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And with the patch, it passes. > > Thanks.  So the earlier culprit/insufficient patch (fe2101e5) went out in > the last release with this test... failing?  Not tested (on fentry systems, > where it makes a difference)?  A known failure in the last release?  (I > forget.) Well, against all odds it actually "works" on older kernels with the quite insufficient patch because by total coincidence the right value happens to be present in the correct arg slot. > > > 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 > > Might as well s/prpbe/probe/ while we're here. Good catch. > > > > @@ -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)); > > > > + } > > I assume we don't worry about the handling of dmp==NULL because it's > "unlikely" (already been checked) and there's nothing more sensible to do > anyhow?  Or load arg1 with 0xdeadbeef or something? Unpredictable result is probably the best we can do right now. Any deliberatea value isn't really any better. We need to look into a future solution to provide proper error reporting during the trampoline creation. > > > > } > > > > 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)