link: do not modify the input object files unless changed

process_obj() and the various implementations of dt_modtext() go to
considerable lengths to not modify the input .o files unless they
actually need to change (dtrace -G differs from a linker in this,
since it modifies its input object files as well as creating an
output).

But if it finds probes, it unconditionally declares the file modified
and (eventually) calls elf_update() on it, even if all the modifications
were already done by a prior invocation of dtrace -G on the same inputs.
This leads to needless re-make(1)s and is entirely unnecessary.  (The
output is still modified regardless, of course.)

(nca: wrote test, commit log)

Orabug: 35417184
Signed-off-by: Nick Alcock <nick.alcock@oracle.com>
Reviewed-by: Kris Van Hees <kris.van.hees@oracle.com>

libdtrace/dt_link.c

@@ -1431,9 +1431,6 @@ process_obj(dtrace_hdl_t *dtp, const char *obj, int *eprobesp)
 				return dt_link_error(dtp, elf, fd, bufs,
 				    "failed to allocate space for probe");
 
-			mod = 1;
-			elf_flagdata(data_tgt, ELF_C_SET, ELF_F_DIRTY);
-
 			/*
 			 * This symbol may already have been marked to
 			 * be ignored by another relocation referencing
@@ -1442,6 +1439,8 @@ process_obj(dtrace_hdl_t *dtp, const char *obj, int *eprobesp)
 			 * invocation.
 			 */
 			if (rsym.st_shndx != SHN_SUNW_IGNORE) {
+				mod = 1;
+				elf_flagdata(data_tgt, ELF_C_SET, ELF_F_DIRTY);
 				rsym.st_shndx = SHN_SUNW_IGNORE;
 				gelf_update_sym(data_sym, ndx, &rsym);
 			}
@@ -1453,15 +1452,19 @@ process_obj(dtrace_hdl_t *dtp, const char *obj, int *eprobesp)
 			 * causes the linker to try to fill in an address on
 			 * top of the NOPs we so carefully planted.
 			 */
-			if (shdr_rel.sh_type == SHT_RELA) {
-				rela.r_info = GELF_R_INFO(ndx, 0);
-				gelf_update_rela(data_rel, i, &rela);
-			} else {
-				GElf_Rel rel;
-
-				rel.r_offset = rela.r_offset;
-				rel.r_info = GELF_R_INFO(ndx, 0);
-				gelf_update_rel(data_rel, i, &rel);
+			if (rela.r_info != GELF_R_INFO(ndx,0)) {
+				mod = 1;
+				elf_flagdata(data_tgt, ELF_C_SET, ELF_F_DIRTY);
+				if (shdr_rel.sh_type == SHT_RELA) {
+					rela.r_info = GELF_R_INFO(ndx, 0);
+					gelf_update_rela(data_rel, i, &rela);
+				} else {
+					GElf_Rel rel;
+
+					rel.r_offset = rela.r_offset;
+					rel.r_info = GELF_R_INFO(ndx, 0);
+					gelf_update_rel(data_rel, i, &rel);
+				}
 			}
 		}
 	}

test/unittest/usdt/tst.link-idempotence.sh

@@ -0,0 +1,66 @@
+#!/bin/bash
+#
+# Oracle Linux DTrace.
+# Copyright (c) 2006, 2023, 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.
+#
+if [ $# != 1 ]; then
+	echo expected one argument: '<'dtrace-path'>'
+	exit 2
+fi
+
+dtrace=$1
+CC=/usr/bin/gcc
+CFLAGS=
+
+DIRNAME="$tmpdir/usdt-link-idempotence.$$.$RANDOM"
+mkdir -p $DIRNAME
+cd $DIRNAME
+
+cat > prov.d <<EOF
+provider test_prov {
+	probe go();
+};
+EOF
+
+$dtrace -h -s prov.d
+if [ $? -ne 0 ]; then
+	echo "failed to generate header file" >& 2
+	exit 1
+fi
+
+cat > test.c <<EOF
+#include <sys/types.h>
+#include "prov.h"
+
+int
+main(int argc, char **argv)
+{
+	if (TEST_PROV_GO_ENABLED()) {
+		TEST_PROV_GO();
+	}
+}
+EOF
+
+${CC} ${CFLAGS} -c test.c
+if [ $? -ne 0 ]; then
+	echo "failed to compile test.c" >& 2
+	exit 1
+fi
+$dtrace -G -s prov.d test.o
+if [ $? -ne 0 ]; then
+	echo "failed to create DOF" >& 2
+	exit 1
+fi
+${CC} ${CFLAGS} -o test test.o prov.o
+DTRACE_DEBUG=t $dtrace -G -s prov.d test.o
+if [ $? -ne 0 ]; then
+	echo "failed to regenerate DOF" >& 2
+	exit 1
+fi
+if [ test.o -nt test ]; then
+	exit 1
+fi
+
+exit 0