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

[Kris Van Hees]

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

Signed-off-by: Kris Van Hees <kris.van.hees at oracle.com>
---
 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.
+	 */
+	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;
 
@@ -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);
 		}
 	}
 
@@ -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));
 		}
 	}
-- 
2.53.0