[PATCH] Relocation processing of STBSZ in ERROR's get_bvar() is too early

[eugene.loh@oracle.com]

From: Eugene Loh <eugene.loh@oracle.com>

When we call dtrace_go(), we do something like this:

    dt_bpf_make_progs()
        dt_program_construct()        // just for ERROR
            dt_link()
                dt_link_construct()
    dt_bpf_gmap_create()
    dt_bpf_load_progs()               // other
            dt_link()
                dt_link_construct()

In dt_link_construct() we dive down and find dt_get_bvar().  One of the
relocations is to supply the value of STBSZ.  The first dt_link() is for
ERROR, while the subsequent calls in dt_bpf_load_progs() are for other
clauses -- that is, two separate versions of dt_get_bvar() are used.
Meanwhile, the value of STBSZ is not set until dt_bpf_gmap_create().
This means that the ERROR copy of dt_get_bvar() does not have STBSZ set
properly. This means that if ERROR accesses probeprov or probename,
dt_get_bvar() returns the beginning of the string table, which is a NUL
terminator.

Change dt_bpf_reloc_prog() -- which performs relocation processing for
BPF maps, necessarily after dt_bpf_gmap_create()! -- to check STBSZ
and set its value if necessary.

Signed-off-by: Eugene Loh <eugene.loh@oracle.com>
---
 libdtrace/dt_bpf.c                          | 17 +++++++++++
 test/unittest/builtinvar/tst.probe_dtrace.d | 34 +++++++++++++++++++++
 test/unittest/builtinvar/tst.probe_dtrace.r |  6 ++++
 3 files changed, 57 insertions(+)
 create mode 100644 test/unittest/builtinvar/tst.probe_dtrace.d
 create mode 100644 test/unittest/builtinvar/tst.probe_dtrace.r

diff --git a/libdtrace/dt_bpf.c b/libdtrace/dt_bpf.c
index 0df1a9b9b..5cce747fe 100644
--- a/libdtrace/dt_bpf.c
+++ b/libdtrace/dt_bpf.c
@@ -1124,6 +1124,23 @@ dt_bpf_reloc_prog(dtrace_hdl_t *dtp, const dtrace_difo_t *dp)
 				text[ioff + 1].imm = 0;
 			} else if (rp->dofr_type == R_BPF_64_32)
 				text[ioff].imm = val;
+		} else if (idp->di_kind == DT_IDENT_SCALAR &&
+			   idp->di_id == DT_CONST_STBSZ) {
+			/*
+			 * There is one other case.  Although STBSZ is not a map
+			 * pointer, the string table size is set when the BPF maps
+			 * are created.  So we may have to set its value here.
+			 */
+			assert(strcmp(name, "STBSZ") == 0);  // FIXME:  extra check for development
+
+			/*
+			 * Worry only about the cases that have not yet
+			 * been handled.
+			 */
+			if (rp->dofr_type != R_BPF_64_64 && rp->dofr_data == 0) {
+				text[ioff].imm = dtp->dt_strmax;
+				text[ioff + 1].imm = 0;
+			}
 		}
 	}
 }
diff --git a/test/unittest/builtinvar/tst.probe_dtrace.d b/test/unittest/builtinvar/tst.probe_dtrace.d
new file mode 100644
index 000000000..d08067ad0
--- /dev/null
+++ b/test/unittest/builtinvar/tst.probe_dtrace.d
@@ -0,0 +1,34 @@
+/*
+ * Oracle Linux DTrace.
+ * Copyright (c) 2024, Oracle and/or its affiliates. All rights reserved.
+ * Licensed under the Universal Permissive License v 1.0 as shown at
+ * http://oss.oracle.com/licenses/upl.
+ */
+
+/*
+ * ASSERTION:
+ * Built-in variables listing provider, module, function, and name
+ * are correct from dtrace-provider probes.
+ *
+ * SECTION: Variables/Built-in Variables
+ */
+
+#pragma D option quiet
+
+/* Check build-in variables from the dtrace-provider probes. */
+BEGIN,ERROR,END
+{
+	printf("%s:%s:%s:%s\n", probeprov, probemod, probefunc, probename);
+}
+
+/* Cause the ERROR probe to fire. */
+BEGIN
+{
+	*((int*)0);
+}
+
+/* Cause the END probe to fire. */
+BEGIN,ERROR
+{
+	exit(0);
+}
diff --git a/test/unittest/builtinvar/tst.probe_dtrace.r b/test/unittest/builtinvar/tst.probe_dtrace.r
new file mode 100644
index 000000000..e6cccc88d
--- /dev/null
+++ b/test/unittest/builtinvar/tst.probe_dtrace.r
@@ -0,0 +1,6 @@
+dtrace:::BEGIN
+dtrace:::ERROR
+dtrace:::END
+
+-- @@stderr --
+dtrace: error in dt_clause_3 for probe ID 1 (dtrace:::BEGIN): invalid address ({ptr}) at BPF pc NNN
-- 
2.43.5