[DTrace-devel] [PATCH] Ensure that strlen() returns a value >= 0

[Eugene Loh]

Reviewed-by: Eugene Loh <eugene.loh at oracle.com>

On 4/8/22 9:53 AM, Kris Van Hees via DTrace-devel wrote:
> Signed-off-by: Kris Van Hees<kris.van.hees at oracle.com>
> ---
>   bpf/strlen.c | 11 +++++++++--
>   1 file changed, 9 insertions(+), 2 deletions(-)
>
> diff --git a/bpf/strlen.c b/bpf/strlen.c
> index 71ddbca4..d39955a2 100644
> --- a/bpf/strlen.c
> +++ b/bpf/strlen.c
> @@ -20,8 +20,15 @@ noinline uint64_t dt_strlen(const dt_dctx_t *dctx, const char *str)
>   	char	*tmp = dctx->strtab + (uint64_t)&STBSZ;
>   	int64_t	len;
>   
> -	len = bpf_probe_read_str(tmp, (uint64_t)&STRSZ + 1, str);
> +	/*
> +	 * The bpf_probe_read_str() helper returns either a negative value (for
> +	 * error conditions) or a positive value (string length + 1 to account
> +	 * for the terminating 0-byte).  It will never return 0, so it is safe
> +	 * to speculatively subtract 1.  Any negative value will be converted
> +	 * into a 0.
> +	 */
> +	len = bpf_probe_read_str(tmp, (uint64_t)&STRSZ + 1, str) - 1;
>   	set_not_neg_bound(len);
>   
> -	return len - 1;		/* bpf_probe_read_str() never returns 0 */
> +	return len;
>   }
> -- 2.34.1 _______________________________________________ DTrace-devel 
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