linux-kernel - Re: [PATCH] bpf: btf: Fix vsnprintf return value check

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Date:   Mon, 11 Jul 2022 23:42:59 +0200
From:   Jiri Olsa <olsajiri@...il.com>
To:     Fedor Tokarev <ftokarev@...il.com>
Cc:     Martin KaFai Lau <martin.lau@...ux.dev>, bpf@...r.kernel.org,
        linux-kernel@...r.kernel.org
Subject: Re: [PATCH] bpf: btf: Fix vsnprintf return value check

On Mon, Jul 11, 2022 at 11:13:17PM +0200, Fedor Tokarev wrote:
> vsnprintf returns the number of characters which would have been written if
> enough space had been available, excluding the terminating null byte. Thus,
> the return value of 'len_left' means that the last character has been
> dropped.

should we have test for this in progs/test_snprintf.c ?

jirka

> 
> Signed-off-by: Fedor Tokarev <ftokarev@...il.com>
> ---
>  kernel/bpf/btf.c | 2 +-
>  1 file changed, 1 insertion(+), 1 deletion(-)
> 
> diff --git a/kernel/bpf/btf.c b/kernel/bpf/btf.c
> index eb12d4f705cc..a9c1c98017d4 100644
> --- a/kernel/bpf/btf.c
> +++ b/kernel/bpf/btf.c
> @@ -6519,7 +6519,7 @@ static void btf_snprintf_show(struct btf_show *show, const char *fmt,
>  	if (len < 0) {
>  		ssnprintf->len_left = 0;
>  		ssnprintf->len = len;
> -	} else if (len > ssnprintf->len_left) {
> +	} else if (len >= ssnprintf->len_left) {
>  		/* no space, drive on to get length we would have written */
>  		ssnprintf->len_left = 0;
>  		ssnprintf->len += len;
> -- 
> 2.25.1
>