netdev - Re: [PATCH iproute2-next] bpf: fix snprintf truncation warning

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Message-ID: <83242eb4-6304-0fcf-2d2a-6ef4de464e81@gmail.com>
Date:   Wed, 4 Sep 2019 16:15:39 -0600
From:   David Ahern <dsahern@...il.com>
To:     Andrea Claudi <aclaudi@...hat.com>, netdev@...r.kernel.org
Cc:     stephen@...workplumber.org, dsahern@...nel.org
Subject: Re: [PATCH iproute2-next] bpf: fix snprintf truncation warning

On 9/4/19 9:50 AM, Andrea Claudi wrote:
> gcc v9.2.1 produces the following warning compiling iproute2:
> 
> bpf.c: In function ‘bpf_get_work_dir’:
> bpf.c:784:49: warning: ‘snprintf’ output may be truncated before the last format character [-Wformat-truncation=]
>   784 |  snprintf(bpf_wrk_dir, sizeof(bpf_wrk_dir), "%s/", mnt);
>       |                                                 ^
> bpf.c:784:2: note: ‘snprintf’ output between 2 and 4097 bytes into a destination of size 4096
>   784 |  snprintf(bpf_wrk_dir, sizeof(bpf_wrk_dir), "%s/", mnt);
>       |  ^~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
> 
> Fix it extending bpf_wrk_dir size by 1 byte for the extra "/" char.
> 
> Signed-off-by: Andrea Claudi <aclaudi@...hat.com>
> ---
>  lib/bpf.c | 2 +-
>  1 file changed, 1 insertion(+), 1 deletion(-)
> 
> diff --git a/lib/bpf.c b/lib/bpf.c
> index 7d2a322ffbaec..95de7894a93ce 100644
> --- a/lib/bpf.c
> +++ b/lib/bpf.c
> @@ -742,7 +742,7 @@ static int bpf_gen_hierarchy(const char *base)
>  static const char *bpf_get_work_dir(enum bpf_prog_type type)
>  {
>  	static char bpf_tmp[PATH_MAX] = BPF_DIR_MNT;
> -	static char bpf_wrk_dir[PATH_MAX];
> +	static char bpf_wrk_dir[PATH_MAX + 1];
>  	static const char *mnt;
>  	static bool bpf_mnt_cached;
>  	const char *mnt_env = getenv(BPF_ENV_MNT);
> 

PATH_MAX is meant to be the max length for a filesystem path including
the null terminator, so I think it would be better to change the
snprintf to 'sizeof(bpf_wrk_dir) - 1'.