Fix handling of null stap semaphores

According to the SystemTap documentation on user-space probes[0], stap
probe points without semaphores are denoted by setting the semaphore
address in the probe's note to zero. At present the code does do a
comparison of the semaphore address against zero, but only after it's
been relocated; as such it will (almost?) always fail, commonly
resulting in GDB trying to overwrite the ELF magic located at the
image's base address.

This commit tests the address as specified in the SDT note rather than
the relocated value in order to correctly detect absent probe
semaphores.

[0]: https://sourceware.org/systemtap/wiki/UserSpaceProbeImplementation

gdb/Changelog:
2020-01-11  George Barrett  <bob@bob131.so>

	* stap-probe.c (stap_modify_semaphore): Don't check for null
	semaphores.
	(stap_probe::set_semaphore, stap_probe::clear_semaphore): Check
	for null semaphores.

gdb/testsuite/ChangeLog:
2020-01-11  George Barrett  <bob@bob131.so>

	* gdb.base/stap-probe.c (relocation_marker): Add dummy variable
	to help in finding the image relocation offset.
	* gdb.base/stap-probe.exp (stap_test): Accept arbitrary compile
	options in arguments.
	(stap_test_no_debuginfo): Likewise.
	(stap-probe-nosem-noopt-pie, stap-probe-nosem-noopt-nopie): Add
	test variants.
	(stap_test): Add null semaphore relocation test.

2 files changed, 33 insertions, 7 deletions

diff --git a/gdb/testsuite/gdb.base/stap-probe.c b/gdb/testsuite/gdb.base/stap-probe.c
index 3d742d1..5cb5494 100644
--- a/gdb/testsuite/gdb.base/stap-probe.c
+++ b/gdb/testsuite/gdb.base/stap-probe.c
@@ -31,6 +31,8 @@ __extension__ unsigned short test_pstr_semaphore __attribute__ ((unused)) __attr
 __extension__ unsigned short test_ps_semaphore __attribute__ ((unused)) __attribute__ ((section (".probes")));
 #else
 
+int relocation_marker __attribute__ ((unused));
+
 #define TEST 1
 #define TEST2 1
 
diff --git a/gdb/testsuite/gdb.base/stap-probe.exp b/gdb/testsuite/gdb.base/stap-probe.exp
index fce5286..398bc89 100644
--- a/gdb/testsuite/gdb.base/stap-probe.exp
+++ b/gdb/testsuite/gdb.base/stap-probe.exp
@@ -18,14 +18,20 @@ standard_testfile
 # Run the tests.  We run the tests two different ways: once with a
 # plain probe, and once with a probe that has an associated semaphore.
 # This returns -1 on failure to compile or start, 0 otherwise.
-proc stap_test {exec_name {arg ""}} {
+proc stap_test {exec_name {args ""}} {
     global testfile hex srcfile
 
     if {[prepare_for_testing "failed to prepare" ${exec_name} $srcfile \
-	   [concat additional_flags=$arg debug]]} {
+	   [concat $args debug]]} {
 	return -1
     }
 
+    set semaphore_addr_var ""
+    if {[string first "-DUSE_SEMAPHORES" $args] == -1} {
+	gdb_test_no_output "set breakpoint always-inserted on"
+	set semaphore_addr_var [get_hexadecimal_valueof "&relocation_marker" "0"]
+    }
+
     if ![runto_main] {
 	return -1
     }
@@ -33,7 +39,7 @@ proc stap_test {exec_name {arg ""}} {
     gdb_test "print \$_probe_argc" "No probe at PC $hex" \
 	"check argument not at probe point"
 
-    if {[string first "-DUSE_SEMAPHORES" $arg] != -1} {
+    if {[string first "-DUSE_SEMAPHORES" $args] != -1} {
 	gdb_test "info probes stap" \
 	    "test *user *$hex *$hex .*"
     } else {
@@ -47,6 +53,20 @@ proc stap_test {exec_name {arg ""}} {
 	fail "run to -pstap test:user"
     }
 
+    if {[string first "-DUSE_SEMAPHORES" $args] == -1} {
+	set relocation_base \
+	   [expr [get_hexadecimal_valueof "&relocation_marker" "0"] - $semaphore_addr_var]
+	if {$relocation_base != 0} {
+	   # Checks that GDB doesn't mistakenly relocate and write to null
+	   # semaphore addresses.  If it were to relocate a zero-valued
+	   # semaphore address and increment the value at that address, we
+	   # would expect to see "\200ELF" here instead.
+	   gdb_test "p (*(char*) $relocation_base)@4" \
+		" = \"\\\\177ELF\"" \
+		"null semaphore relocation"
+	}
+    }
+
     # Test probe arguments.
     gdb_test "print \$_probe_argc" " = 1" \
     "print \$_probe_argc for probe user"
@@ -97,11 +117,11 @@ proc stap_test {exec_name {arg ""}} {
     return 0
 }
 
-proc stap_test_no_debuginfo {exec_name {arg ""}} {
+proc stap_test_no_debuginfo {exec_name {args ""}} {
     global testfile hex
 
     if {[prepare_for_testing "failed to prepare" ${exec_name} ${testfile}.c \
-	   [concat additional_flags=$arg nodebug optimize=-O2]]} {
+	   [concat $args nodebug optimize=-O2]]} {
 	return -1
     }
 
@@ -168,10 +188,14 @@ with_test_prefix "without semaphore, not optimized" {
 	untested "stap probe test failed"
 	  return -1
     }
+
+    foreach_with_prefix pie { "nopie" "pie" } {
+	stap_test "stap-probe-nosem-noopt-$pie" $pie
+    }
 }
 
 with_test_prefix "with semaphore, not optimized" {
-    stap_test "stap-probe-sem-noopt" "-DUSE_SEMAPHORES"
+    stap_test "stap-probe-sem-noopt" additional_flags=-DUSE_SEMAPHORES
 }
 
 with_test_prefix "without semaphore, optimized" {
@@ -179,5 +203,5 @@ with_test_prefix "without semaphore, optimized" {
 }
 
 with_test_prefix "with semaphore, optimized" {
-    stap_test_no_debuginfo "stap-probe-sem-opt" "-DUSE_SEMAPHORES"
+    stap_test_no_debuginfo "stap-probe-sem-opt" additional_flags=-DUSE_SEMAPHORES
 }