[DTrace-devel] [PATCH 6/7] Populate sdt probe argument data based on argument types

[Eugene Loh]

Reviewed-by: Eugene Loh <eugene.loh at oracle.com>
with earlier comments about renaming dt_cg_load() and moving 
tst.args-match-argN.d to patch 5 adding a trigger.
I am curious about one thing...

On 5/26/22 11:24, Kris Van Hees via DTrace-devel wrote:
> diff --git a/libdtrace/dt_prov_sdt.c b/libdtrace/dt_prov_sdt.c
> @@ -143,17 +145,41 @@ static void trampoline(dt_pcb_t *pcb)
>   	dt_cg_tramp_clear_regs(pcb);
>   
>   	/*
> -	 *	for (i = 0; i < argc; i++)
> -	 *		dctx->mst->argv[i] = ((uint64_t *)ctx)[i + 1];
> -	 *				//     (first value is private)
> -	 *				// lddw %r0, [%r8 + (i + 1) * 8]
> -	 *				// stdw [%r7 + DMST_ARG(i)], 0
> +	 * Decode the arguments of the underlying tracepoint (from the context)
> +	 * and store them in the cached argument list in the mstate.
> +	 *
> +	 * Skip the first argument (a private pointer that we are not allowed
> +	 * to access from BPF).
What is being skipped?  The loop starts at i=0.
>   	 */
> -	for (i = 0; i < pcb->pcb_pinfo.dtp_argc; i++) {
> -		emit(dlp, BPF_LOAD(BPF_DW, BPF_REG_0, BPF_REG_8, (i + 1) * 8));
> -		emit(dlp, BPF_STORE(BPF_DW, BPF_REG_7, DMST_ARG(i), BPF_REG_0));
> +	off = sizeof(uint64_t);
> +	for (i = 0; i < prp->argc; i++) {
> +		dt_node_t	*anp = prp->xargv[i];
> +		ssize_t		size = ctf_type_size(anp->dn_ctfp,
> +						     anp->dn_type);
> +		ssize_t		align = ctf_type_align(anp->dn_ctfp,
> +						       anp->dn_type);
> +
> +		off = P2ROUNDUP(off, align);
> +		if (dt_node_is_scalar(anp)) {
> +			uint_t	ldsz = dt_cg_load(anp, anp->dn_ctfp,
> +						  anp->dn_type, NULL);
> +
> +			emit(dlp, BPF_LOAD(ldsz, BPF_REG_0, BPF_REG_8, off));
> +			emit(dlp, BPF_STORE(BPF_DW, BPF_REG_7, DMST_ARG(i), BPF_REG_0));
> +		} else {
> +			emit(dlp, BPF_MOV_REG(BPF_REG_0, BPF_REG_8));
> +			emit(dlp, BPF_ALU64_IMM(BPF_ADD, BPF_REG_0, off));
> +			emit(dlp, BPF_STORE(BPF_DW, BPF_REG_7, DMST_ARG(i), BPF_REG_0));
> +		}
> +		off += size;
>   	}
> + */
> +
> +#pragma D option quiet
> +
> +write:entry {
> +	printf("%x vs %x\n", args[0], arg0);
> +	printf("%x vs %x\n", (uint64_t)args[1], arg1);
> +	printf("%x vs %x\n", args[2], arg2);
> +	exit(args[0] == arg0 && (uint64_t)args[1] == arg1 && args[2] == arg2 ? 0 : 1);
> +}
> +
> +ERROR {
> +	exit(1);
> +}