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Yes, the testsuite already contains a test (tst.return1.d) whose failure prompted the need for this patch. And with the patch, it passes. > > On 8/1/24 01:19, Kris Van Hees wrote: > > Commit fe2101e5 ("fbt: implement based on fentry/fexit probes") failed > > to provide a proper implementation to pass the function return value as > > arg1 for FBT return probes based on fexit probes. > > > > Signed-off-by: Kris Van Hees > > --- > > libdtrace/dt_prov_fbt.c | 20 +++++++++++++++++++- > > 1 file changed, 19 insertions(+), 1 deletion(-) > > > > diff --git a/libdtrace/dt_prov_fbt.c b/libdtrace/dt_prov_fbt.c > > index fa888ed8..99b42586 100644 > > --- a/libdtrace/dt_prov_fbt.c > > +++ b/libdtrace/dt_prov_fbt.c > > @@ -172,6 +172,7 @@ static int populate(dtrace_hdl_t *dtp) > > */ > > static int fprobe_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; > > @@ -185,6 +186,8 @@ static int fprobe_trampoline(dt_pcb_t *pcb, uint_t exitlbl) > > emit(dlp, BPF_STORE(BPF_DW, BPF_REG_7, DMST_ARG(i), BPF_REG_0)); > > } > > } else { > > + dt_module_t *dmp; > > + > > /* > > * fbt:::return arg0 should be the function offset for the > > * return instruction. The fexit prpbe fires at a point where > > @@ -194,6 +197,21 @@ static int fprobe_trampoline(dt_pcb_t *pcb, uint_t exitlbl) > > */ > > dt_cg_xsetx(dlp, NULL, DT_LBL_NONE, BPF_REG_0, -1); > > emit(dlp, BPF_STORE(BPF_DW, BPF_REG_7, DMST_ARG(0), BPF_REG_0)); > > + > > + /* > > + * The return value is provided by the fexit probe as an > > + * argument slot past the last function argument. We can get > > + * the number of function arguments using the BTF id that has > > + * been stored as the tracepoint event id. > > + */ > > + dmp = dt_module_lookup_by_name(dtp, prp->desc->mod); > > + if (dmp != NULL) { > > + int32_t btf_id = dt_tp_get_event_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_STORE(BPF_DW, BPF_REG_7, DMST_ARG(1), BPF_REG_0)); > > + } > > } > > dt_cg_tramp_epilogue(pcb); > > @@ -274,7 +292,7 @@ static int fprobe_prog_load(dtrace_hdl_t *dtp, const dt_probe_t *prp, > > dt_module_t *dmp; > > atype = strcmp(desc->prb, "entry") == 0 ? BPF_TRACE_FENTRY > > - : BPF_TRACE_FEXIT; > > + : BPF_TRACE_FEXIT; > > dmp = dt_module_lookup_by_name(dtp, desc->mod); > > if (dmp == NULL)