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tls.c

/*
 * Copyright (C) 2005 Paolo 'Blaisorblade' Giarrusso <blaisorblade@yahoo.it>
 * Licensed under the GPL
 */

#include "linux/percpu.h"
#include "linux/sched.h"
#include "asm/uaccess.h"
#include "os.h"
#include "skas.h"
#include "sysdep/tls.h"

/*
 * If needed we can detect when it's uninitialized.
 *
 * These are initialized in an initcall and unchanged thereafter.
 */
static int host_supports_tls = -1;
int host_gdt_entry_tls_min;

int do_set_thread_area(struct user_desc *info)
{
      int ret;
      u32 cpu;

      cpu = get_cpu();
      ret = os_set_thread_area(info, userspace_pid[cpu]);
      put_cpu();
      return ret;
}

int do_get_thread_area(struct user_desc *info)
{
      int ret;
      u32 cpu;

      cpu = get_cpu();
      ret = os_get_thread_area(info, userspace_pid[cpu]);
      put_cpu();
      return ret;
}

/*
 * sys_get_thread_area: get a yet unused TLS descriptor index.
 * XXX: Consider leaving one free slot for glibc usage at first place. This must
 * be done here (and by changing GDT_ENTRY_TLS_* macros) and nowhere else.
 *
 * Also, this must be tested when compiling in SKAS mode with dynamic linking
 * and running against NPTL.
 */
static int get_free_idx(struct task_struct* task)
{
      struct thread_struct *t = &task->thread;
      int idx;

      if (!t->arch.tls_array)
            return GDT_ENTRY_TLS_MIN;

      for (idx = 0; idx < GDT_ENTRY_TLS_ENTRIES; idx++)
            if (!t->arch.tls_array[idx].present)
                  return idx + GDT_ENTRY_TLS_MIN;
      return -ESRCH;
}

static inline void clear_user_desc(struct user_desc* info)
{
      /* Postcondition: LDT_empty(info) returns true. */
      memset(info, 0, sizeof(*info));

      /*
       * Check the LDT_empty or the i386 sys_get_thread_area code - we obtain
       * indeed an empty user_desc.
       */
      info->read_exec_only = 1;
      info->seg_not_present = 1;
}

#define O_FORCE 1

static int load_TLS(int flags, struct task_struct *to)
{
      int ret = 0;
      int idx;

      for (idx = GDT_ENTRY_TLS_MIN; idx < GDT_ENTRY_TLS_MAX; idx++) {
            struct uml_tls_struct* curr =
                  &to->thread.arch.tls_array[idx - GDT_ENTRY_TLS_MIN];

            /*
             * Actually, now if it wasn't flushed it gets cleared and
             * flushed to the host, which will clear it.
             */
            if (!curr->present) {
                  if (!curr->flushed) {
                        clear_user_desc(&curr->tls);
                        curr->tls.entry_number = idx;
                  } else {
                        WARN_ON(!LDT_empty(&curr->tls));
                        continue;
                  }
            }

            if (!(flags & O_FORCE) && curr->flushed)
                  continue;

            ret = do_set_thread_area(&curr->tls);
            if (ret)
                  goto out;

            curr->flushed = 1;
      }
out:
      return ret;
}

/*
 * Verify if we need to do a flush for the new process, i.e. if there are any
 * present desc's, only if they haven't been flushed.
 */
static inline int needs_TLS_update(struct task_struct *task)
{
      int i;
      int ret = 0;

      for (i = GDT_ENTRY_TLS_MIN; i < GDT_ENTRY_TLS_MAX; i++) {
            struct uml_tls_struct* curr =
                  &task->thread.arch.tls_array[i - GDT_ENTRY_TLS_MIN];

            /*
             * Can't test curr->present, we may need to clear a descriptor
             * which had a value.
             */
            if (curr->flushed)
                  continue;
            ret = 1;
            break;
      }
      return ret;
}

/*
 * On a newly forked process, the TLS descriptors haven't yet been flushed. So
 * we mark them as such and the first switch_to will do the job.
 */
void clear_flushed_tls(struct task_struct *task)
{
      int i;

      for (i = GDT_ENTRY_TLS_MIN; i < GDT_ENTRY_TLS_MAX; i++) {
            struct uml_tls_struct* curr =
                  &task->thread.arch.tls_array[i - GDT_ENTRY_TLS_MIN];

            /*
             * Still correct to do this, if it wasn't present on the host it
             * will remain as flushed as it was.
             */
            if (!curr->present)
                  continue;

            curr->flushed = 0;
      }
}

/*
 * In SKAS0 mode, currently, multiple guest threads sharing the same ->mm have a
 * common host process. So this is needed in SKAS0 too.
 *
 * However, if each thread had a different host process (and this was discussed
 * for SMP support) this won't be needed.
 *
 * And this will not need be used when (and if) we'll add support to the host
 * SKAS patch.
 */

int arch_switch_tls(struct task_struct *from, struct task_struct *to)
{
      if (!host_supports_tls)
            return 0;

      /*
       * We have no need whatsoever to switch TLS for kernel threads; beyond
       * that, that would also result in us calling os_set_thread_area with
       * userspace_pid[cpu] == 0, which gives an error.
       */
      if (likely(to->mm))
            return load_TLS(O_FORCE, to);

      return 0;
}

static int set_tls_entry(struct task_struct* task, struct user_desc *info,
                   int idx, int flushed)
{
      struct thread_struct *t = &task->thread;

      if (idx < GDT_ENTRY_TLS_MIN || idx > GDT_ENTRY_TLS_MAX)
            return -EINVAL;

      t->arch.tls_array[idx - GDT_ENTRY_TLS_MIN].tls = *info;
      t->arch.tls_array[idx - GDT_ENTRY_TLS_MIN].present = 1;
      t->arch.tls_array[idx - GDT_ENTRY_TLS_MIN].flushed = flushed;

      return 0;
}

int arch_copy_tls(struct task_struct *new)
{
      struct user_desc info;
      int idx, ret = -EFAULT;

      if (copy_from_user(&info,
                     (void __user *) UPT_ESI(&new->thread.regs.regs),
                     sizeof(info)))
            goto out;

      ret = -EINVAL;
      if (LDT_empty(&info))
            goto out;

      idx = info.entry_number;

      ret = set_tls_entry(new, &info, idx, 0);
out:
      return ret;
}

/* XXX: use do_get_thread_area to read the host value? I'm not at all sure! */
static int get_tls_entry(struct task_struct* task, struct user_desc *info, int idx)
{
      struct thread_struct *t = &task->thread;

      if (!t->arch.tls_array)
            goto clear;

      if (idx < GDT_ENTRY_TLS_MIN || idx > GDT_ENTRY_TLS_MAX)
            return -EINVAL;

      if (!t->arch.tls_array[idx - GDT_ENTRY_TLS_MIN].present)
            goto clear;

      *info = t->arch.tls_array[idx - GDT_ENTRY_TLS_MIN].tls;

out:
      /*
       * Temporary debugging check, to make sure that things have been
       * flushed. This could be triggered if load_TLS() failed.
       */
      if (unlikely(task == current &&
                 !t->arch.tls_array[idx - GDT_ENTRY_TLS_MIN].flushed)) {
            printk(KERN_ERR "get_tls_entry: task with pid %d got here "
                        "without flushed TLS.", current->pid);
      }

      return 0;
clear:
      /*
       * When the TLS entry has not been set, the values read to user in the
       * tls_array are 0 (because it's cleared at boot, see
       * arch/i386/kernel/head.S:cpu_gdt_table). Emulate that.
       */
      clear_user_desc(info);
      info->entry_number = idx;
      goto out;
}

asmlinkage int sys_set_thread_area(struct user_desc __user *user_desc)
{
      struct user_desc info;
      int idx, ret;

      if (!host_supports_tls)
            return -ENOSYS;

      if (copy_from_user(&info, user_desc, sizeof(info)))
            return -EFAULT;

      idx = info.entry_number;

      if (idx == -1) {
            idx = get_free_idx(current);
            if (idx < 0)
                  return idx;
            info.entry_number = idx;
            /* Tell the user which slot we chose for him.*/
            if (put_user(idx, &user_desc->entry_number))
                  return -EFAULT;
      }

      ret = do_set_thread_area(&info);
      if (ret)
            return ret;
      return set_tls_entry(current, &info, idx, 1);
}

/*
 * Perform set_thread_area on behalf of the traced child.
 * Note: error handling is not done on the deferred load, and this differ from
 * i386. However the only possible error are caused by bugs.
 */
int ptrace_set_thread_area(struct task_struct *child, int idx,
            struct user_desc __user *user_desc)
{
      struct user_desc info;

      if (!host_supports_tls)
            return -EIO;

      if (copy_from_user(&info, user_desc, sizeof(info)))
            return -EFAULT;

      return set_tls_entry(child, &info, idx, 0);
}

asmlinkage int sys_get_thread_area(struct user_desc __user *user_desc)
{
      struct user_desc info;
      int idx, ret;

      if (!host_supports_tls)
            return -ENOSYS;

      if (get_user(idx, &user_desc->entry_number))
            return -EFAULT;

      ret = get_tls_entry(current, &info, idx);
      if (ret < 0)
            goto out;

      if (copy_to_user(user_desc, &info, sizeof(info)))
            ret = -EFAULT;

out:
      return ret;
}

/*
 * Perform get_thread_area on behalf of the traced child.
 */
int ptrace_get_thread_area(struct task_struct *child, int idx,
            struct user_desc __user *user_desc)
{
      struct user_desc info;
      int ret;

      if (!host_supports_tls)
            return -EIO;

      ret = get_tls_entry(child, &info, idx);
      if (ret < 0)
            goto out;

      if (copy_to_user(user_desc, &info, sizeof(info)))
            ret = -EFAULT;
out:
      return ret;
}


/*
 * XXX: This part is probably common to i386 and x86-64. Don't create a common
 * file for now, do that when implementing x86-64 support.
 */
static int __init __setup_host_supports_tls(void)
{
      check_host_supports_tls(&host_supports_tls, &host_gdt_entry_tls_min);
      if (host_supports_tls) {
            printk(KERN_INFO "Host TLS support detected\n");
            printk(KERN_INFO "Detected host type: ");
            switch (host_gdt_entry_tls_min) {
                  case GDT_ENTRY_TLS_MIN_I386:
                        printk("i386\n");
                        break;
                  case GDT_ENTRY_TLS_MIN_X86_64:
                        printk("x86_64\n");
                        break;
            }
      } else
            printk(KERN_ERR "  Host TLS support NOT detected! "
                        "TLS support inside UML will not work\n");
      return 0;
}

__initcall(__setup_host_supports_tls);

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