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@xxxxxxxxxx>
>> Signed-off-by: Xu Kuohai <xukuohai@xxxxxxxxxx>
>> ---
>> 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
>>
> .
