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

[Kris Van Hees]

On Fri, May 27, 2022 at 10:43:46AM -0700, Eugene Loh via DTrace-devel wrote:
> 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.

See below...

> >   	 */
> > -	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);

... this skips the first argument that we get from the tracepoint.

> > +	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);
> > +}
> 
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