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

This patch is part of a series that has the aim of making the code
that, for x86, reads the target description for a native process
shared 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.

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 could 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>

2 files changed, 75 insertions, 0 deletions

diff --git a/gdb/nat/x86-linux.c b/gdb/nat/x86-linux.c
index e61f4d7..4242a1b 100644
--- a/gdb/nat/x86-linux.c
+++ b/gdb/nat/x86-linux.c
@@ -20,6 +20,8 @@
 #include "gdbsupport/common-defs.h"
 #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.  */
 
@@ -80,3 +82,48 @@ 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 */