[PATCH bpf-next v5 2/6] ftrace: Fix deadloop caused by direct call in ftrace selftest

[Xu Kuohai]

On 5/25/2022 9:43 PM, Mark Rutland wrote:
> On Wed, May 18, 2022 at 09:16:34AM -0400, Xu Kuohai wrote:
>> After direct call is enabled for arm64, ftrace selftest enters a
>> dead loop:
> 
> IIUC this means that patch 1 alone is broken, and presumably this patch should
> have been part of it?

No, patch 1 is not broken. This patch fixes bug in the selftest
trampoline, not bug in patch 1.

> 
>> <trace_selftest_dynamic_test_func>:
>> 00  bti     c
>> 01  mov     x9, x30                            <trace_direct_tramp>:
>> 02  bl      <trace_direct_tramp>    ---------->     ret
>>                                                      |
>>                                          lr/x30 is 03, return to 03
>>                                                      |
>> 03  mov     w0, #0x0   <-----------------------------|
>>      |                                               |
>>      |                   dead loop!                  |
>>      |                                               |
>> 04  ret   ---- lr/x30 is still 03, go back to 03 ----|
>>
>> The reason is that when the direct caller trace_direct_tramp() returns
>> to the patched function trace_selftest_dynamic_test_func(), lr is still
>> the address after the instrumented instruction in the patched function,
>> so when the patched function exits, it returns to itself!
>>
>> To fix this issue, we need to restore lr before trace_direct_tramp()
>> exits, so rewrite a dedicated trace_direct_tramp() for arm64.
> 
> As mentioned on patch 1 I'd prefer we solved this through indirection, which
> would avoid the need for this and would make things more robust generally by
> keeping the unusual calling convention private to the patch-site and regular
> trampoline.
>

IIUC, we still need to restore x30 before returning from the trampoline
even through indirection, so this bug is still there.

> Thanks,
> Mark.
> 
>> Reported-by: Li Huafei <lihuafei1 at huawei.com>
>> Signed-off-by: Xu Kuohai <xukuohai at huawei.com>
>> ---
>>  arch/arm64/include/asm/ftrace.h  | 10 ++++++++++
>>  arch/arm64/kernel/entry-ftrace.S | 10 ++++++++++
>>  kernel/trace/trace_selftest.c    |  2 ++
>>  3 files changed, 22 insertions(+)
>>
>> diff --git a/arch/arm64/include/asm/ftrace.h b/arch/arm64/include/asm/ftrace.h
>> index 14a35a5df0a1..6f6b184e72fb 100644
>> --- a/arch/arm64/include/asm/ftrace.h
>> +++ b/arch/arm64/include/asm/ftrace.h
>> @@ -126,6 +126,16 @@ static inline bool arch_syscall_match_sym_name(const char *sym,
>>  	 */
>>  	return !strcmp(sym + 8, name);
>>  }
>> +
>> +#ifdef CONFIG_FTRACE_SELFTEST
>> +#ifdef CONFIG_DYNAMIC_FTRACE_WITH_DIRECT_CALLS
>> +
>> +#define trace_direct_tramp trace_direct_tramp
>> +extern void trace_direct_tramp(void);
>> +
>> +#endif /* CONFIG_DYNAMIC_FTRACE_WITH_DIRECT_CALLS */
>> +#endif /* CONFIG_FTRACE_SELFTEST */
>> +
>>  #endif /* ifndef __ASSEMBLY__ */
>>  
>>  #endif /* __ASM_FTRACE_H */
>> diff --git a/arch/arm64/kernel/entry-ftrace.S b/arch/arm64/kernel/entry-ftrace.S
>> index dfe62c55e3a2..a47e87d4d3dd 100644
>> --- a/arch/arm64/kernel/entry-ftrace.S
>> +++ b/arch/arm64/kernel/entry-ftrace.S
>> @@ -357,3 +357,13 @@ SYM_CODE_START(return_to_handler)
>>  	ret
>>  SYM_CODE_END(return_to_handler)
>>  #endif /* CONFIG_FUNCTION_GRAPH_TRACER */
>> +
>> +#ifdef CONFIG_FTRACE_SELFTEST
>> +#ifdef CONFIG_DYNAMIC_FTRACE_WITH_DIRECT_CALLS
>> +SYM_FUNC_START(trace_direct_tramp)
>> +	mov x10, x30
>> +	mov x30, x9
>> +	ret x10
>> +SYM_FUNC_END(trace_direct_tramp)
>> +#endif /* CONFIG_DYNAMIC_FTRACE_WITH_DIRECT_CALLS */
>> +#endif /* CONFIG_FTRACE_SELFTEST */
>> diff --git a/kernel/trace/trace_selftest.c b/kernel/trace/trace_selftest.c
>> index abcadbe933bb..e7ccd0d10c39 100644
>> --- a/kernel/trace/trace_selftest.c
>> +++ b/kernel/trace/trace_selftest.c
>> @@ -785,8 +785,10 @@ static struct fgraph_ops fgraph_ops __initdata  = {
>>  };
>>  
>>  #ifdef CONFIG_DYNAMIC_FTRACE_WITH_DIRECT_CALLS
>> +#ifndef trace_direct_tramp
>>  noinline __noclone static void trace_direct_tramp(void) { }
>>  #endif
>> +#endif
>>  
>>  /*
>>   * Pretty much the same than for the function tracer from which the selftest
>> -- 
>> 2.30.2
>>
> .