Re: [PATCH] riscv: Fix early ftrace nop patching

Alexandre Ghiti
2024-06-17T13:23:11Z

Hi Conor,

Sorry for the delay here.

On 13/06/2024 09:48, Conor Dooley wrote:
> On Thu, May 23, 2024 at 01:51:34PM +0200, Alexandre Ghiti wrote:
>> Commit c97bf629963e ("riscv: Fix text patching when IPI are used")
>> converted ftrace_make_nop() to use patch_insn_write() which does not
>> emit any icache flush relying entirely on __ftrace_modify_code() to do
>> that.
>>
>> But we missed that ftrace_make_nop() was called very early directly when
>> converting mcount calls into nops (actually on riscv it converts 2B nops
>> emitted by the compiler into 4B nops).
>>
>> This caused crashes on multiple HW as reported by Conor and Björn since
>> the booting core could have half-patched instructions in its icache
>> which would trigger an illegal instruction trap: fix this by emitting a
>> local flush icache when early patching nops.
>>
>> Fixes: c97bf629963e ("riscv: Fix text patching when IPI are used")
>> Signed-off-by: Alexandre Ghiti <alexghiti@rivosinc.com>
>> ---
>> arch/riscv/include/asm/cacheflush.h | 6 ++++++
>> arch/riscv/kernel/ftrace.c | 3 +++
>> 2 files changed, 9 insertions(+)
>>
>> diff --git a/arch/riscv/include/asm/cacheflush.h b/arch/riscv/include/asm/cacheflush.h
>> index dd8d07146116..ce79c558a4c8 100644
>> --- a/arch/riscv/include/asm/cacheflush.h
>> +++ b/arch/riscv/include/asm/cacheflush.h
>> @@ -13,6 +13,12 @@ static inline void local_flush_icache_all(void)
>> asm volatile ("fence.i" ::: "memory");
>> }
>>
>> +static inline void local_flush_icache_range(unsigned long start,
>> + unsigned long end)
>> +{
>> + local_flush_icache_all();
>> +}
>> +
>> #define PG_dcache_clean PG_arch_1
>>
>> static inline void flush_dcache_folio(struct folio *folio)
>> diff --git a/arch/riscv/kernel/ftrace.c b/arch/riscv/kernel/ftrace.c
>> index 4f4987a6d83d..32e7c401dfb4 100644
>> --- a/arch/riscv/kernel/ftrace.c
>> +++ b/arch/riscv/kernel/ftrace.c
>> @@ -120,6 +120,9 @@ int ftrace_init_nop(struct module *mod, struct dyn_ftrace *rec)
>> out = ftrace_make_nop(mod, rec, MCOUNT_ADDR);
>> mutex_unlock(&text_mutex);
> So, turns out that this patch is not sufficient. I've seen some more
> crashes, seemingly due to initialising nops on this mutex_unlock().
> Palmer suggested moving the if (!mod) ... into the lock, which fixed
> the problem for me.

Ok, it makes sense, I completely missed that. I'll send a fix for that
shortly so that it can be merged in rc5.

Thanks,

Alex

>
>>
>> + if (!mod)
>> + local_flush_icache_range(rec->ip, rec->ip + MCOUNT_INSN_SIZE);
>> +
>> return out;
>> }
>>
>> --
>> 2.39.2
>>
>>
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