Alternative handling of tracemem's dynamic size.

The tracemem() action takes a constant size argument that gives the size
of the buffer used.

Optionally, it also takes another size argument that is dynamically
computed.  Currently, in this DTrace port, this dsize is used only by the
consumer, to limit how much data is displayed.  The documentation is
unclear if dsize should also limit how much data is written into the
buffer in the first place.  Arguably, it should:

*)  Copying less data might mean faster run time, although the
    savings would be very small and would incur the cost of extra
    BPF instructions.

*)  Copying less data might protect against accessing memory
    illegally.

Using dsize to cap how much data is written into the buffer is
implemented in this patch.

Since any changes in behavior would not be seen in our testing, no test
changes are made.

Signed-off-by: Eugene Loh <eugene.loh@oracle.com>
Signed-off-by: Kris Van Hees <kris.van.hees@oracle.com>
Reviewed-by: Kris Van Hees <kris.van.hees@oracle.com>

libdtrace/dt_cg.c

@@ -2018,12 +2018,42 @@ dt_cg_act_tracemem(dt_pcb_t *pcb, dt_node_t *dnp, dtrace_actkind_t kind)
 	dt_cg_node(addr, dlp, drp);
 	dt_cg_node(nsiz, dlp, drp);
 
-	off = dt_rec_add(dtp, dt_cg_fill_gap, DTRACEACT_TRACEMEM, nsiz->dn_value, 1, NULL,
-	    dsiz ? DTRACE_TRACEMEM_DYNAMIC : DTRACE_TRACEMEM_STATIC);
+	off = dt_rec_add(dtp, dt_cg_fill_gap, DTRACEACT_TRACEMEM,
+			 nsiz->dn_value, 1, NULL,
+			 dsiz ? DTRACE_TRACEMEM_DYNAMIC
+			      : DTRACE_TRACEMEM_STATIC);
 
-	if (dsiz != NULL)
+	if (dsiz != NULL) {
+		uint_t	Lokay = dt_irlist_label(dlp);
+
+		/* The dt_cg_store_val() call evaluates dsiz. */
 		dt_cg_store_val(pcb, dsiz, DTRACEACT_TRACEMEM, NULL,
-		    dsiz->dn_flags & DT_NF_SIGNED ? DTRACE_TRACEMEM_SSIZE : DTRACE_TRACEMEM_SIZE);
+				dsiz->dn_flags & DT_NF_SIGNED
+					? DTRACE_TRACEMEM_SSIZE
+					: DTRACE_TRACEMEM_SIZE);
+
+		/*
+		 * This is quite naughty.  We know dt_cg_store_val() just
+		 * free'd dsiz->dn_reg and no register allocation took place
+		 * after that, so we know we can re-alloc that register.
+		 */
+		dt_regset_xalloc(drp, dsiz->dn_reg);
+
+		/*
+		 * Decide whether to reduce the size of the data that is
+		 * actually being recorded.  We do this only if the dynamic
+		 * size is non-negative and less than the nsiz.  Note that the
+		 * dsiz->dn_reg value has been sign-extended to 64 bits and we
+		 * will never have an nsiz that is 1<<63 or greater.  So, just
+		 * treat dsiz->dn_reg as an unsigned value.
+		 */
+		emit(dlp,  BPF_BRANCH_REG(BPF_JLE, nsiz->dn_reg, dsiz->dn_reg, Lokay));
+		emit(dlp,  BPF_MOV_REG(nsiz->dn_reg, dsiz->dn_reg));
+		emitl(dlp, Lokay,
+			   BPF_NOP());
+
+		dt_regset_free(drp, dsiz->dn_reg);
+	}
 
 	if (dt_regset_xalloc_args(drp) == -1)
 		longjmp(yypcb->pcb_jmpbuf, EDT_NOREG);
@@ -2037,8 +2067,8 @@ dt_cg_act_tracemem(dt_pcb_t *pcb, dt_node_t *dnp, dtrace_actkind_t kind)
 
 	dt_regset_xalloc(drp, BPF_REG_0);
 	emit(dlp, BPF_CALL_HELPER(BPF_FUNC_probe_read));
-	dt_regset_free_args(drp);
 	dt_regset_free(drp, BPF_REG_0);
+	dt_regset_free_args(drp);
 }
 
 static void