[PATCH] Fix handling of null stap semaphores

[George Barrett <bob@bob131.so>]

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:

	* stap-probe.c: Fix handling of null stap semaphores
---
 gdb/stap-probe.c | 7 ++++---
 1 file changed, 4 insertions(+), 3 deletions(-)

diff --git a/gdb/stap-probe.c b/gdb/stap-probe.c
index ba927790a5..e5e3cceacd 100644
--- a/gdb/stap-probe.c
+++ b/gdb/stap-probe.c
@@ -1425,9 +1425,6 @@ stap_modify_semaphore (CORE_ADDR address, int set, struct gdbarch *gdbarch)
   struct type *type = builtin_type (gdbarch)->builtin_unsigned_short;
   ULONGEST value;
 
-  if (address == 0)
-    return;
-
   /* Swallow errors.  */
   if (target_read_memory (address, bytes, TYPE_LENGTH (type)) != 0)
     {
@@ -1461,6 +1458,8 @@ stap_modify_semaphore (CORE_ADDR address, int set, struct gdbarch *gdbarch)
 void
 stap_probe::set_semaphore (struct objfile *objfile, struct gdbarch *gdbarch)
 {
+  if (m_sem_addr == 0)
+    return;
   stap_modify_semaphore (relocate_address (m_sem_addr, objfile), 1, gdbarch);
 }
 
@@ -1469,6 +1468,8 @@ stap_probe::set_semaphore (struct objfile *objfile, struct gdbarch *gdbarch)
 void
 stap_probe::clear_semaphore (struct objfile *objfile, struct gdbarch *gdbarch)
 {
+  if (m_sem_addr == 0)
+    return;
   stap_modify_semaphore (relocate_address (m_sem_addr, objfile), 0, gdbarch);
 }
 
-- 
2.23.0