[PATCH] uml: maintain own LDT entries

Patch imlements full LDT handling in SKAS:
 * UML holds it's own LDT table, used to deliver data on
   modify_ldt(READ)
 * UML disables the default_ldt, inherited from the host (SKAS3)
   or resets LDT entries, set by host's clib and inherited in
   SKAS0
 * A new global variable skas_needs_stub is inserted, that
   can be used to decide, whether stub-pages must be supported
   or not.
 * Uses the syscall-stub to replace missing PTRACE_LDT (therefore,
   write_ldt_entry needs to be modified)

Signed-off-by: Bodo Stroesser <bstroesser@fujitsu-siemens.com>
Signed-off-by: Jeff Dike <jdike@addtoit.com>
Cc: Paolo Giarrusso <blaisorblade@yahoo.it>
Signed-off-by: Andrew Morton <akpm@osdl.org>
Signed-off-by: Linus Torvalds <torvalds@osdl.org>

11 files changed, 586 insertions, 141 deletions

diff --git a/arch/um/kernel/skas/include/mmu-skas.h b/arch/um/kernel/skas/include/mmu-skas.h
index 09536f81ee42..44110c521e49 100644
--- a/arch/um/kernel/skas/include/mmu-skas.h
+++ b/arch/um/kernel/skas/include/mmu-skas.h
@@ -8,6 +8,7 @@
 
 #include "linux/config.h"
 #include "mm_id.h"
+#include "asm/ldt.h"
 
 struct mmu_context_skas {
         struct mm_id id;
@@ -15,6 +16,7 @@ struct mmu_context_skas {
 #ifdef CONFIG_3_LEVEL_PGTABLES
         unsigned long last_pmd;
 #endif
+        uml_ldt_t ldt;
 };
 
 extern void switch_mm_skas(struct mm_id * mm_idp);
diff --git a/arch/um/kernel/skas/include/skas.h b/arch/um/kernel/skas/include/skas.h
index 060934740f9f..daa2f85b684c 100644
--- a/arch/um/kernel/skas/include/skas.h
+++ b/arch/um/kernel/skas/include/skas.h
@@ -10,7 +10,8 @@
 #include "sysdep/ptrace.h"
 
 extern int userspace_pid[];
-extern int proc_mm, ptrace_faultinfo;
+extern int proc_mm, ptrace_faultinfo, ptrace_ldt;
+extern int skas_needs_stub;
 
 extern void switch_threads(void *me, void *next);
 extern void thread_wait(void *sw, void *fb);
diff --git a/arch/um/kernel/skas/mem.c b/arch/um/kernel/skas/mem.c
index 147466d7ff4f..88ab96c609ce 100644
--- a/arch/um/kernel/skas/mem.c
+++ b/arch/um/kernel/skas/mem.c
@@ -20,7 +20,7 @@ unsigned long set_task_sizes_skas(int arg, unsigned long *host_size_out,
         *task_size_out = CONFIG_HOST_TASK_SIZE;
 #else
         *host_size_out = top;
-        if (proc_mm && ptrace_faultinfo)
+        if (!skas_needs_stub)
                 *task_size_out = top;
         else *task_size_out = CONFIG_STUB_START & PGDIR_MASK;
 #endif
diff --git a/arch/um/kernel/skas/mmu.c b/arch/um/kernel/skas/mmu.c
index 9e5e39cea821..677871f1b37c 100644
--- a/arch/um/kernel/skas/mmu.c
+++ b/arch/um/kernel/skas/mmu.c
@@ -15,6 +15,7 @@
 #include "asm/mmu.h"
 #include "asm/pgalloc.h"
 #include "asm/pgtable.h"
+#include "asm/ldt.h"
 #include "os.h"
 #include "skas.h"
 
@@ -74,13 +75,12 @@ static int init_stub_pte(struct mm_struct *mm, unsigned long proc,
 
 int init_new_context_skas(struct task_struct *task, struct mm_struct *mm)
 {
-        struct mm_struct *cur_mm = current->mm;
-        struct mm_id *cur_mm_id = &cur_mm->context.skas.id;
-        struct mm_id *mm_id = &mm->context.skas.id;
+        struct mmu_context_skas *from_mm = NULL;
+        struct mmu_context_skas *to_mm = &mm->context.skas;
         unsigned long stack = 0;
-        int from, ret = -ENOMEM;
+        int from_fd, ret = -ENOMEM;
 
-        if(!proc_mm || !ptrace_faultinfo){
+        if(skas_needs_stub){
                 stack = get_zeroed_page(GFP_KERNEL);
                 if(stack == 0)
                         goto out;
@@ -102,33 +102,43 @@ int init_new_context_skas(struct task_struct *task, struct mm_struct *mm)
 
                 mm->nr_ptes--;
         }
-        mm_id->stack = stack;
+
+        to_mm->id.stack = stack;
+        if(current->mm != NULL && current->mm != &init_mm)
+                from_mm = &current->mm->context.skas;
 
         if(proc_mm){
-                if((cur_mm != NULL) && (cur_mm != &init_mm))
-                        from = cur_mm_id->u.mm_fd;
-                else from = -1;
+                if(from_mm)
+                        from_fd = from_mm->id.u.mm_fd;
+                else from_fd = -1;
 
-                ret = new_mm(from, stack);
+                ret = new_mm(from_fd, stack);
                 if(ret < 0){
                         printk("init_new_context_skas - new_mm failed, "
                                "errno = %d\n", ret);
                         goto out_free;
                 }
-                mm_id->u.mm_fd = ret;
+                to_mm->id.u.mm_fd = ret;
         }
         else {
-                if((cur_mm != NULL) && (cur_mm != &init_mm))
-                        mm_id->u.pid = copy_context_skas0(stack,
-                                                          cur_mm_id->u.pid);
-                else mm_id->u.pid = start_userspace(stack);
+                if(from_mm)
+                        to_mm->id.u.pid = copy_context_skas0(stack,
+                                                             from_mm->id.u.pid);
+                else to_mm->id.u.pid = start_userspace(stack);
+        }
+
+        ret = init_new_ldt(to_mm, from_mm);
+        if(ret < 0){
+                printk("init_new_context_skas - init_ldt"
+                       " failed, errno = %d\n", ret);
+                goto out_free;
         }
 
         return 0;
 
  out_free:
-        if(mm_id->stack != 0)
-                free_page(mm_id->stack);
+        if(to_mm->id.stack != 0)
+                free_page(to_mm->id.stack);
  out:
         return ret;
 }
diff --git a/arch/um/kernel/skas/process.c b/arch/um/kernel/skas/process.c
index 42f2da687dc8..599d679bd4fc 100644
--- a/arch/um/kernel/skas/process.c
+++ b/arch/um/kernel/skas/process.c
@@ -381,9 +381,9 @@ int copy_context_skas0(unsigned long new_stack, int pid)
 }
 
 /*
- * This is used only, if proc_mm is available, while PTRACE_FAULTINFO
- * isn't. Opening /proc/mm creates a new mm_context, which lacks the stub-pages
- * Thus, we map them using /proc/mm-fd
+ * This is used only, if stub pages are needed, while proc_mm is
+ * availabl. Opening /proc/mm creates a new mm_context, which lacks
+ * the stub-pages. Thus, we map them using /proc/mm-fd
  */
 void map_stub_pages(int fd, unsigned long code,
                     unsigned long data, unsigned long stack)
diff --git a/arch/um/kernel/skas/process_kern.c b/arch/um/kernel/skas/process_kern.c
index efe92e8aa2a9..9c990253966c 100644
--- a/arch/um/kernel/skas/process_kern.c
+++ b/arch/um/kernel/skas/process_kern.c
@@ -145,7 +145,7 @@ int new_mm(int from, unsigned long stack)
                                "err = %d\n", -n);
         }
 
-        if(!ptrace_faultinfo)
+        if(skas_needs_stub)
                 map_stub_pages(fd, CONFIG_STUB_CODE, CONFIG_STUB_DATA, stack);
 
         return(fd);
diff --git a/arch/um/os-Linux/start_up.c b/arch/um/os-Linux/start_up.c
index b99ab414542f..553a09c7d0bc 100644
--- a/arch/um/os-Linux/start_up.c
+++ b/arch/um/os-Linux/start_up.c
@@ -135,7 +135,9 @@ static int stop_ptraced_child(int pid, void *stack, int exitcode,
 }
 
 int ptrace_faultinfo = 1;
+int ptrace_ldt = 1;
 int proc_mm = 1;
+int skas_needs_stub = 0;
 
 static int __init skas0_cmd_param(char *str, int* add)
 {
@@ -352,14 +354,26 @@ __uml_setup("noptracefaultinfo", noptracefaultinfo_cmd_param,
 "    it. To support PTRACE_FAULTINFO, the host needs to be patched\n"
 "    using the current skas3 patch.\n\n");
 
+static int __init noptraceldt_cmd_param(char *str, int* add)
+{
+        ptrace_ldt = 0;
+        return 0;
+}
+
+__uml_setup("noptraceldt", noptraceldt_cmd_param,
+"noptraceldt\n"
+"    Turns off usage of PTRACE_LDT, even if host supports it.\n"
+"    To support PTRACE_LDT, the host needs to be patched using\n"
+"    the current skas3 patch.\n\n");
+
 #ifdef UML_CONFIG_MODE_SKAS
-static inline void check_skas3_ptrace_support(void)
+static inline void check_skas3_ptrace_faultinfo(void)
 {
         struct ptrace_faultinfo fi;
         void *stack;
         int pid, n;
 
-        printf("Checking for the skas3 patch in the host...");
+        printf("  - PTRACE_FAULTINFO...");
         pid = start_ptraced_child(&stack);
 
         n = ptrace(PTRACE_FAULTINFO, pid, 0, &fi);
@@ -381,9 +395,49 @@ static inline void check_skas3_ptrace_support(void)
         stop_ptraced_child(pid, stack, 1, 1);
 }
 
-int can_do_skas(void)
+static inline void check_skas3_ptrace_ldt(void)
+{
+#ifdef PTRACE_LDT
+        void *stack;
+        int pid, n;
+        unsigned char ldtbuf[40];
+        struct ptrace_ldt ldt_op = (struct ptrace_ldt) {
+                .func = 2, /* read default ldt */
+                .ptr = ldtbuf,
+                .bytecount = sizeof(ldtbuf)};
+
+        printf("  - PTRACE_LDT...");
+        pid = start_ptraced_child(&stack);
+
+        n = ptrace(PTRACE_LDT, pid, 0, (unsigned long) &ldt_op);
+        if (n < 0) {
+                if(errno == EIO)
+                        printf("not found\n");
+                else {
+                        perror("not found");
+                }
+                ptrace_ldt = 0;
+        }
+        else {
+                if(ptrace_ldt)
+                        printf("found\n");
+                else
+                        printf("found, but use is disabled\n");
+        }
+
+        stop_ptraced_child(pid, stack, 1, 1);
+#else
+        /* PTRACE_LDT might be disabled via cmdline option.
+         * We want to override this, else we might use the stub
+         * without real need
+         */
+        ptrace_ldt = 1;
+#endif
+}
+
+static inline void check_skas3_proc_mm(void)
 {
-        printf("Checking for /proc/mm...");
+        printf("  - /proc/mm...");
         if (os_access("/proc/mm", OS_ACC_W_OK) < 0) {
                 proc_mm = 0;
                 printf("not found\n");
@@ -394,8 +448,19 @@ int can_do_skas(void)
                 else
                         printf("found\n");
         }
+}
+
+int can_do_skas(void)
+{
+        printf("Checking for the skas3 patch in the host:\n");
+
+        check_skas3_proc_mm();
+        check_skas3_ptrace_faultinfo();
+        check_skas3_ptrace_ldt();
+
+        if(!proc_mm || !ptrace_faultinfo || !ptrace_ldt)
+                skas_needs_stub = 1;
 
-        check_skas3_ptrace_support();
         return 1;
 }
 #else
diff --git a/arch/um/scripts/Makefile.rules b/arch/um/scripts/Makefile.rules
index 651d9d88b656..b3fbf125709b 100644
--- a/arch/um/scripts/Makefile.rules
+++ b/arch/um/scripts/Makefile.rules
@@ -26,8 +26,13 @@ define unprofile
         $(patsubst -pg,,$(patsubst -fprofile-arcs -ftest-coverage,,$(1)))
 endef
 
+# cmd_make_link checks to see if the $(foo-dir) variable starts with a /.  If
+# so, it's considered to be a path relative to $(srcdir) rather than
+# $(srcdir)/arch/$(SUBARCH).  This is because x86_64 wants to get ldt.c from
+# arch/um/sys-i386 rather than arch/i386 like the other borrowed files.  So,
+# it sets $(ldt.c-dir) to /arch/um/sys-i386.
 quiet_cmd_make_link = SYMLINK $@
-cmd_make_link       = ln -sf $(srctree)/arch/$(SUBARCH)/$($(notdir $@)-dir)/$(notdir $@) $@
+cmd_make_link       = rm -f $@; ln -sf $(srctree)$(if $(filter-out /%,$($(notdir $@)-dir)),/arch/$(SUBARCH))/$($(notdir $@)-dir)/$(notdir $@) $@
 
 # this needs to be before the foreach, because targets does not accept
 # complete paths like $(obj)/$(f). To make sure this works, use a := assignment
diff --git a/arch/um/sys-i386/ldt.c b/arch/um/sys-i386/ldt.c
index 36b5c2c13289..6360f1c958d0 100644
--- a/arch/um/sys-i386/ldt.c
+++ b/arch/um/sys-i386/ldt.c
@@ -3,53 +3,26 @@
  * Licensed under the GPL
  */
 
+#include "linux/stddef.h"
 #include "linux/config.h"
 #include "linux/sched.h"
 #include "linux/slab.h"
 #include "linux/types.h"
+#include "linux/errno.h"
 #include "asm/uaccess.h"
-#include "asm/ptrace.h"
 #include "asm/smp.h"
 #include "asm/ldt.h"
+#include "asm/unistd.h"
 #include "choose-mode.h"
 #include "kern.h"
 #include "mode_kern.h"
 
-#ifdef CONFIG_MODE_TT
-
 extern int modify_ldt(int func, void *ptr, unsigned long bytecount);
 
-static int do_modify_ldt_tt(int func, void *ptr, unsigned long bytecount)
-{
-        return modify_ldt(func, ptr, bytecount);
-}
-
-#endif
-
-#ifdef CONFIG_MODE_SKAS
-
-#include "skas.h"
-#include "skas_ptrace.h"
-
-static int do_modify_ldt_skas(int func, void *ptr, unsigned long bytecount)
-{
-        struct ptrace_ldt ldt;
-        u32 cpu;
-        int res;
-
-        ldt = ((struct ptrace_ldt) { .func      = func,
-                                     .ptr       = ptr,
-                                     .bytecount = bytecount });
-
-        cpu = get_cpu();
-        res = ptrace(PTRACE_LDT, userspace_pid[cpu], 0, (unsigned long) &ldt);
-        put_cpu();
-
-        return res;
-}
-#endif
+#ifdef CONFIG_MODE_TT
 
-int sys_modify_ldt(int func, void __user *ptr, unsigned long bytecount)
+static long do_modify_ldt_tt(int func, void __user *ptr,
+                              unsigned long bytecount)
 {
         struct user_desc info;
         int res = 0;
@@ -89,8 +62,7 @@ int sys_modify_ldt(int func, void __user *ptr, unsigned long bytecount)
                 goto out;
         }
 
-        res = CHOOSE_MODE_PROC(do_modify_ldt_tt, do_modify_ldt_skas, func,
-                                p, bytecount);
+        res = modify_ldt(func, p, bytecount);
         if(res < 0)
                 goto out;
 
@@ -108,3 +80,467 @@ out:
         kfree(buf);
         return res;
 }
+
+#endif
+
+#ifdef CONFIG_MODE_SKAS
+
+#include "skas.h"
+#include "skas_ptrace.h"
+#include "asm/mmu_context.h"
+
+long write_ldt_entry(struct mm_id * mm_idp, int func, struct user_desc * desc,
+                     void **addr, int done)
+{
+        long res;
+
+        if(proc_mm){
+                /* This is a special handling for the case, that the mm to
+                 * modify isn't current->active_mm.
+                 * If this is called directly by modify_ldt,
+                 *     (current->active_mm->context.skas.u == mm_idp)
+                 * will be true. So no call to switch_mm_skas(mm_idp) is done.
+                 * If this is called in case of init_new_ldt or PTRACE_LDT,
+                 * mm_idp won't belong to current->active_mm, but child->mm.
+                 * So we need to switch child's mm into our userspace, then
+                 * later switch back.
+                 *
+                 * Note: I'm unshure: should interrupts be disabled here?
+                 */
+                if(!current->active_mm || current->active_mm == &init_mm ||
+                   mm_idp != &current->active_mm->context.skas.id)
+                        switch_mm_skas(mm_idp);
+        }
+
+        if(ptrace_ldt) {
+                struct ptrace_ldt ldt_op = (struct ptrace_ldt) {
+                        .func = func,
+                        .ptr = desc,
+                        .bytecount = sizeof(*desc)};
+                u32 cpu;
+                int pid;
+
+                if(!proc_mm)
+                        pid = mm_idp->u.pid;
+                else {
+                        cpu = get_cpu();
+                        pid = userspace_pid[cpu];
+                }
+
+                res = ptrace(PTRACE_LDT, pid, 0, (unsigned long) &ldt_op);
+                if(res)
+                        res = errno;
+
+                if(proc_mm)
+                        put_cpu();
+        }
+        else {
+                void *stub_addr;
+                res = syscall_stub_data(mm_idp, (unsigned long *)desc,
+                                        (sizeof(*desc) + sizeof(long) - 1) &
+                                            ~(sizeof(long) - 1),
+                                        addr, &stub_addr);
+                if(!res){
+                        unsigned long args[] = { func,
+                                                 (unsigned long)stub_addr,
+                                                 sizeof(*desc),
+                                                 0, 0, 0 };
+                        res = run_syscall_stub(mm_idp, __NR_modify_ldt, args,
+                                               0, addr, done);
+                }
+        }
+
+        if(proc_mm){
+                /* This is the second part of special handling, that makes
+                 * PTRACE_LDT possible to implement.
+                 */
+                if(current->active_mm && current->active_mm != &init_mm &&
+                   mm_idp != &current->active_mm->context.skas.id)
+                        switch_mm_skas(&current->active_mm->context.skas.id);
+        }
+
+        return res;
+}
+
+static long read_ldt_from_host(void __user * ptr, unsigned long bytecount)
+{
+        int res, n;
+        struct ptrace_ldt ptrace_ldt = (struct ptrace_ldt) {
+                        .func = 0,
+                        .bytecount = bytecount,
+                        .ptr = (void *)kmalloc(bytecount, GFP_KERNEL)};
+        u32 cpu;
+
+        if(ptrace_ldt.ptr == NULL)
+                return -ENOMEM;
+
+        /* This is called from sys_modify_ldt only, so userspace_pid gives
+         * us the right number
+         */
+
+        cpu = get_cpu();
+        res = ptrace(PTRACE_LDT, userspace_pid[cpu], 0,
+                     (unsigned long) &ptrace_ldt);
+        put_cpu();
+        if(res < 0)
+                goto out;
+
+        n = copy_to_user(ptr, ptrace_ldt.ptr, res);
+        if(n != 0)
+                res = -EFAULT;
+
+  out:
+        kfree(ptrace_ldt.ptr);
+
+        return res;
+}
+
+/*
+ * In skas mode, we hold our own ldt data in UML.
+ * Thus, the code implementing sys_modify_ldt_skas
+ * is very similar to (and mostly stolen from) sys_modify_ldt
+ * for arch/i386/kernel/ldt.c
+ * The routines copied and modified in part are:
+ * - read_ldt
+ * - read_default_ldt
+ * - write_ldt
+ * - sys_modify_ldt_skas
+ */
+
+static int read_ldt(void __user * ptr, unsigned long bytecount)
+{
+        int i, err = 0;
+        unsigned long size;
+        uml_ldt_t * ldt = &current->mm->context.skas.ldt;
+
+        if(!ldt->entry_count)
+                goto out;
+        if(bytecount > LDT_ENTRY_SIZE*LDT_ENTRIES)
+                bytecount = LDT_ENTRY_SIZE*LDT_ENTRIES;
+        err = bytecount;
+
+        if(ptrace_ldt){
+                return read_ldt_from_host(ptr, bytecount);
+        }
+
+        down(&ldt->semaphore);
+        if(ldt->entry_count <= LDT_DIRECT_ENTRIES){
+                size = LDT_ENTRY_SIZE*LDT_DIRECT_ENTRIES;
+                if(size > bytecount)
+                        size = bytecount;
+                if(copy_to_user(ptr, ldt->entries, size))
+                        err = -EFAULT;
+                bytecount -= size;
+                ptr += size;
+        }
+        else {
+                for(i=0; i<ldt->entry_count/LDT_ENTRIES_PER_PAGE && bytecount;
+                         i++){
+                        size = PAGE_SIZE;
+                        if(size > bytecount)
+                                size = bytecount;
+                        if(copy_to_user(ptr, ldt->pages[i], size)){
+                                err = -EFAULT;
+                                break;
+                        }
+                        bytecount -= size;
+                        ptr += size;
+                }
+        }
+        up(&ldt->semaphore);
+
+        if(bytecount == 0 || err == -EFAULT)
+                goto out;
+
+        if(clear_user(ptr, bytecount))
+                err = -EFAULT;
+
+out:
+        return err;
+}
+
+static int read_default_ldt(void __user * ptr, unsigned long bytecount)
+{
+        int err;
+
+        if(bytecount > 5*LDT_ENTRY_SIZE)
+                bytecount = 5*LDT_ENTRY_SIZE;
+
+        err = bytecount;
+        /* UML doesn't support lcall7 and lcall27.
+         * So, we don't really have a default ldt, but emulate
+         * an empty ldt of common host default ldt size.
+         */
+        if(clear_user(ptr, bytecount))
+                err = -EFAULT;
+
+        return err;
+}
+
+static int write_ldt(void __user * ptr, unsigned long bytecount, int func)
+{
+        uml_ldt_t * ldt = &current->mm->context.skas.ldt;
+        struct mm_id * mm_idp = &current->mm->context.skas.id;
+        int i, err;
+        struct user_desc ldt_info;
+        struct ldt_entry entry0, *ldt_p;
+        void *addr = NULL;
+
+        err = -EINVAL;
+        if(bytecount != sizeof(ldt_info))
+                goto out;
+        err = -EFAULT;
+        if(copy_from_user(&ldt_info, ptr, sizeof(ldt_info)))
+                goto out;
+
+        err = -EINVAL;
+        if(ldt_info.entry_number >= LDT_ENTRIES)
+                goto out;
+        if(ldt_info.contents == 3){
+                if (func == 1)
+                        goto out;
+                if (ldt_info.seg_not_present == 0)
+                        goto out;
+        }
+
+        if(!ptrace_ldt)
+                down(&ldt->semaphore);
+
+        err = write_ldt_entry(mm_idp, func, &ldt_info, &addr, 1);
+        if(err)
+                goto out_unlock;
+        else if(ptrace_ldt) {
+        /* With PTRACE_LDT available, this is used as a flag only */
+                ldt->entry_count = 1;
+                goto out;
+        }
+
+        if(ldt_info.entry_number >= ldt->entry_count &&
+           ldt_info.entry_number >= LDT_DIRECT_ENTRIES){
+                for(i=ldt->entry_count/LDT_ENTRIES_PER_PAGE;
+                    i*LDT_ENTRIES_PER_PAGE <= ldt_info.entry_number;
+                    i++){
+                        if(i == 0)
+                                memcpy(&entry0, ldt->entries, sizeof(entry0));
+                        ldt->pages[i] = (struct ldt_entry *)
+                                        __get_free_page(GFP_KERNEL|__GFP_ZERO);
+                        if(!ldt->pages[i]){
+                                err = -ENOMEM;
+                                /* Undo the change in host */
+                                memset(&ldt_info, 0, sizeof(ldt_info));
+                                write_ldt_entry(mm_idp, 1, &ldt_info, &addr, 1);
+                                goto out_unlock;
+                        }
+                        if(i == 0) {
+                                memcpy(ldt->pages[0], &entry0, sizeof(entry0));
+                                memcpy(ldt->pages[0]+1, ldt->entries+1,
+                                       sizeof(entry0)*(LDT_DIRECT_ENTRIES-1));
+                        }
+                        ldt->entry_count = (i + 1) * LDT_ENTRIES_PER_PAGE;
+                }
+        }
+        if(ldt->entry_count <= ldt_info.entry_number)
+                ldt->entry_count = ldt_info.entry_number + 1;
+
+        if(ldt->entry_count <= LDT_DIRECT_ENTRIES)
+                ldt_p = ldt->entries + ldt_info.entry_number;
+        else
+                ldt_p = ldt->pages[ldt_info.entry_number/LDT_ENTRIES_PER_PAGE] +
+                        ldt_info.entry_number%LDT_ENTRIES_PER_PAGE;
+
+        if(ldt_info.base_addr == 0 && ldt_info.limit == 0 &&
+           (func == 1 || LDT_empty(&ldt_info))){
+                ldt_p->a = 0;
+                ldt_p->b = 0;
+        }
+        else{
+                if (func == 1)
+                        ldt_info.useable = 0;
+                ldt_p->a = LDT_entry_a(&ldt_info);
+                ldt_p->b = LDT_entry_b(&ldt_info);
+        }
+        err = 0;
+
+out_unlock:
+        up(&ldt->semaphore);
+out:
+        return err;
+}
+
+static long do_modify_ldt_skas(int func, void __user *ptr,
+                               unsigned long bytecount)
+{
+        int ret = -ENOSYS;
+
+        switch (func) {
+                case 0:
+                        ret = read_ldt(ptr, bytecount);
+                        break;
+                case 1:
+                case 0x11:
+                        ret = write_ldt(ptr, bytecount, func);
+                        break;
+                case 2:
+                        ret = read_default_ldt(ptr, bytecount);
+                        break;
+        }
+        return ret;
+}
+
+short dummy_list[9] = {0, -1};
+short * host_ldt_entries = NULL;
+
+void ldt_get_host_info(void)
+{
+        long ret;
+        struct ldt_entry * ldt;
+        int i, size, k, order;
+
+        host_ldt_entries = dummy_list+1;
+
+        for(i = LDT_PAGES_MAX-1, order=0; i; i>>=1, order++);
+
+        ldt = (struct ldt_entry *)
+              __get_free_pages(GFP_KERNEL|__GFP_ZERO, order);
+        if(ldt == NULL) {
+                printk("ldt_get_host_info: couldn't allocate buffer for host ldt\n");
+                return;
+        }
+
+        ret = modify_ldt(0, ldt, (1<<order)*PAGE_SIZE);
+        if(ret < 0) {
+                printk("ldt_get_host_info: couldn't read host ldt\n");
+                goto out_free;
+        }
+        if(ret == 0) {
+                /* default_ldt is active, simply write an empty entry 0 */
+                host_ldt_entries = dummy_list;
+                goto out_free;
+        }
+
+        for(i=0, size=0; i<ret/LDT_ENTRY_SIZE; i++){
+                if(ldt[i].a != 0 || ldt[i].b != 0)
+                        size++;
+        }
+
+        if(size < sizeof(dummy_list)/sizeof(dummy_list[0])) {
+                host_ldt_entries = dummy_list;
+        }
+        else {
+                size = (size + 1) * sizeof(dummy_list[0]);
+                host_ldt_entries = (short *)kmalloc(size, GFP_KERNEL);
+                if(host_ldt_entries == NULL) {
+                        printk("ldt_get_host_info: couldn't allocate host ldt list\n");
+                        goto out_free;
+                }
+        }
+
+        for(i=0, k=0; i<ret/LDT_ENTRY_SIZE; i++){
+                if(ldt[i].a != 0 || ldt[i].b != 0) {
+                        host_ldt_entries[k++] = i;
+                }
+        }
+        host_ldt_entries[k] = -1;
+
+out_free:
+        free_pages((unsigned long)ldt, order);
+}
+
+long init_new_ldt(struct mmu_context_skas * new_mm,
+                  struct mmu_context_skas * from_mm)
+{
+        struct user_desc desc;
+        short * num_p;
+        int i;
+        long page, err=0;
+        void *addr = NULL;
+
+        memset(&desc, 0, sizeof(desc));
+
+        if(!ptrace_ldt)
+                init_MUTEX(&new_mm->ldt.semaphore);
+
+        if(!from_mm){
+                /*
+                 * We have to initialize a clean ldt.
+                 */
+                if(proc_mm) {
+                        /*
+                         * If the new mm was created using proc_mm, host's
+                         * default-ldt currently is assigned, which normally
+                         * contains the call-gates for lcall7 and lcall27.
+                         * To remove these gates, we simply write an empty
+                         * entry as number 0 to the host.
+                         */
+                        err = write_ldt_entry(&new_mm->id, 1, &desc,
+                                              &addr, 1);
+                }
+                else{
+                        /*
+                         * Now we try to retrieve info about the ldt, we
+                         * inherited from the host. All ldt-entries found
+                         * will be reset in the following loop
+                         */
+                        if(host_ldt_entries == NULL)
+                                ldt_get_host_info();
+                        for(num_p=host_ldt_entries; *num_p != -1; num_p++){
+                                desc.entry_number = *num_p;
+                                err = write_ldt_entry(&new_mm->id, 1, &desc,
+                                                      &addr, *(num_p + 1) == -1);
+                                if(err)
+                                        break;
+                        }
+                }
+                new_mm->ldt.entry_count = 0;
+        }
+        else if (!ptrace_ldt) {
+                /* Our local LDT is used to supply the data for
+                 * modify_ldt(READLDT), if PTRACE_LDT isn't available,
+                 * i.e., we have to use the stub for modify_ldt, which
+                 * can't handle the big read buffer of up to 64kB.
+                 */
+                down(&from_mm->ldt.semaphore);
+                if(from_mm->ldt.entry_count <= LDT_DIRECT_ENTRIES){
+                        memcpy(new_mm->ldt.entries, from_mm->ldt.entries,
+                               sizeof(new_mm->ldt.entries));
+                }
+                else{
+                        i = from_mm->ldt.entry_count / LDT_ENTRIES_PER_PAGE;
+                        while(i-->0){
+                                page = __get_free_page(GFP_KERNEL|__GFP_ZERO);
+                                if (!page){
+                                        err = -ENOMEM;
+                                        break;
+                                }
+                                new_mm->ldt.pages[i] = (struct ldt_entry*)page;
+                                memcpy(new_mm->ldt.pages[i],
+                                       from_mm->ldt.pages[i], PAGE_SIZE);
+                        }
+                }
+                new_mm->ldt.entry_count = from_mm->ldt.entry_count;
+                up(&from_mm->ldt.semaphore);
+        }
+
+        return err;
+}
+
+
+void free_ldt(struct mmu_context_skas * mm)
+{
+        int i;
+
+        if(!ptrace_ldt && mm->ldt.entry_count > LDT_DIRECT_ENTRIES){
+                i = mm->ldt.entry_count / LDT_ENTRIES_PER_PAGE;
+                while(i-- > 0){
+                        free_page((long )mm->ldt.pages[i]);
+                }
+        }
+        mm->ldt.entry_count = 0;
+}
+#endif
+
+int sys_modify_ldt(int func, void __user *ptr, unsigned long bytecount)
+{
+        return(CHOOSE_MODE_PROC(do_modify_ldt_tt, do_modify_ldt_skas, func,
+                                ptr, bytecount));
+}
diff --git a/arch/um/sys-x86_64/Makefile b/arch/um/sys-x86_64/Makefile
index 06c3633457a2..ea977df395a1 100644
--- a/arch/um/sys-x86_64/Makefile
+++ b/arch/um/sys-x86_64/Makefile
@@ -5,7 +5,7 @@
 #
 
 #XXX: why into lib-y?
-lib-y = bitops.o bugs.o csum-partial.o delay.o fault.o mem.o memcpy.o \
+lib-y = bitops.o bugs.o csum-partial.o delay.o fault.o ldt.o mem.o memcpy.o \
         ptrace.o ptrace_user.o sigcontext.o signal.o stub.o \
         stub_segv.o syscalls.o syscall_table.o sysrq.o thunk.o
 
@@ -14,7 +14,7 @@ obj-$(CONFIG_MODULES) += module.o um_module.o
 
 USER_OBJS := ptrace_user.o sigcontext.o
 
-SYMLINKS = bitops.c csum-copy.S csum-partial.c csum-wrappers.c memcpy.S \
+SYMLINKS = bitops.c csum-copy.S csum-partial.c csum-wrappers.c ldt.c memcpy.S \
         thunk.S module.c
 
 include arch/um/scripts/Makefile.rules
@@ -23,6 +23,7 @@ bitops.c-dir = lib
 csum-copy.S-dir = lib
 csum-partial.c-dir = lib
 csum-wrappers.c-dir = lib
+ldt.c-dir = /arch/um/sys-i386
 memcpy.S-dir = lib
 thunk.S-dir = lib
 module.c-dir = kernel
diff --git a/arch/um/sys-x86_64/syscalls.c b/arch/um/sys-x86_64/syscalls.c
index 3259a4db4534..6acee5c4ada6 100644
--- a/arch/um/sys-x86_64/syscalls.c
+++ b/arch/um/sys-x86_64/syscalls.c
@@ -29,81 +29,6 @@ asmlinkage long sys_uname64(struct new_utsname __user * name)
 }
 
 #ifdef CONFIG_MODE_TT
-extern int modify_ldt(int func, void *ptr, unsigned long bytecount);
-
-long sys_modify_ldt_tt(int func, void *ptr, unsigned long bytecount)
-{
-        /* XXX This should check VERIFY_WRITE depending on func, check this
-         * in i386 as well.
-         */
-        if (!access_ok(VERIFY_READ, ptr, bytecount))
-                return -EFAULT;
-        return(modify_ldt(func, ptr, bytecount));
-}
-#endif
-
-#ifdef CONFIG_MODE_SKAS
-extern int userspace_pid[];
-
-#include "skas_ptrace.h"
-
-long sys_modify_ldt_skas(int func, void *ptr, unsigned long bytecount)
-{
-        struct ptrace_ldt ldt;
-        void *buf;
-        int res, n;
-
-        buf = kmalloc(bytecount, GFP_KERNEL);
-        if(buf == NULL)
-                return(-ENOMEM);
-
-        res = 0;
-
-        switch(func){
-        case 1:
-        case 0x11:
-                res = copy_from_user(buf, ptr, bytecount);
-                break;
-        }
-
-        if(res != 0){
-                res = -EFAULT;
-                goto out;
-        }
-
-        ldt = ((struct ptrace_ldt) { .func      = func,
-                                     .ptr       = buf,
-                                     .bytecount = bytecount });
-#warning Need to look up userspace_pid by cpu
-        res = ptrace(PTRACE_LDT, userspace_pid[0], 0, (unsigned long) &ldt);
-        if(res < 0)
-                goto out;
-
-        switch(func){
-        case 0:
-        case 2:
-                n = res;
-                res = copy_to_user(ptr, buf, n);
-                if(res != 0)
-                        res = -EFAULT;
-                else
-                        res = n;
-                break;
-        }
-
- out:
-        kfree(buf);
-        return(res);
-}
-#endif
-
-long sys_modify_ldt(int func, void *ptr, unsigned long bytecount)
-{
-        return(CHOOSE_MODE_PROC(sys_modify_ldt_tt, sys_modify_ldt_skas, func,
-                                ptr, bytecount));
-}
-
-#ifdef CONFIG_MODE_TT
 extern long arch_prctl(int code, unsigned long addr);
 
 static long arch_prctl_tt(int code, unsigned long addr)