[PATCH 1/20] UML - Runtime detection of host VMSPLIT on i386

From: Jeff Dike
Date: Thu Jan 17 2008 - 16:47:34 EST

Calculate TASK_SIZE at run-time by figuring out the host's VMSPLIT -
this is needed on i386 if UML is to run on hosts with varying VMSPLITs
without recompilation.

TASK_SIZE is now defined in terms of a variable, task_size. This gets
rid of an include of pgtable.h from processor.h, which can cause
include loops.

On i386, task_size is calculated early in boot by probing the address
space in a binary search to figure out where the boundary between
usable and non-usable memory is. This tries to make sure that a page
that is considered to be in userspace is, or can be made, read-write.
I'm concerned about a system-global VDSO page in kernel memory being
hit and considered to be a userspace page.

On x86_64, task_size is just the old value of CONFIG_TOP_ADDR.

A bunch of config variable are gone now. CONFIG_TOP_ADDR is directly
replaced by TASK_SIZE. NEST_LEVEL is gone since the relocation of
the stubs makes it irrelevant. All the HOST_VMSPLIT stuff is gone.
All references to these in arch/um/Makefile are also gone.

I noticed and fixed a missing extern in os.h when adding
os_get_task_size.

Note: This has been revised to fix the 32-bit UML on 64-bit host bug
that Miklos ran into.

Signed-off-by: Jeff Dike <jdike@xxxxxxxxxxxxxxx>
---
arch/um/Kconfig | 11 --
arch/um/Kconfig.i386 | 37 ---------
arch/um/Kconfig.x86_64 | 4 -
arch/um/Makefile | 11 --
arch/um/defconfig | 3
arch/um/include/as-layout.h | 2
arch/um/include/os.h | 5 +
arch/um/kernel/exec.c | 2
arch/um/kernel/um_arch.c | 16 +++-
arch/um/os-Linux/sys-i386/Makefile | 2
arch/um/os-Linux/sys-i386/task_size.c | 120 ++++++++++++++++++++++++++++++++
arch/um/os-Linux/sys-x86_64/Makefile | 2
arch/um/os-Linux/sys-x86_64/task_size.c | 5 +
include/asm-um/fixmap.h | 3
include/asm-um/processor-generic.h | 5 -
15 files changed, 153 insertions(+), 75 deletions(-)

Index: linux-2.6-git/arch/um/kernel/um_arch.c
===================================================================
--- linux-2.6-git.orig/arch/um/kernel/um_arch.c 2008-01-17 14:32:56.000000000 -0500
+++ linux-2.6-git/arch/um/kernel/um_arch.c 2008-01-17 14:34:15.000000000 -0500
@@ -224,6 +224,11 @@ static void __init uml_postsetup(void)
}

/* Set during early boot */
+unsigned long task_size;
+EXPORT_SYMBOL(task_size);
+
+unsigned long host_task_size;
+
unsigned long brk_start;
unsigned long end_iomem;
EXPORT_SYMBOL(end_iomem);
@@ -250,6 +255,13 @@ int __init linux_main(int argc, char **a
if (have_root == 0)
add_arg(DEFAULT_COMMAND_LINE);

+ host_task_size = os_get_task_size();
+ /*
+ * TASK_SIZE needs to be PGDIR_SIZE aligned or else exit_mmap craps
+ * out
+ */
+ task_size = host_task_size & PGDIR_MASK;
+
/* OS sanity checks that need to happen before the kernel runs */
os_early_checks();

@@ -286,7 +298,7 @@ int __init linux_main(int argc, char **a

highmem = 0;
iomem_size = (iomem_size + PAGE_SIZE - 1) & PAGE_MASK;
- max_physmem = CONFIG_TOP_ADDR - uml_physmem - iomem_size - MIN_VMALLOC;
+ max_physmem = TASK_SIZE - uml_physmem - iomem_size - MIN_VMALLOC;

/*
* Zones have to begin on a 1 << MAX_ORDER page boundary,
@@ -318,7 +330,7 @@ int __init linux_main(int argc, char **a
}

virtmem_size = physmem_size;
- avail = CONFIG_TOP_ADDR - start_vm;
+ avail = TASK_SIZE - start_vm;
if (physmem_size > avail)
virtmem_size = avail;
end_vm = start_vm + virtmem_size;
Index: linux-2.6-git/include/asm-um/processor-generic.h
===================================================================
--- linux-2.6-git.orig/include/asm-um/processor-generic.h 2008-01-17 14:32:56.000000000 -0500
+++ linux-2.6-git/include/asm-um/processor-generic.h 2008-01-17 14:34:15.000000000 -0500
@@ -11,7 +11,6 @@ struct pt_regs;
struct task_struct;

#include "asm/ptrace.h"
-#include "asm/pgtable.h"
#include "registers.h"
#include "sysdep/archsetjmp.h"

@@ -94,7 +93,9 @@ static inline void mm_copy_segments(stru
/*
* User space process size: 3GB (default).
*/
-#define TASK_SIZE (CONFIG_TOP_ADDR & PGDIR_MASK)
+extern unsigned long task_size;
+
+#define TASK_SIZE (task_size)

/* This decides where the kernel will search for a free chunk of vm
* space during mmap's.
Index: linux-2.6-git/arch/um/include/os.h
===================================================================
--- linux-2.6-git.orig/arch/um/include/os.h 2008-01-17 14:32:56.000000000 -0500
+++ linux-2.6-git/arch/um/include/os.h 2008-01-17 14:34:15.000000000 -0500
@@ -302,6 +302,9 @@ extern void sig_handler_common_skas(int
extern int os_arch_prctl(int pid, int code, unsigned long *addr);

/* tty.c */
-int get_pty(void);
+extern int get_pty(void);
+
+/* sys-$ARCH/task_size.c */
+extern unsigned long os_get_task_size(void);

#endif
Index: linux-2.6-git/arch/um/os-Linux/sys-i386/Makefile
===================================================================
--- linux-2.6-git.orig/arch/um/os-Linux/sys-i386/Makefile 2008-01-17 14:32:56.000000000 -0500
+++ linux-2.6-git/arch/um/os-Linux/sys-i386/Makefile 2008-01-17 14:34:15.000000000 -0500
@@ -3,7 +3,7 @@
# Licensed under the GPL
#

-obj-y = registers.o signal.o tls.o
+obj-y = registers.o signal.o task_size.o tls.o

USER_OBJS := $(obj-y)

Index: linux-2.6-git/arch/um/os-Linux/sys-i386/task_size.c
===================================================================
--- /dev/null 1970-01-01 00:00:00.000000000 +0000
+++ linux-2.6-git/arch/um/os-Linux/sys-i386/task_size.c 2008-01-17 14:34:15.000000000 -0500
@@ -0,0 +1,120 @@
+#include <stdio.h>
+#include <stdlib.h>
+#include <signal.h>
+#include <sys/mman.h>
+#include "longjmp.h"
+#include "kern_constants.h"
+
+static jmp_buf buf;
+
+static void segfault(int sig)
+{
+ longjmp(buf, 1);
+}
+
+static int page_ok(unsigned long page)
+{
+ unsigned long *address = (unsigned long *) (page << UM_KERN_PAGE_SHIFT);
+ unsigned long n = ~0UL;
+ void *mapped = NULL;
+ int ok = 0;
+
+ /*
+ * First see if the page is readable. If it is, it may still
+ * be a VDSO, so we go on to see if it's writable. If not
+ * then try mapping memory there. If that fails, then we're
+ * still in the kernel area. As a sanity check, we'll fail if
+ * the mmap succeeds, but gives us an address different from
+ * what we wanted.
+ */
+ if (setjmp(buf) == 0)
+ n = *address;
+ else {
+ mapped = mmap(address, UM_KERN_PAGE_SIZE,
+ PROT_READ | PROT_WRITE,
+ MAP_FIXED | MAP_PRIVATE | MAP_ANONYMOUS, -1, 0);
+ if (mapped == MAP_FAILED)
+ return 0;
+ if (mapped != address)
+ goto out;
+ }
+
+ /*
+ * Now, is it writeable? If so, then we're in user address
+ * space. If not, then try mprotecting it and try the write
+ * again.
+ */
+ if (setjmp(buf) == 0) {
+ *address = n;
+ ok = 1;
+ goto out;
+ } else if (mprotect(address, UM_KERN_PAGE_SIZE,
+ PROT_READ | PROT_WRITE) != 0)
+ goto out;
+
+ if (setjmp(buf) == 0) {
+ *address = n;
+ ok = 1;
+ }
+
+ out:
+ if (mapped != NULL)
+ munmap(mapped, UM_KERN_PAGE_SIZE);
+ return ok;
+}
+
+unsigned long os_get_task_size(void)
+{
+ struct sigaction sa, old;
+ unsigned long bottom = 0;
+ /*
+ * A 32-bit UML on a 64-bit host gets confused about the VDSO at
+ * 0xffffe000. It is mapped, is readable, can be reprotected writeable
+ * and written. However, exec discovers later that it can't be
+ * unmapped. So, just set the highest address to be checked to just
+ * below it. This might waste some address space on 4G/4G 32-bit
+ * hosts, but shouldn't hurt otherwise.
+ */
+ unsigned long top = 0xffffd000 >> UM_KERN_PAGE_SHIFT;
+ unsigned long test;
+
+ printf("Locating the top of the address space ... ");
+ fflush(stdout);
+
+ /*
+ * We're going to be longjmping out of the signal handler, so
+ * SA_DEFER needs to be set.
+ */
+ sa.sa_handler = segfault;
+ sigemptyset(&sa.sa_mask);
+ sa.sa_flags = SA_NODEFER;
+ sigaction(SIGSEGV, &sa, &old);
+
+ if (!page_ok(bottom)) {
+ fprintf(stderr, "Address 0x%x no good?\n",
+ bottom << UM_KERN_PAGE_SHIFT);
+ exit(1);
+ }
+
+ /* This could happen with a 4G/4G split */
+ if (page_ok(top))
+ goto out;
+
+ do {
+ test = bottom + (top - bottom) / 2;
+ if (page_ok(test))
+ bottom = test;
+ else
+ top = test;
+ } while (top - bottom > 1);
+
+out:
+ /* Restore the old SIGSEGV handling */
+ sigaction(SIGSEGV, &old, NULL);
+
+ top <<= UM_KERN_PAGE_SHIFT;
+ printf("0x%x\n", top);
+ fflush(stdout);
+
+ return top;
+}
Index: linux-2.6-git/arch/um/os-Linux/sys-x86_64/Makefile
===================================================================
--- linux-2.6-git.orig/arch/um/os-Linux/sys-x86_64/Makefile 2008-01-17 14:32:56.000000000 -0500
+++ linux-2.6-git/arch/um/os-Linux/sys-x86_64/Makefile 2008-01-17 14:34:15.000000000 -0500
@@ -3,7 +3,7 @@
# Licensed under the GPL
#

-obj-y = registers.o prctl.o signal.o
+obj-y = registers.o prctl.o signal.o task_size.o

USER_OBJS := $(obj-y)

Index: linux-2.6-git/arch/um/os-Linux/sys-x86_64/task_size.c
===================================================================
--- /dev/null 1970-01-01 00:00:00.000000000 +0000
+++ linux-2.6-git/arch/um/os-Linux/sys-x86_64/task_size.c 2008-01-17 14:34:15.000000000 -0500
@@ -0,0 +1,5 @@
+unsigned long os_get_task_size(unsigned long shift)
+{
+ /* The old value of CONFIG_TOP_ADDR */
+ return 0x7fc0000000;
+}
Index: linux-2.6-git/arch/um/Kconfig
===================================================================
--- linux-2.6-git.orig/arch/um/Kconfig 2008-01-17 14:32:56.000000000 -0500
+++ linux-2.6-git/arch/um/Kconfig 2008-01-17 14:34:15.000000000 -0500
@@ -216,17 +216,6 @@ config NR_CPUS
depends on SMP
default "32"

-config NEST_LEVEL
- int "Nesting level"
- default "0"
- help
- This is set to the number of layers of UMLs that this UML will be run
- in. Normally, this is zero, meaning that it will run directly on the
- host. Setting it to one will build a UML that can run inside a UML
- that is running on the host. Generally, if you intend this UML to run
- inside another UML, set CONFIG_NEST_LEVEL to one more than the host
- UML.
-
config HIGHMEM
bool "Highmem support (EXPERIMENTAL)"
depends on !64BIT && EXPERIMENTAL
Index: linux-2.6-git/arch/um/Kconfig.i386
===================================================================
--- linux-2.6-git.orig/arch/um/Kconfig.i386 2008-01-17 14:32:56.000000000 -0500
+++ linux-2.6-git/arch/um/Kconfig.i386 2008-01-17 14:34:15.000000000 -0500
@@ -23,43 +23,6 @@ config SEMAPHORE_SLEEPERS
bool
default y

-choice
- prompt "Host memory split"
- default HOST_VMSPLIT_3G
- help
- This is needed when the host kernel on which you run has a non-default
- (like 2G/2G) memory split, instead of the customary 3G/1G. If you did
- not recompile your own kernel but use the default distro's one, you can
- safely accept the "Default split" option.
-
- It can be enabled on recent (>=2.6.16-rc2) vanilla kernels via
- CONFIG_VM_SPLIT_*, or on previous kernels with special patches (-ck
- patchset by Con Kolivas, or other ones) - option names match closely the
- host CONFIG_VM_SPLIT_* ones.
-
- A lower setting (where 1G/3G is lowest and 3G/1G is higher) will
- tolerate even more "normal" host kernels, but an higher setting will be
- stricter.
-
- So, if you do not know what to do here, say 'Default split'.
-
-config HOST_VMSPLIT_3G
- bool "Default split (3G/1G user/kernel host split)"
-config HOST_VMSPLIT_3G_OPT
- bool "3G/1G user/kernel host split (for full 1G low memory)"
-config HOST_VMSPLIT_2G
- bool "2G/2G user/kernel host split"
-config HOST_VMSPLIT_1G
- bool "1G/3G user/kernel host split"
-endchoice
-
-config TOP_ADDR
- hex
- default 0xB0000000 if HOST_VMSPLIT_3G_OPT
- default 0x78000000 if HOST_VMSPLIT_2G
- default 0x40000000 if HOST_VMSPLIT_1G
- default 0xC0000000
-
config 3_LEVEL_PGTABLES
bool "Three-level pagetables (EXPERIMENTAL)"
default n
Index: linux-2.6-git/arch/um/Kconfig.x86_64
===================================================================
--- linux-2.6-git.orig/arch/um/Kconfig.x86_64 2008-01-17 14:32:56.000000000 -0500
+++ linux-2.6-git/arch/um/Kconfig.x86_64 2008-01-17 14:34:15.000000000 -0500
@@ -15,10 +15,6 @@ config SEMAPHORE_SLEEPERS
bool
default y

-config TOP_ADDR
- hex
- default 0x7fc0000000
-
config 3_LEVEL_PGTABLES
bool
default y
Index: linux-2.6-git/arch/um/Makefile
===================================================================
--- linux-2.6-git.orig/arch/um/Makefile 2008-01-17 14:32:56.000000000 -0500
+++ linux-2.6-git/arch/um/Makefile 2008-01-17 14:34:15.000000000 -0500
@@ -79,13 +79,6 @@ KERNEL_DEFINES = $(strip -Derrno=kernel_
KBUILD_CFLAGS += $(KERNEL_DEFINES)
KBUILD_CFLAGS += $(call cc-option,-fno-unit-at-a-time,)

-# These are needed for clean and mrproper, since in that case .config is not
-# included; the values here are meaningless
-
-CONFIG_NEST_LEVEL ?= 0
-
-SIZE = ($(CONFIG_NEST_LEVEL) * 0x20000000)
-
PHONY += linux

all: linux
@@ -120,10 +113,6 @@ CFLAGS_NO_HARDENING := $(call cc-option,
CONFIG_KERNEL_STACK_ORDER ?= 2
STACK_SIZE := $(shell echo $$[ 4096 * (1 << $(CONFIG_KERNEL_STACK_ORDER)) ] )

-ifndef START
- START = $(shell echo $$[ $(TOP_ADDR) - $(SIZE) ] )
-endif
-
CPPFLAGS_vmlinux.lds = -U$(SUBARCH) -DSTART=$(START) -DELF_ARCH=$(ELF_ARCH) \
-DELF_FORMAT="$(ELF_FORMAT)" -DKERNEL_STACK_SIZE=$(STACK_SIZE)

Index: linux-2.6-git/arch/um/defconfig
===================================================================
--- linux-2.6-git.orig/arch/um/defconfig 2008-01-17 14:32:56.000000000 -0500
+++ linux-2.6-git/arch/um/defconfig 2008-01-17 14:34:15.000000000 -0500
@@ -56,8 +56,6 @@ CONFIG_X86_TSC=y
CONFIG_UML_X86=y
# CONFIG_64BIT is not set
CONFIG_SEMAPHORE_SLEEPERS=y
-# CONFIG_HOST_2G_2G is not set
-CONFIG_TOP_ADDR=0xc0000000
# CONFIG_3_LEVEL_PGTABLES is not set
CONFIG_ARCH_HAS_SC_SIGNALS=y
CONFIG_ARCH_REUSE_HOST_VSYSCALL_AREA=y
@@ -81,7 +79,6 @@ CONFIG_BINFMT_MISC=m
# CONFIG_HPPFS is not set
CONFIG_MCONSOLE=y
CONFIG_MAGIC_SYSRQ=y
-CONFIG_NEST_LEVEL=0
# CONFIG_HIGHMEM is not set
CONFIG_KERNEL_STACK_ORDER=0

Index: linux-2.6-git/include/asm-um/fixmap.h
===================================================================
--- linux-2.6-git.orig/include/asm-um/fixmap.h 2008-01-17 14:32:56.000000000 -0500
+++ linux-2.6-git/include/asm-um/fixmap.h 2008-01-17 14:34:15.000000000 -0500
@@ -1,6 +1,7 @@
#ifndef __UM_FIXMAP_H
#define __UM_FIXMAP_H

+#include <asm/processor.h>
#include <asm/system.h>
#include <asm/kmap_types.h>
#include <asm/archparam.h>
@@ -57,7 +58,7 @@ extern void __set_fixmap (enum fixed_add
* at the top of mem..
*/

-#define FIXADDR_TOP (CONFIG_TOP_ADDR - 2 * PAGE_SIZE)
+#define FIXADDR_TOP (TASK_SIZE - 2 * PAGE_SIZE)
#define FIXADDR_SIZE (__end_of_fixed_addresses << PAGE_SHIFT)
#define FIXADDR_START (FIXADDR_TOP - FIXADDR_SIZE)

Index: linux-2.6-git/arch/um/include/as-layout.h
===================================================================
--- linux-2.6-git.orig/arch/um/include/as-layout.h 2008-01-17 14:28:26.000000000 -0500
+++ linux-2.6-git/arch/um/include/as-layout.h 2008-01-17 14:34:15.000000000 -0500
@@ -57,6 +57,8 @@ extern unsigned long _stext, _etext, _sd
extern unsigned long _unprotected_end;
extern unsigned long brk_start;

+extern unsigned long host_task_size;
+
extern int linux_main(int argc, char **argv);

extern void (*sig_info[])(int, struct uml_pt_regs *);
Index: linux-2.6-git/arch/um/kernel/exec.c
===================================================================
--- linux-2.6-git.orig/arch/um/kernel/exec.c 2008-01-17 14:28:50.000000000 -0500
+++ linux-2.6-git/arch/um/kernel/exec.c 2008-01-17 14:34:43.000000000 -0500
@@ -25,7 +25,7 @@ void flush_thread(void)

ret = unmap(&current->mm->context.id, 0, STUB_START, 0, &data);
ret = ret || unmap(&current->mm->context.id, STUB_END,
- TASK_SIZE - STUB_END, 1, &data);
+ host_task_size - STUB_END, 1, &data);
if (ret) {
printk(KERN_ERR "flush_thread - clearing address space failed, "
"err = %d\n", ret);
--
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