Use sysctl KERN_PROC_SIGTRAMP to retrieve the signal trampoline

location for the native amd64 ABI.  This fixes unwinding over the
signal frame after trampoline was moved to the shared page.

The code would be more correct if using sysctl for the target process
instead of inspecting gdb' own trampoline, but the current change is
least intrusive and currently, we always initialize the native ABI
sysvec first, which means that trampoline location for FreeBSD/amd64
ABI is relatively stable.

Similar change will benefit libunwind.

Analyzed by:	avg
Sponsored by:	The FreeBSD Foundation
MFC after:	1 week

contrib/gdb/gdb/amd64fbsd-nat.c

@@ -29,6 +29,7 @@
 #include <sys/types.h>
 #include <sys/ptrace.h>
 #include <sys/sysctl.h>
+#include <sys/user.h>
 #include <machine/reg.h>
 
 #ifdef HAVE_SYS_PROCFS_H
@@ -212,24 +213,23 @@ Please report this to <[email protected]>.",
 
   SC_RBP_OFFSET = offset;
 
-  /* FreeBSD provides a kern.ps_strings sysctl that we can use to
+  /* FreeBSD provides a kern.proc.sigtramp sysctl that we can use to
      locate the sigtramp.  That way we can still recognize a sigtramp
-     if its location is changed in a new kernel.  Of course this is
-     still based on the assumption that the sigtramp is placed
-     directly under the location where the program arguments and
-     environment can be found.  */
+     if its location is changed in a new kernel. */
   {
-    int mib[2];
-    long ps_strings;
+    int mib[4];
+    struct kinfo_sigtramp kst;
     size_t len;
 
     mib[0] = CTL_KERN;
-    mib[1] = KERN_PS_STRINGS;
-    len = sizeof (ps_strings);
-    if (sysctl (mib, 2, &ps_strings, &len, NULL, 0) == 0)
+    mib[1] = KERN_PROC;
+    mib[2] = KERN_PROC_SIGTRAMP;
+    mib[3] = getpid();
+    len = sizeof (kst);
+    if (sysctl (mib, sizeof(mib) / sizeof(mib[0]), &kst, &len, NULL, 0) == 0)
       {
-	amd64fbsd_sigtramp_start_addr = ps_strings - 32;
-	amd64fbsd_sigtramp_end_addr = ps_strings;
+	amd64fbsd_sigtramp_start_addr = kst.ksigtramp_start;
+	amd64fbsd_sigtramp_end_addr = kst.ksigtramp_end;
       }
   }
 }