void *)source, call[0], replaced[0]);
 			return -EINVAL;
-- 
2.34.1


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