Re: [DTrace-devel] [PATCH v2 6/7] tcp: support non-libctf environments

Reviewed-by: Eugene Loh <[email protected]>

On 11/18/25 16:08, Kris Van Hees via DTrace-devel wrote:
> Commit 0fa93b3ac ("tcp: new provider") and commit c08a5fcf2
> ("tcp provider: support tcp:::accept-established in absence of skb") both
> introduced code that uses ctf_func_type_info() without a pre-processor
> conditional to also support the case when libctf is not present on the
> system.
>
> When libctf is not present, it is assumed that the system is quite old
> and therefore is running an older kernel.  Use of libdtrace-ctf (which
> has been deprecated in favour of libctf) on systems with recent kernels
> is not supported and may lead to unexpected results.
>
> Signed-off-by: Kris Van Hees <[email protected]>
> ---
>   libdtrace/dt_prov_tcp.c | 27 ++++++++++++++++-----------
>   1 file changed, 16 insertions(+), 11 deletions(-)
>
> diff --git a/libdtrace/dt_prov_tcp.c b/libdtrace/dt_prov_tcp.c
> index 44ed2bc9..c3d5553e 100644
> --- a/libdtrace/dt_prov_tcp.c
> +++ b/libdtrace/dt_prov_tcp.c
> @@ -160,17 +160,19 @@ static int populate(dtrace_hdl_t *dtp)
>    */
>   static int trampoline(dt_pcb_t *pcb, uint_t exitlbl)
>   {
> -	dtrace_hdl_t	*dtp = pcb->pcb_hdl;
>   	dt_irlist_t	*dlp = &pcb->pcb_ir;
>   	dt_probe_t	*prp = pcb->pcb_probe;
>   	dt_probe_t	*uprp = pcb->pcb_parent_probe;
>   	int		direction, have_iphdr;
>   	int		skarg = 0, skbarg = 1, tcparg = 0;
> -	int		skarg_maybe_null, have_skb = 1;
> +	int		skarg_maybe_null = 0, have_skb = 1;
>   	int		skstate = 0;
> +#ifdef HAVE_LIBCTF
> +	dtrace_hdl_t	*dtp = pcb->pcb_hdl;
>   	dtrace_typeinfo_t sym;
>   	ctf_funcinfo_t	fi;
>   	int		rc;
> +#endif
>   
>   	/*
>   	 * We construct the tcp::: probe arguments as follows:
> @@ -262,6 +264,7 @@ static int trampoline(dt_pcb_t *pcb, uint_t exitlbl)
>   
>   	if (strcmp(prp->desc->prb, "accept-established") == 0) {
>   		direction = NET_PROBE_INBOUND;
> +#ifdef HAVE_LIBCTF
>   		have_iphdr = 1;
>   		/* on older (5.4) kernels, tcp_init_transfer() only has 2
>   		 * args, i.e. no struct skb * third argument.
> @@ -274,11 +277,14 @@ static int trampoline(dt_pcb_t *pcb, uint_t exitlbl)
>   		    fi.ctc_argc > 2) {
>   			/* skb in arg2 not arg1 */
>   			skbarg = 2;
> -			skarg_maybe_null = 0;
>   		} else {
>   			have_skb = 0;
>   			have_iphdr = 0;
>   		}
> +#else
> +		have_skb = 0;
> +		have_iphdr = 0;
> +#endif
>   		/* ensure arg1 is BPF_SOCK_OPS_PASSIVE_ESTABLISHED_CB */
>   		emit(dlp, BPF_LOAD(BPF_DW, BPF_REG_6, BPF_REG_7, DMST_ARG(1)));
>   		emit(dlp, BPF_BRANCH_IMM(BPF_JNE, BPF_REG_6,
> @@ -291,20 +297,17 @@ static int trampoline(dt_pcb_t *pcb, uint_t exitlbl)
>   		if (strcmp(uprp->desc->fun, "tcp_v4_send_reset") == 0 ||
>   		    strcmp(uprp->desc->fun, "tcp_v6_send_reset") == 0)
>   			skarg_maybe_null = 1;
> -		else
> -			skarg_maybe_null = 0;
>   	} else if (strcmp(prp->desc->prb, "connect-established") == 0) {
>   		direction = NET_PROBE_INBOUND;
>   		have_iphdr = 1;
> -		skarg_maybe_null = 0;
>   	} else if (strcmp(prp->desc->prb, "connect-refused") == 0) {
>   		direction = NET_PROBE_INBOUND;
>   		have_iphdr = 1;
> -		skarg_maybe_null = 0;
>   		skstate = BPF_TCP_SYN_SENT;
>   	} else {
>   		direction = NET_PROBE_OUTBOUND;
>   		if (strcmp(uprp->desc->fun, "ip_send_unicast_reply") == 0) {
> +#ifdef HAVE_LIBCTF
>   			/* Newer kernels pass the original socket as second
>   			 * arg to ip_send_unicast_reply(); if that function
>   			 * has an extra (> 9) argument we know we have to
> @@ -329,6 +332,11 @@ static int trampoline(dt_pcb_t *pcb, uint_t exitlbl)
>   				skbarg = 1;
>   				tcparg = 5;
>   			}
> +#else
> +			skarg = 0;
> +			skbarg = 1;
> +			tcparg = 5;
> +#endif
>   			have_iphdr = 1;
>   			tcparg = 6;
>   			skarg_maybe_null = 1;
> @@ -340,11 +348,8 @@ static int trampoline(dt_pcb_t *pcb, uint_t exitlbl)
>   		} else if (strcmp(prp->desc->prb, "connect-request") == 0) {
>   			skstate = BPF_TCP_SYN_SENT;
>   			have_iphdr = 0;
> -			skarg_maybe_null = 0;
> -		} else {
> +		} else
>   			have_iphdr = 0;
> -			skarg_maybe_null = 0;
> -		}
>   	}
>   
>   	/* first save sk to args[3]; this avoids overwriting it when we