[PATCH] uml: check for differences in host support

If running on a host not supporting TLS (for instance 2.4) we should report
that cleanly to the user, instead of printing not comprehensible "error 5" for
that.

Additionally, i386 and x86_64 support different ranges for
user_desc->entry_number, and we must account for that; we couldn't pass
ourselves -1 because we need to override previously existing TLS descriptors
which glibc has possibly set, so test at startup the range to use.

x86 and x86_64 existing ranges are hardcoded.

Signed-off-by: Paolo 'Blaisorblade' Giarrusso <blaisorblade@yahoo.it>
Acked-by: Jeff Dike <jdike@addtoit.com>
Signed-off-by: Andrew Morton <akpm@osdl.org>
Signed-off-by: Linus Torvalds <torvalds@osdl.org>

arch/um/include/os.h

@@ -173,6 +173,7 @@ extern int os_fchange_dir(int fd);
 extern void os_early_checks(void);
 extern int can_do_skas(void);
 extern void os_check_bugs(void);
+extern void check_host_supports_tls(int *supports_tls, int *tls_min);
 
 /* Make sure they are clear when running in TT mode. Required by
  * SEGV_MAYBE_FIXABLE */

arch/um/include/sysdep-i386/tls.h

@@ -25,4 +25,8 @@ typedef struct um_dup_user_desc {
 typedef struct user_desc user_desc_t;
 
 # endif /* __KERNEL__ */
+
+#define GDT_ENTRY_TLS_MIN_I386 6
+#define GDT_ENTRY_TLS_MIN_X86_64 12
+
 #endif /* _SYSDEP_TLS_H */

arch/um/include/user_util.h

@@ -8,6 +8,9 @@
 
 #include "sysdep/ptrace.h"
 
+/* Copied from kernel.h */
+#define ARRAY_SIZE(x) (sizeof(x) / sizeof((x)[0]))
+
 #define CATCH_EINTR(expr) while ((errno = 0, ((expr) < 0)) && (errno == EINTR))
 
 extern int mode_tt;

arch/um/os-Linux/sys-i386/Makefile

@@ -3,7 +3,7 @@
 # Licensed under the GPL
 #
 
-obj-$(CONFIG_MODE_SKAS) = registers.o
+obj-$(CONFIG_MODE_SKAS) = registers.o tls.o
 
 USER_OBJS := $(obj-y)
 

arch/um/os-Linux/sys-i386/tls.c

@@ -0,0 +1,33 @@
+#include <linux/unistd.h>
+#include "sysdep/tls.h"
+#include "user_util.h"
+
+static _syscall1(int, get_thread_area, user_desc_t *, u_info);
+
+/* Checks whether host supports TLS, and sets *tls_min according to the value
+ * valid on the host.
+ * i386 host have it == 6; x86_64 host have it == 12, for i386 emulation. */
+void check_host_supports_tls(int *supports_tls, int *tls_min) {
+	/* Values for x86 and x86_64.*/
+	int val[] = {GDT_ENTRY_TLS_MIN_I386, GDT_ENTRY_TLS_MIN_X86_64};
+	int i;
+
+	for (i = 0; i < ARRAY_SIZE(val); i++) {
+		user_desc_t info;
+		info.entry_number = val[i];
+
+		if (get_thread_area(&info) == 0) {
+			*tls_min = val[i];
+			*supports_tls = 1;
+			return;
+		} else {
+			if (errno == EINVAL)
+				continue;
+			else if (errno == ENOSYS)
+				*supports_tls = 0;
+				return;
+		}
+	}
+
+	*supports_tls = 0;
+}

arch/um/os-Linux/tls.c

@@ -48,8 +48,8 @@ int os_get_thread_area(user_desc_t *info, int pid)
 #ifdef UML_CONFIG_MODE_TT
 #include "linux/unistd.h"
 
-_syscall1(int, get_thread_area, user_desc_t *, u_info);
-_syscall1(int, set_thread_area, user_desc_t *, u_info);
+static _syscall1(int, get_thread_area, user_desc_t *, u_info);
+static _syscall1(int, set_thread_area, user_desc_t *, u_info);
 
 int do_set_thread_area_tt(user_desc_t *info)
 {

arch/um/sys-i386/tls.c

@@ -24,6 +24,10 @@
 #include "skas.h"
 #endif
 
+/* If needed we can detect when it's uninitialized. */
+static int host_supports_tls = -1;
+int host_gdt_entry_tls_min = -1;
+
 #ifdef CONFIG_MODE_SKAS
 int do_set_thread_area_skas(struct user_desc *info)
 {
@@ -157,11 +161,20 @@ void clear_flushed_tls(struct task_struct *task)
 	}
 }
 
-/* This in SKAS0 does not need to be used, since we have different host
- * processes. Nor will this need to be used when we'll add support to the host
+/* 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_skas(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. */
@@ -173,6 +186,9 @@ int arch_switch_tls_skas(struct task_struct *from, struct task_struct *to)
 
 int arch_switch_tls_tt(struct task_struct *from, struct task_struct *to)
 {
+	if (!host_supports_tls)
+		return 0;
+
 	if (needs_TLS_update(to))
 		return load_TLS(0, to);
 
@@ -256,6 +272,9 @@ 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;
 
@@ -287,6 +306,9 @@ int ptrace_set_thread_area(struct task_struct *child, int idx,
 {
 	struct user_desc info;
 
+	if (!host_supports_tls)
+		return -EIO;
+
 	if (copy_from_user(&info, user_desc, sizeof(info)))
 		return -EFAULT;
 
@@ -298,6 +320,9 @@ 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;
 
@@ -321,6 +346,9 @@ int ptrace_get_thread_area(struct task_struct *child, int idx,
 	struct user_desc info;
 	int ret;
 
+	if (!host_supports_tls)
+		return -EIO;
+
 	ret = get_tls_entry(child, &info, idx);
 	if (ret < 0)
 		goto out;
@@ -331,3 +359,26 @@ int ptrace_get_thread_area(struct task_struct *child, int idx,
 	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 1;
+}
+
+__initcall(__setup_host_supports_tls);

include/asm-um/segment.h

@@ -1,6 +1,10 @@
 #ifndef __UM_SEGMENT_H
 #define __UM_SEGMENT_H
 
-#include "asm/arch/segment.h"
+extern int host_gdt_entry_tls_min;
+
+#define GDT_ENTRY_TLS_ENTRIES 3
+#define GDT_ENTRY_TLS_MIN host_gdt_entry_tls_min
+#define GDT_ENTRY_TLS_MAX (GDT_ENTRY_TLS_MIN + GDT_ENTRY_TLS_ENTRIES - 1)
 
 #endif