Internal, but disabled, TLS support for i386

This commit shows how internal TLS address lookup support could
be implemented for the i386 target.

Unfortunately, it doesn't work due to I386_GSBASE_REGNUM being
unavailable for Linux targets.  I looked at trying to access the
gsbase register via PTRACE_GET_THREAD_AREA, but did not understand
it well enough to finish it.  Since the i386 target is much less
important than it used to be, I gave up working on it.

I don't want to leave this disabled code in our sources, so I
will delete it in the next commit, however, this commit will be
in our git repo, so it'll be available for someone with sufficient
interest in the i386 target to look at.

Tested-By: Luis Machado <luis.machado@arm.com>
Approved-By: Luis Machado <luis.machado@arm.com>

1 files changed, 46 insertions, 0 deletions

diff --git a/gdb/i386-linux-tdep.c b/gdb/i386-linux-tdep.c
index 4b05cc6..7ad3acb 100644
--- a/gdb/i386-linux-tdep.c
+++ b/gdb/i386-linux-tdep.c
@@ -1237,6 +1237,47 @@ i386_linux_displaced_step_copy_insn (struct gdbarch *gdbarch,
   return closure_;
 }
 
+#if 0
+/* Disabled because fetching I386_GSBASE_REGNUM causes an internal
+   error.  */
+
+#include "svr4-tls-tdep.h"
+
+/* Fetch and return the TLS DTV (dynamic thread vector) address for PTID.
+   Throw a suitable TLS error if something goes wrong.  */
+
+static CORE_ADDR
+i386_linux_get_tls_dtv_addr (struct gdbarch *gdbarch, ptid_t ptid,
+			     enum svr4_tls_libc libc)
+{
+  /* On i386, the thread pointer is found in the gsbase register.  */
+  regcache *regcache
+    = get_thread_arch_regcache (current_inferior (), ptid, gdbarch);
+  target_fetch_registers (regcache, I386_GSBASE_REGNUM);
+  ULONGEST gsbase;
+  if (regcache->cooked_read (I386_GSBASE_REGNUM, &gsbase) != REG_VALID)
+    throw_error (TLS_GENERIC_ERROR, _("Unable to fetch thread pointer"));
+
+  /* The thread pointer (gsbase) points at the TCB (thread control
+     block).  The first two members of this struct are both pointers,
+     where the first will be a pointer to the TCB (i.e. it points at
+     itself) and the second will be a pointer to the DTV (dynamic
+     thread vector).  There are many other fields too, but the one
+     we care about here is the DTV pointer.  Compute the address
+     of the DTV pointer, fetch it, and convert it to an address.  */
+  CORE_ADDR dtv_ptr_addr
+    = gsbase + gdbarch_ptr_bit (gdbarch) / TARGET_CHAR_BIT;
+  gdb::byte_vector buf (gdbarch_ptr_bit (gdbarch) / TARGET_CHAR_BIT);
+  if (target_read_memory (dtv_ptr_addr, buf.data (), buf.size ()) != 0)
+    throw_error (TLS_GENERIC_ERROR, _("Unable to fetch DTV address"));
+
+  const struct builtin_type *builtin = builtin_type (gdbarch);
+  CORE_ADDR dtv_addr = gdbarch_pointer_to_address
+			 (gdbarch, builtin->builtin_data_ptr, buf.data ());
+  return dtv_addr;
+}
+#endif
+
 static void
 i386_linux_init_abi (struct gdbarch_info info, struct gdbarch *gdbarch)
 {
@@ -1472,6 +1513,11 @@ i386_linux_init_abi (struct gdbarch_info info, struct gdbarch *gdbarch)
   /* Enable TLS support.  */
   set_gdbarch_fetch_tls_load_module_address (gdbarch,
 					     svr4_fetch_objfile_link_map);
+#if 0
+  set_gdbarch_get_thread_local_address (gdbarch,
+					svr4_tls_get_thread_local_address);
+  svr4_tls_register_tls_methods (info, gdbarch, i386_linux_get_tls_dtv_addr);
+#endif
 
   /* Core file support.  */
   set_gdbarch_iterate_over_regset_sections