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

On 2026/8/7 11:12, Paul Walmsley wrote:
> Hi,
> 
> On Sun, 2 Aug 2026, Pu Lehui wrote:
> 
>> 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
> 
> I guess you meant
> 
> echo 'f do_nanosleep' >> dynamic_events
> 
> instead?
> 
>> 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]>
> 
> I queued the patch with the reproduction sequence updated as mentioned
> above for v7.2-rc.  Please let me know ASAP if you don't agree -

Thanks Paul, total agree to update.

> 
> 
> - Paul


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