Re: [PATCH 4/4] fbt: do not provide untraceable functins (for fprobes)

On 14 Apr 2026, Kris Van Hees uttered the following:

> Subject: Re: [PATCH 4/4] fbt: do not provide untraceable functins (for fprobes)

Typo.

> Ensure that we do not provide FBT probes for functions that cannot be
> probed using BPF fentry/fexit probes if fprobes are the default.

I guess falling back to kprobes for those single functions alone is
either too complicated or has unpleasantly visible side-effects?

> Signed-off-by: Kris Van Hees <[email protected]>
> ---
>  libdtrace/dt_prov_fbt.c | 47 ++++++++++++++++++++++++++---------------
>  1 file changed, 30 insertions(+), 17 deletions(-)
>
> diff --git a/libdtrace/dt_prov_fbt.c b/libdtrace/dt_prov_fbt.c
> index 59e4583b2..a2f20c9bc 100644
> --- a/libdtrace/dt_prov_fbt.c
> +++ b/libdtrace/dt_prov_fbt.c
> @@ -86,14 +86,30 @@ static int populate(dtrace_hdl_t *dtp)
>  }
>  
>  /* Create a probe (if it does not exist yet). */
> -static int provide_probe(dtrace_hdl_t *dtp, const dtrace_probedesc_t *pdp)
> +static int provide_probe(dtrace_hdl_t *dtp, dt_module_t *dmp,
> +			 dtrace_probedesc_t *pdp)
>  {
>  	dt_provider_t	*prv = dt_provider_lookup(dtp, pdp->prv);
>  
>  	if (prv == NULL)
>  		return 0;
> +	pdp->mod = dmp->dm_name;
>  	if (dt_probe_lookup(dtp, pdp) != NULL)
>  		return 0;
> +
> +	/*
> +	 * The BPF verifier does not allow fentry/fexit probing of variadic
> +	 * functions.

This comment is wrong now that this is being implemented via another
function: the fact that dt_btf_func_is_traceable() checks variadicity
belongs above that function (and is indeed there).  Something like

/*
 * Do not provide probes for functions with BTF that indicates that they
 * are not traceable.
 */

maybe?

> +	 */
> +	if (prv->impl->prog_type == BPF_PROG_TYPE_TRACING) {
> +		int32_t	btf_id;
> +
> +		btf_id = dt_btf_lookup_name_kind(dtp, dmp, pdp->fun, BTF_KIND_FUNC);
> +		if (btf_id <= 0 ||
> +		    !dt_btf_func_is_traceable(dtp, dmp->dm_btf, btf_id))
> +			return -1;
> +	}
> +
>  	if (dt_tp_probe_insert(dtp, prv, pdp->prv, pdp->mod, pdp->fun, pdp->prb))
>  		return 1;
>  

The hunks below appear to be doing something not called out in the
commit log. I think this is getting the symbol module right?

> @@ -173,16 +189,15 @@ static int provide(dtrace_hdl_t *dtp, const dtrace_probedesc_t *pdp)
>  
>  			pd.id = DTRACE_IDNONE;
>  			pd.prv = pdp->prv;
> -			pd.mod = dmp->dm_name;
>  			pd.fun = pdp->fun;
>  
>  			if (prb & FBT_ENTRY) {
>  				pd.prb = "entry";
> -				n += provide_probe(dtp, &pd);
> +				n += provide_probe(dtp, dmp, &pd);
>  			}
>  			if (prb & FBT_RETURN) {
>  				pd.prb = "return";
> -				n += provide_probe(dtp, &pd);
> +				n += provide_probe(dtp, dmp, &pd);
>  			}
>  
>  			return n;
> @@ -190,22 +205,20 @@ static int provide(dtrace_hdl_t *dtp, const dtrace_probedesc_t *pdp)
>  
>  		sym = dt_symbol_by_name(dtp, pdp->fun);
>  		while (sym != NULL) {
> -			const char	*mod = dt_symbol_module(sym)->dm_name;
> -
> +			dmp = dt_symbol_module(sym);
>  			if (dt_symbol_traceable(sym) &&
> -			    dt_gmatch(mod, pdp->mod)) {
> +			    dt_gmatch(dmp->dm_name, pdp->mod)) {
>  				pd.id = DTRACE_IDNONE;
>  				pd.prv = pdp->prv;
> -				pd.mod = mod;
>  				pd.fun = pdp->fun;
>  
>  				if (prb & FBT_ENTRY) {
>  					pd.prb = "entry";
> -					n += provide_probe(dtp, &pd);
> +					n += provide_probe(dtp, dmp, &pd);
>  				}
>  				if (prb & FBT_RETURN) {
>  					pd.prb = "return";
> -					n += provide_probe(dtp, &pd);
> +					n += provide_probe(dtp, dmp, &pd);
>  				}
>  
>  			}
> @@ -242,16 +255,15 @@ static int provide(dtrace_hdl_t *dtp, const dtrace_probedesc_t *pdp)
>  
>  		pd.id = DTRACE_IDNONE;
>  		pd.prv = pdp->prv;
> -		pd.mod = smp->dm_name;
>  		pd.fun = fun;
>  
>  		if (prb & FBT_ENTRY) {
>  			pd.prb = "entry";
> -			n += provide_probe(dtp, &pd);
> +			n += provide_probe(dtp, smp, &pd);
>  		}
>  		if (prb & FBT_RETURN) {
>  			pd.prb = "return";
> -			n += provide_probe(dtp, &pd);
> +			n += provide_probe(dtp, smp, &pd);
>  		}
>  	}
>  

The need for this trampoline change is also rather opaque.

> @@ -323,10 +335,11 @@ static int fprobe_trampoline(dt_pcb_t *pcb, uint_t exitlbl)
>  		 */
>  		dmp = dt_module_lookup_by_name(dtp, prp->desc->mod);
>  		if (dmp && prp->argc == 2) {
> -			int32_t	btf_id = dt_tp_probe_get_id(prp);
> -			int	i = dt_btf_func_argc(dtp, dmp->dm_btf, btf_id);
> -
> -			emit(dlp, BPF_LOAD(BPF_DW, BPF_REG_0, BPF_REG_8, i * 8));
> +			emit(dlp, BPF_MOV_REG(BPF_REG_1, BPF_REG_8));
> +			emit(dlp, BPF_MOV_REG(BPF_REG_2, BPF_REG_FP));
> +			emit(dlp, BPF_ALU64_IMM(BPF_ADD, BPF_REG_2, DT_TRAMP_SP_SLOT(0)));
> +			emit(dlp, BPF_CALL_HELPER(dtp->dt_bpfhelper[BPF_FUNC_get_func_ret]));
> +			emit(dlp, BPF_LOAD(BPF_DW, BPF_REG_0, BPF_REG_FP, DT_TRAMP_SP_SLOT(0)));
>  			emit(dlp, BPF_STORE(BPF_DW, BPF_REG_7, DMST_ARG(1), BPF_REG_0));
>  		}
>  	}

-- 
NULL && (void)