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, 27 insertions, 1 deletions

diff --git a/gdb/nat/linux-btrace.c b/gdb/nat/linux-btrace.c
index 3093a5c..e7aff5e 100644
--- a/gdb/nat/linux-btrace.c
+++ b/gdb/nat/linux-btrace.c
@@ -38,6 +38,7 @@
 #include <sys/ptrace.h>
 #include <sys/types.h>
 #include <signal.h>
+#include <sys/utsname.h>
 
 /* A branch trace record in perf_event.  */
 struct perf_event_bts
@@ -102,6 +103,31 @@ perf_event_new_data (const struct perf_event_buffer *pev)
   return *pev->data_head != pev->last_head;
 }
 
+/* Try to determine the size of a pointer in bits for the OS.
+
+   This is the same as the size of a pointer for the inferior process
+   except when a 32-bit inferior is running on a 64-bit OS.  */
+
+static int
+linux_determine_kernel_ptr_bits (void)
+{
+  struct utsname utsn;
+  int errcode;
+
+  memset (&utsn, 0, sizeof (utsn));
+
+  errcode = uname (&utsn);
+  if (errcode < 0)
+    return 0;
+
+  /* We only need to handle the 64-bit host case, here.  For 32-bit host,
+     the pointer size can be filled in later based on the inferior.  */
+  if (strcmp (utsn.machine, "x86_64") == 0)
+    return 64;
+
+  return 0;
+}
+
 /* Check whether an address is in the kernel.  */
 
 static inline int
@@ -434,7 +460,7 @@ linux_enable_bts (ptid_t ptid, const struct btrace_config_bts *conf)
 
   tinfo = xzalloc (sizeof (*tinfo));
   tinfo->ptid = ptid;
-  tinfo->ptr_bits = 0;
+  tinfo->ptr_bits = linux_determine_kernel_ptr_bits ();
 
   tinfo->conf.format = BTRACE_FORMAT_BTS;
   bts = &tinfo->variant.bts;