[DTrace-devel] [PATCH] fbt: implement return value support for fexit-based FBT return probes

[Eugene Loh]

Is there a test?

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 <kris.van.hees at oracle.com>
> ---
>   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)