[PATCH] riscv: ftrace: Properly acquire text_mutex to fix a race condition

[Palmer Dabbelt]

On Tue, 08 Jul 2025 01:33:48 PDT (-0700), alexghiti at rivosinc.com wrote:
> As reported by lockdep, some patching was done without acquiring
> text_mutex, so there could be a race when mapping the page to patch
> since we use the same fixmap entry.
>
> Reported-by: Han Gao <rabenda.cn at gmail.com>
> Reported-by: Vivian Wang <wangruikang at iscas.ac.cn>
> Reported-by: Yao Zi <ziyao at disroot.org>
> Closes: https://lore.kernel.org/linux-riscv/aGODMpq7TGINddzM@pie.lan/
> Tested-by: Yao Zi <ziyao at disroot.org>
> Signed-off-by: Alexandre Ghiti <alexghiti at rivosinc.com>
> ---
>  arch/riscv/kernel/ftrace.c | 16 ++++++++++++----
>  1 file changed, 12 insertions(+), 4 deletions(-)
>
> diff --git a/arch/riscv/kernel/ftrace.c b/arch/riscv/kernel/ftrace.c
> index 4c6c24380cfd9d6c51f0e4340cd674160b83a610..22e7bdf8de2b6ca950cf2c8b734bc82ea46ba8bf 100644
> --- a/arch/riscv/kernel/ftrace.c
> +++ b/arch/riscv/kernel/ftrace.c
> @@ -14,6 +14,16 @@
>  #include <asm/text-patching.h>
>
>  #ifdef CONFIG_DYNAMIC_FTRACE
> +void ftrace_arch_code_modify_prepare(void)
> +{
> +	mutex_lock(&text_mutex);
> +}
> +
> +void ftrace_arch_code_modify_post_process(void)
> +{
> +	mutex_unlock(&text_mutex);
> +}

IIRC there's a reason we don't do it this way, we had (or had tried to) 
have it before.  It's been a while, though, and I'm just having some a 
coffee so may I'm just wrong...

>  unsigned long ftrace_call_adjust(unsigned long addr)
>  {
>  	if (IS_ENABLED(CONFIG_DYNAMIC_FTRACE_WITH_CALL_OPS))
> @@ -29,10 +39,8 @@ unsigned long arch_ftrace_get_symaddr(unsigned long fentry_ip)
>
>  void arch_ftrace_update_code(int command)
>  {
> -	mutex_lock(&text_mutex);
>  	command |= FTRACE_MAY_SLEEP;
>  	ftrace_modify_all_code(command);
> -	mutex_unlock(&text_mutex);
>  	flush_icache_all();
>  }
>
> @@ -149,6 +157,8 @@ int ftrace_init_nop(struct module *mod, struct dyn_ftrace *rec)
>  	unsigned int nops[2], offset;
>  	int ret;
>
> +	guard(mutex)(&text_mutex);
> +
>  	ret = ftrace_rec_set_nop_ops(rec);
>  	if (ret)
>  		return ret;
> @@ -157,9 +167,7 @@ int ftrace_init_nop(struct module *mod, struct dyn_ftrace *rec)
>  	nops[0] = to_auipc_t0(offset);
>  	nops[1] = RISCV_INSN_NOP4;
>
> -	mutex_lock(&text_mutex);
>  	ret = patch_insn_write((void *)pc, nops, 2 * MCOUNT_INSN_SIZE);
> -	mutex_unlock(&text_mutex);
>
>  	return ret;
>  }
>
> ---
> base-commit: d7b8f8e20813f0179d8ef519541a3527e7661d3a
> change-id: 20250708-alex-fixes-1e719b9899f3
>
> Best regards,