linux-kernel - Re: [PATCH] tracing/eprobe: Replace kzalloc with kmalloc

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Date:   Sat, 7 Jan 2023 09:42:40 +0100
From:   Christophe JAILLET <christophe.jaillet@...adoo.fr>
To:     Quanfa Fu <quanfafu@...il.com>, rostedt@...dmis.org,
        mhiramat@...nel.org
Cc:     linux-kernel@...r.kernel.org, linux-trace-kernel@...r.kernel.org
Subject: Re: [PATCH] tracing/eprobe: Replace kzalloc with kmalloc

Le 07/01/2023 à 04:45, Quanfa Fu a écrit :
> Since this memory will be filled soon below, I feel that there is
> no need for a memory of all zeros here. 'snprintf' does not return
> negative num according to ISO C99, so I feel that no judgment is
> needed here.
> 
> No functional change intended.
> 
> Signed-off-by: Quanfa Fu <quanfafu@...il.com>
> ---
>   kernel/trace/trace_eprobe.c | 4 +---
>   1 file changed, 1 insertion(+), 3 deletions(-)
> 
> diff --git a/kernel/trace/trace_eprobe.c b/kernel/trace/trace_eprobe.c
> index 352b65e2b910..cd1d271a74e7 100644
> --- a/kernel/trace/trace_eprobe.c
> +++ b/kernel/trace/trace_eprobe.c
> @@ -917,15 +917,13 @@ static int trace_eprobe_parse_filter(struct trace_eprobe *ep, int argc, const ch
>   	for (i = 0; i < argc; i++)
>   		len += strlen(argv[i]) + 1;
>   
> -	ep->filter_str = kzalloc(len, GFP_KERNEL);
> +	ep->filter_str = kmalloc(len, GFP_KERNEL);
>   	if (!ep->filter_str)
>   		return -ENOMEM;
>   
>   	p = ep->filter_str;
>   	for (i = 0; i < argc; i++) {
>   		ret = snprintf(p, len, "%s ", argv[i]);
> -		if (ret < 0)
> -			goto error;
>   		if (ret > len) {

Hi,

as per [1]:
  * The return value is the number of characters which would be
  * generated for the given input, excluding the trailing null,
  * as per ISO C99.  If the return is greater than *or equal* to
  * @size, the resulting string is truncated.

So, should this test be:
     if (ret >= len)
            ~~~~


Also, isn't the p[-1] = '\0' after the loop eating the last character?
    argc = 1;
    argv[0] = "a";

    Before the loop:
    ===============
    len = 1 + 1 = 2;
    ep->filter_str = 0x00 0x00
                     ^
                     |___ p

    After the loop:
    ===============
    ep->filter_str = 0x61 0x00
                          ^
                          |___ p
    len = 1;

    After p[-1]:
    ============
    ep->filter_str = 0x00 0x00
                       ~~ ^
                          |___ p

Did I miss something obvious?
I don't know the intent here, or if it is an issue at all, but it looks odd.

CJ


[1]: https://elixir.bootlin.com/linux/v6.2-rc1/source/lib/vsprintf.c#L2925
>   			ret = -E2BIG;
>   			goto error;