gdb/x86: move reading of cs and ds state into gdb/nat directory

This patch is part of a series that has the aim sharing the x86 Linux
target description creation code between GDB and gdbserver.

Within GDB part of this process involves reading the cs and ds state
from the 'struct user_regs_struct' using a ptrace call.

This isn't done by gdbserver, which is part of the motivation for this
whole series; the approach gdbserver takes is inferior to the approach
GDB takes (gdbserver relies on reading the file being debugged, and
extracting similar information from the file headers).

This commit moves the reading of cs and ds, which is used to figure
out if a thread is 32-bit or 64-bit (or in x32 mode), into the gdb/nat
directory so that the code can be shared with gdbserver, but at this
point I'm not actually using the code in gdbserver, that will come
later.

As such there should be no user visible changes after this commit, GDB
continues to do things as it did before (reading cs/ds), while
gdbserver continues to use its own approach (which doesn't require
reading cs/ds).

Approved-By: John Baldwin <jhb@FreeBSD.org>
Reviewed-By: Felix Willgerodt <felix.willgerodt@intel.com>

3 files changed, 83 insertions, 37 deletions

diff --git a/gdb/nat/x86-linux.c b/gdb/nat/x86-linux.c
index 7a21c8f..ad3ed3c 100644
--- a/gdb/nat/x86-linux.c
+++ b/gdb/nat/x86-linux.c
@@ -19,6 +19,8 @@
 
 #include "x86-linux.h"
 #include "x86-linux-dregs.h"
+#include "nat/gdb_ptrace.h"
+#include <sys/user.h>
 
 /* Per-thread arch-specific data we want to keep.  */
 
@@ -79,3 +81,48 @@ x86_linux_prepare_to_resume (struct lwp_info *lwp)
 {
   x86_linux_update_debug_registers (lwp);
 }
+
+#ifdef __x86_64__
+/* Value of CS segment register:
+     64bit process: 0x33
+     32bit process: 0x23  */
+#define AMD64_LINUX_USER64_CS 0x33
+
+/* Value of DS segment register:
+     LP64 process: 0x0
+     X32 process: 0x2b  */
+#define AMD64_LINUX_X32_DS 0x2b
+#endif
+
+/* See nat/x86-linux.h.  */
+
+x86_linux_arch_size
+x86_linux_ptrace_get_arch_size (int tid)
+{
+#ifdef __x86_64__
+  unsigned long cs;
+  unsigned long ds;
+
+  /* Get CS register.  */
+  errno = 0;
+  cs = ptrace (PTRACE_PEEKUSER, tid,
+	       offsetof (struct user_regs_struct, cs), 0);
+  if (errno != 0)
+    perror_with_name (_("Couldn't get CS register"));
+
+  bool is_64bit = cs == AMD64_LINUX_USER64_CS;
+
+  /* Get DS register.  */
+  errno = 0;
+  ds = ptrace (PTRACE_PEEKUSER, tid,
+	       offsetof (struct user_regs_struct, ds), 0);
+  if (errno != 0)
+    perror_with_name (_("Couldn't get DS register"));
+
+  bool is_x32 = ds == AMD64_LINUX_X32_DS;
+
+  return x86_linux_arch_size (is_64bit, is_x32);
+#else
+  return x86_linux_arch_size (false, false);
+#endif
+}
diff --git a/gdb/nat/x86-linux.h b/gdb/nat/x86-linux.h
index 8228821..15153ea 100644
--- a/gdb/nat/x86-linux.h
+++ b/gdb/nat/x86-linux.h
@@ -47,4 +47,32 @@ extern void x86_linux_delete_thread (struct arch_lwp_info *arch_lwp);
 
 extern void x86_linux_prepare_to_resume (struct lwp_info *lwp);
 
+/* Return value from x86_linux_ptrace_get_arch_size function.  Indicates if
+   a thread is 32-bit, 64-bit, or x32.  */
+
+struct x86_linux_arch_size
+{
+  explicit x86_linux_arch_size (bool is_64bit, bool is_x32)
+    : m_is_64bit (is_64bit),
+      m_is_x32 (is_x32)
+  {
+    /* Nothing.  */
+  }
+
+  bool is_64bit () const
+  { return m_is_64bit; }
+
+  bool is_x32 () const
+  { return m_is_x32; }
+
+private:
+  bool m_is_64bit = false;
+  bool m_is_x32 = false;
+};
+
+/* Use ptrace calls to figure out if thread TID is 32-bit, 64-bit, or
+   64-bit running in x32 mode.  */
+
+extern x86_linux_arch_size x86_linux_ptrace_get_arch_size (int tid);
+
 #endif /* NAT_X86_LINUX_H */
diff --git a/gdb/x86-linux-nat.c b/gdb/x86-linux-nat.c
index 762151f..ed76c17 100644
--- a/gdb/x86-linux-nat.c
+++ b/gdb/x86-linux-nat.c
@@ -90,18 +90,6 @@ x86_linux_nat_target::post_startup_inferior (ptid_t ptid)
   linux_nat_target::post_startup_inferior (ptid);
 }
 
-#ifdef __x86_64__
-/* Value of CS segment register:
-     64bit process: 0x33
-     32bit process: 0x23  */
-#define AMD64_LINUX_USER64_CS 0x33
-
-/* Value of DS segment register:
-     LP64 process: 0x0
-     X32 process: 0x2b  */
-#define AMD64_LINUX_X32_DS 0x2b
-#endif
-
 /* Get Linux/x86 target description from running target.  */
 
 const struct target_desc *
@@ -121,31 +109,14 @@ x86_linux_nat_target::read_description ()
   tid = inferior_ptid.pid ();
 
 #ifdef __x86_64__
-  {
-    unsigned long cs;
-    unsigned long ds;
-
-    /* Get CS register.  */
-    errno = 0;
-    cs = ptrace (PTRACE_PEEKUSER, tid,
-		 offsetof (struct user_regs_struct, cs), 0);
-    if (errno != 0)
-      perror_with_name (_("Couldn't get CS register"));
-
-    is_64bit = cs == AMD64_LINUX_USER64_CS;
-
-    /* Get DS register.  */
-    errno = 0;
-    ds = ptrace (PTRACE_PEEKUSER, tid,
-		 offsetof (struct user_regs_struct, ds), 0);
-    if (errno != 0)
-      perror_with_name (_("Couldn't get DS register"));
-
-    is_x32 = ds == AMD64_LINUX_X32_DS;
-
-    if (sizeof (void *) == 4 && is_64bit && !is_x32)
-      error (_("Can't debug 64-bit process with 32-bit GDB"));
-  }
+
+  x86_linux_arch_size arch_size = x86_linux_ptrace_get_arch_size (tid);
+  is_64bit = arch_size.is_64bit ();
+  is_x32 = arch_size.is_x32 ();
+
+  if (sizeof (void *) == 4 && is_64bit && !is_x32)
+    error (_("Can't debug 64-bit process with 32-bit GDB"));
+
 #elif HAVE_PTRACE_GETFPXREGS
   if (have_ptrace_getfpxregs == TRIBOOL_UNKNOWN)
     {