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

[Conor Dooley <conor.dooley@microchip.com>]

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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.

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

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