[glibc] hurd: Fix x86_64 _hurd_tls_fork

[Samuel Thibault <sthibaul@sourceware.org>]

https://sourceware.org/git/gitweb.cgi?p=glibc.git;h=2f8ecb58a59eb82c43214d000842d99644a662d1

commit 2f8ecb58a59eb82c43214d000842d99644a662d1
Author: Sergey Bugaev <bugaevc@gmail.com>
Date:   Wed May 17 22:14:33 2023 +0300

    hurd: Fix x86_64 _hurd_tls_fork
    
    It is illegal to call thread_get_state () on mach_thread_self (), so
    this codepath cannot be used as-is to fork the calling thread's TLS.
    Fortunately we can use THREAD_SELF (aka %fs:0x0) to find out the value
    of our fs_base without calling into the kernel.
    
    Fixes: f6cf701efc61c9ad910372bda14b9a235db310a8
    "hurd: Implement TLS for x86_64"
    
    Checked on x86_64-gnu: fork () now works!
    
    Signed-off-by: Sergey Bugaev <bugaevc@gmail.com>
    Message-Id: <20230517191436.73636-8-bugaevc@gmail.com>

Diff:
---
 sysdeps/mach/hurd/x86_64/tls.h | 25 +++++++++++++++++++------
 1 file changed, 19 insertions(+), 6 deletions(-)

diff --git a/sysdeps/mach/hurd/x86_64/tls.h b/sysdeps/mach/hurd/x86_64/tls.h
index 35dcef447e..6487ed3580 100644
--- a/sysdeps/mach/hurd/x86_64/tls.h
+++ b/sysdeps/mach/hurd/x86_64/tls.h
@@ -140,12 +140,25 @@ _hurd_tls_fork (thread_t child, thread_t orig,
   error_t err;
   struct i386_fsgs_base_state state;
   mach_msg_type_number_t state_count = i386_FSGS_BASE_STATE_COUNT;
-  err = __thread_get_state (orig, i386_FSGS_BASE_STATE,
-                            (thread_state_t) &state,
-                            &state_count);
-  if (err)
-    return err;
-  assert (state_count == i386_FSGS_BASE_STATE_COUNT);
+
+  extern thread_t hurd_thread_self (void);
+  if (orig != hurd_thread_self ())
+    {
+      err = __thread_get_state (orig, i386_FSGS_BASE_STATE,
+                                (thread_state_t) &state,
+                                &state_count);
+      if (err)
+        return err;
+      assert (state_count == i386_FSGS_BASE_STATE_COUNT);
+    }
+  else
+    {
+      /* It is illegal to call thread_get_state () on mach_thread_self ().
+         But we're only interested in the value of fs_base, and since we're
+         this thread, we know it points to our TCB.  */
+      state.fs_base = (unsigned long) THREAD_SELF;
+      state.gs_base = 0;
+    }
 
   return __thread_set_state (child, i386_FSGS_BASE_STATE,
                              (thread_state_t) &state,