Add support for built-in variable stackdepth

Add space to the "mem" BPF map where we can write a stack of maximum
depth.  (This approach should be replaced eventually with a more general
mechanism for using scratch space.)

Since precompiled BPF code needs to verify the use of the new buffer, we
we make its offset and size available to get_bvar() via STKOFF and STKSIZ.

For DIF_VAR_STACKDEPTH, add code to write the stack and return its length.

Fix test/unittest/stackdepth/tst.value.d:

  *)  Use a probe that has a kernel stack.

  *)  Change the postprocessing to check properly whether a line is empty.

  *)  Remove xfail.

Consolidate stackdepth tests with other built-in-variable tests:

  *)  stackdepth/tst.default.d was already simply a subset of
      variables/bvar/tst.stackdepth.d and so can be deleted.

  *)  stackdepth/tst.value.* is moved to variables/bvar/tst.stackdepth2.*

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

bpf/get_bvar.c

@@ -25,6 +25,8 @@ extern struct bpf_map_def probes;
 extern struct bpf_map_def state;
 
 extern uint64_t STBSZ;
+extern uint64_t STKOFF;
+extern uint64_t STKSIZ;
 
 #define error(dctx, fault, illval) \
 	({ \
@@ -54,8 +56,26 @@ noinline uint64_t dt_get_bvar(dt_dctx_t *dctx, uint32_t id)
 	case DIF_VAR_ARG9:
 		return mst->argv[id - DIF_VAR_ARG0];
 	case DIF_VAR_STACKDEPTH: {
-		/* FIXME: no stack() yet */
-		return 0;
+		uint32_t bufsiz = (uint32_t) (uint64_t) (&STKSIZ);
+		uint64_t flags = 0 & BPF_F_SKIP_FIELD_MASK;
+		char *buf = ((char *) dctx->buf) + ((uint64_t) &STKOFF);
+		uint64_t stacksize;
+
+		stacksize = bpf_get_stack(dctx->ctx, buf, bufsiz, flags);
+		if (stacksize < 0)
+			return error(dctx, DTRACEFLT_BADSTACK, 0 /* FIXME */);
+
+		/*
+		 * While linux/bpf.h does not describe the meaning of
+		 * bpf_get_stack()'s return value outside of its sign,
+		 * it is presumably the length of the copied stack.
+		 *
+		 * If stacksize==bufsiz, presumably the stack is larger than
+		 * what we can retrieve.  But it's also possible that the
+		 * buffer was exactly large enough.  So, leave it to the user
+		 * to interpret the result.
+		 */
+		return stacksize / sizeof(uint64_t);
 	}
 	case DIF_VAR_CALLER: {
 		uint64_t flags = 0 & BPF_F_SKIP_FIELD_MASK;

libdtrace/dt_bpf.c

@@ -273,8 +273,10 @@ dt_bpf_gmap_create(dtrace_hdl_t *dtp)
 
 	if (create_gmap(dtp, "mem", BPF_MAP_TYPE_PERCPU_ARRAY,
 			sizeof(uint32_t),
-			roundup(sizeof(dt_mstate_t), 8) + 8 +
-				roundup(dtp->dt_maxreclen, 8), 1) == -1)
+			roundup(sizeof(dt_mstate_t), 8) + 8
+			  + roundup(dtp->dt_maxreclen, 8)
+			  + 8 * dtp->dt_options[DTRACEOPT_MAXFRAMES],
+			1) == -1)
 		return -1;		/* dt_errno is set for us */
 
 	/*

libdtrace/dt_bpf.h

@@ -23,6 +23,8 @@ extern "C" {
 #define DT_CONST_CLID	3
 #define DT_CONST_ARGC	4
 #define DT_CONST_STBSZ	5
+#define DT_CONST_STKOFF	6
+#define DT_CONST_STKSIZ	7
 
 extern int perf_event_open(struct perf_event_attr *attr, pid_t pid, int cpu,
 			   int group_fd, unsigned long flags);

libdtrace/dt_cc.c

@@ -2331,6 +2331,13 @@ dt_link_construct(dtrace_hdl_t *dtp, const dt_probe_t *prp, dtrace_difo_t *dp,
 			case DT_CONST_STBSZ:
 				nrp->dofr_data = dtp->dt_strlen;
 				continue;
+			case DT_CONST_STKOFF:
+				nrp->dofr_data = roundup(dtp->dt_maxreclen, 8);
+				continue;
+			case DT_CONST_STKSIZ:
+				nrp->dofr_data = sizeof(uint64_t)
+				    * dtp->dt_options[DTRACEOPT_MAXFRAMES];
+				continue;
 			default:
 				/* probe name -> value is probe id */
 				if (strchr(idp->di_name, ':') != NULL)

libdtrace/dt_dlibs.c

@@ -78,6 +78,8 @@ static const dt_ident_t		dt_bpf_symbols[] = {
 	DT_BPF_SYMBOL_ID(CLID, DT_IDENT_SCALAR, DT_CONST_CLID),
 	DT_BPF_SYMBOL_ID(ARGC, DT_IDENT_SCALAR, DT_CONST_ARGC),
 	DT_BPF_SYMBOL_ID(STBSZ, DT_IDENT_SCALAR, DT_CONST_STBSZ),
+	DT_BPF_SYMBOL_ID(STKOFF, DT_IDENT_SCALAR, DT_CONST_STKOFF),
+	DT_BPF_SYMBOL_ID(STKSIZ, DT_IDENT_SCALAR, DT_CONST_STKSIZ),
 	/* End-of-list marker */
 	{ NULL, }
 };

test/unittest/stackdepth/tst.value.d → test/unittest/variables/bvar/tst.stackdepth2.d

@@ -4,7 +4,6 @@
  * Licensed under the Universal Permissive License v 1.0 as shown at
  * http://oss.oracle.com/licenses/upl.
  */
-/* @@xfail: dtv2 */
 
 #pragma D option destructive
 #pragma D option quiet