btrace: support 32-bit inferior on 64-bit host

The heuristic for filtering out kernel addressess in BTS trace checks the
most significant bit in each address.  This works fine for 32-bit and 64-bit
mode.

For 32-bit compatibility mode, i.e. a 32-bit inferior running on 64-bit
host, we need to check bit 63 (or any bit bigger than 31), not bit 31.

Use the machine field in struct utsname provided by a uname call to
determine whether we are running on a 64-bit host.

Thanks to Jan Kratochvil for reporting the issue.

gdb/
	* nat/linux-btrace.c: Include sys/utsname.h.
	(linux_determine_kernel_ptr_bits): New.
	(linux_enable_bts): Call linux_determine_kernel_ptr_bits.
	* x86-linux-nat.c (x86_linux_enable_btrace): Do not overwrite non-zero
	ptr_bits.

gdbserver/
	* linux-low.c (linux_low_enable_btrace): Do not overwrite non-zero
	ptr_bits.

1 files changed, 5 insertions, 3 deletions

diff --git a/gdb/x86-linux-nat.c b/gdb/x86-linux-nat.c
index c58c01a..7f038f0 100644
--- a/gdb/x86-linux-nat.c
+++ b/gdb/x86-linux-nat.c
@@ -450,9 +450,11 @@ x86_linux_enable_btrace (struct target_ops *self, ptid_t ptid,
 	   target_pid_to_str (ptid), safe_strerror (errno));
 
   /* Fill in the size of a pointer in bits.  */
-  gdbarch = target_thread_architecture (ptid);
-  tinfo->ptr_bits = gdbarch_ptr_bit (gdbarch);
-
+  if (tinfo->ptr_bits == 0)
+    {
+      gdbarch = target_thread_architecture (ptid);
+      tinfo->ptr_bits = gdbarch_ptr_bit (gdbarch);
+    }
   return tinfo;
 }