[PATCH v2 (w/ ChangeLog)] Fix disabling of solib probes when LD_AUDITing

[George Barrett <bob@bob131.so>]

The SVR4 solib event handler determines whether an event is related to a
non-base link namespace by comparing the event's debug struct address
to the debug struct address of the initial program image. However, this
can fail when using LD_AUDIT as audit libraries are loaded before the
loader has initialised the initial program image's debug struct. When
the event handler fails to find the debug struct, the probe-based
debugger interface is disabled and a warning is flagged to the user.

This commit adds a fallback test to help determine whether an event is
for a foreign link namespace when the debug struct isn't available.

gdb/ChangeLog:
2019-12-15  George Barrett  <bob@bob131.so>

	* solib-svr4.c (svr4_handle_solib_event): Add fallback link
	namespace test for when the debug struct isn't available.
---
 gdb/solib-svr4.c | 22 +++++++++++++++++++++-
 1 file changed, 21 insertions(+), 1 deletion(-)

diff --git a/gdb/solib-svr4.c b/gdb/solib-svr4.c
index de765576d0..f0c7769ac2 100644
--- a/gdb/solib-svr4.c
+++ b/gdb/solib-svr4.c
@@ -1942,7 +1942,27 @@ svr4_handle_solib_event (void)
     /* Always locate the debug struct, in case it moved.  */
     info->debug_base = 0;
     if (locate_base (info) == 0)
-      return;
+      {
+	/* It's possible for the reloc_complete probe to be triggered before
+	   the linker has set the DT_DEBUG pointer (for example, when the
+	   linker has finished relocating an LD_AUDIT library or its
+	   dependencies).  Since we can't yet handle libraries from other link
+	   namespaces, we don't lose anything by ignoring them here.  */
+	struct value *link_map_id_val;
+	try
+	  {
+	    link_map_id_val = pa->prob->evaluate_argument (0, frame);
+	  }
+	catch (const gdb_exception_error)
+	  {
+	    link_map_id_val = NULL;
+	  }
+	/* glibc and illumos' libc both define LM_ID_BASE as zero.  */
+	if (link_map_id_val != NULL && value_as_long (link_map_id_val) != 0)
+	  action = DO_NOTHING;
+	else
+	  return;
+      }
 
     /* GDB does not currently support libraries loaded via dlmopen
        into namespaces other than the initial one.  We must ignore
-- 
2.23.0