Re: [PATCH bpf v2] ftrace: fix incorrect hash size in register_ftrace_direct()

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On Sun, 13 Apr 2025 09:44:44 +0800
Menglong Dong <menglong8.dong@xxxxxxxxx> wrote:

> The maximum of the ftrace hash bits is made fls(32) in
> register_ftrace_direct(), which seems illogical. So, we fix it by making
> the max hash bits FTRACE_HASH_MAX_BITS instead.
> 

Loogs good to me.

Acked-by: Masami Hiramatsu (Google) <mhiramat@xxxxxxxxxx>

Thanks!

> Fixes: d05cb470663a ("ftrace: Fix modification of direct_function hash while in use")
> Signed-off-by: Menglong Dong <dongml2@xxxxxxxxxxxxxxx>
> ---
> v2:
> - thanks for Steven's advice, we fix the problem by making the max hash
>   bits FTRACE_HASH_MAX_BITS instead.
> ---
>  kernel/trace/ftrace.c | 7 ++++---
>  1 file changed, 4 insertions(+), 3 deletions(-)
> 
> diff --git a/kernel/trace/ftrace.c b/kernel/trace/ftrace.c
> index 1a48aedb5255..d153ad13e0e0 100644
> --- a/kernel/trace/ftrace.c
> +++ b/kernel/trace/ftrace.c
> @@ -5914,9 +5914,10 @@ int register_ftrace_direct(struct ftrace_ops *ops, unsigned long addr)
>  
>  	/* Make a copy hash to place the new and the old entries in */
>  	size = hash->count + direct_functions->count;
> -	if (size > 32)
> -		size = 32;
> -	new_hash = alloc_ftrace_hash(fls(size));
> +	size = fls(size);
> +	if (size > FTRACE_HASH_MAX_BITS)
> +		size = FTRACE_HASH_MAX_BITS;
> +	new_hash = alloc_ftrace_hash(size);
>  	if (!new_hash)
>  		goto out_unlock;
>  
> -- 
> 2.39.5
> 
> 


-- 
Masami Hiramatsu (Google) <mhiramat@xxxxxxxxxx>