Support ia32 exec domains without CONFIG_IA32_SUPPORT

The attached patch enables the alternate root feature for programs running with a different personality such as PER_LINUX32, even though in-kernel 32 bit support is turned off.

Although this feature can be emulated at the user level, it's more efficient to do it in the kernel.

Tony, please apply.

-Arun

Register the ia32 exec domain even though CONFIG_IA32_SUPPORT and CONFIG_COMPAT are turned off. This is necessary to support the alternate root feature for programs running with PER_LINUX32.

@@ -11,6 +11,8 @@
 
 #ifndef __ASSEMBLY__
 
+extern int register_ia32_exec_domain(void);
+
 # ifdef CONFIG_IA32_SUPPORT
 
 extern void ia32_cpu_init (void);

@@ -765,3 +765,19 @@
 }
 
 EXPORT_SYMBOL(machine_power_off);
+
+struct exec_domain ia32_exec_domain;
+
+int __init
+register_ia32_exec_domain()
+{
+	ia32_exec_domain.name = "Linux/x86";
+	ia32_exec_domain.handler = NULL;
+	ia32_exec_domain.pers_low = PER_LINUX32;
+	ia32_exec_domain.pers_high = PER_LINUX32;
+	ia32_exec_domain.signal_map = default_exec_domain.signal_map;
+	ia32_exec_domain.signal_invmap = default_exec_domain.signal_invmap;
+	return register_exec_domain(&ia32_exec_domain);
+}
+
+__initcall(register_ia32_exec_domain);

@@ -29,7 +29,6 @@
 
 extern void die_if_kernel (char *str, struct pt_regs *regs, long err);
 
-struct exec_domain ia32_exec_domain;
 struct page *ia32_shared_page[NR_CPUS];
 unsigned long *ia32_boot_gdt;
 unsigned long *cpu_gdt_table[NR_CPUS];

@@ -211,14 +210,6 @@
 static int __init
 ia32_init (void)
 {
-	ia32_exec_domain.name = "Linux/x86";
-	ia32_exec_domain.handler = NULL;
-	ia32_exec_domain.pers_low = PER_LINUX32;
-	ia32_exec_domain.pers_high = PER_LINUX32;
-	ia32_exec_domain.signal_map = default_exec_domain.signal_map;
-	ia32_exec_domain.signal_invmap = default_exec_domain.signal_invmap;
-	register_exec_domain(&ia32_exec_domain);
-
 #if PAGE_SHIFT > IA32_PAGE_SHIFT
 	{
 		extern kmem_cache_t *partial_page_cachep;

This was rejected for qemu on various arches already. Please do it in your userland emulator.

I agree that there is no functionality issue here, but there is a performance issue. Doing it in the userland emulator is much slower. What's wrong with the patch ?

-Arun

PS: For others following this thread, the qemu stuff is at:
http://groups.google.com/groups?hl=en&lr=&threadm 030407063012%243336%40gated-at.bofh.it

Arun> I agree that there is no functionality issue here, but there Arun> is a performance issue. Doing it in the userland emulator is Arun> much slower. What's wrong with the patch ?

Arun> -Arun

Arun> PS: For others following this thread, the qemu stuff is at:
Arun> http://groups.google.com/groups?hl=en&lr=&threadm 030407063012%243336%40gated-at.bofh.it

Unfortunately, that thread ran out in a rather unhelpful manner, as far as I can see. Rusty seemed to agree that the performance-hit of doing it all in user-level was unacceptably high, but I didn't see any actual numbers. There was a suggestion to decouple the altroot from the personality which makes some sense, but nobody actually did anything about it?

	--david

I'd really like this issue to be resolved one way or the other. I'm not sure I've heard a convincing argument on why my original patch(which adds a new exec domain unconditionally) should not be applied.

I'm fine with the attached patch to set the altroot via a system call as well.

-Arun

Index: linux-2.6-cvs/arch/ia64/kernel/entry.S
===================================================================
RCS file: /home/adsharma/disk2/cvs/linux-2.5/arch/ia64/kernel/entry.S,v retrieving revision 1.49

diff -u -r1.49 entry.S
@@ -1527,7 +1527,7 @@
 	data8 sys_mq_getsetattr
 	data8 sys_ni_syscall			// reserved for kexec_load
 	data8 sys_ni_syscall
-	data8 sys_ni_syscall			// 1270
+	data8 sys_altroot			// 1270
 	data8 sys_ni_syscall
 	data8 sys_ni_syscall
 	data8 sys_ni_syscall

Index: linux-2.6-cvs/fs/namei.c
===================================================================
RCS file: /home/adsharma/disk2/cvs/linux-2.5/fs/namei.c,v retrieving revision 1.110

diff -u -r1.110 namei.c
@@ -897,7 +897,7 @@
 	return 1;
 }
 
-void set_fs_altroot(void)
+int set_fs_altroot(const char __user *altroot)
 {
 	char *emul = __emul_prefix();
 	struct nameidata nd;

@@ -905,12 +905,20 @@
 	struct dentry *dentry = NULL, *olddentry;
 	int err;
 
+	if (altroot) {
+		emul = getname(altroot);
+		if (IS_ERR(emul))
+			return PTR_ERR(emul);
+	}
+
 	if (!emul)
 		goto set_it;
 	err = path_lookup(emul, LOOKUP_FOLLOW|LOOKUP_DIRECTORY|LOOKUP_NOALT, &nd);
 	if (!err) {
 		mnt = nd.mnt;
 		dentry = nd.dentry;
+	} else {
+		return err;
 	}
 set_it:
 	write_lock(&current->fs->lock);

@@ -923,6 +931,33 @@
 		dput(olddentry);
 		mntput(oldmnt);
 	}
+	return 0;
+}
+
+asmlinkage int sys_altroot(const char __user * altroot)
+{
+	if (atomic_read(&current->fs->count) != 1) {
+		struct fs_struct *fsp, *ofsp;
+
+		fsp = copy_fs_struct(current->fs);
+		if (fsp == NULL) {
+			return -ENOMEM;
+		}
+
+		task_lock(current);
+		ofsp = current->fs;
+		current->fs = fsp;
+		task_unlock(current);
+
+		put_fs_struct(ofsp);
+	}
+
+	/*
+	 * At that point we are guaranteed to be the sole owner of
+	 * current->fs.
+	 */
+
+	return set_fs_altroot(altroot);
 }
 
 int fastcall path_lookup(const char *name, unsigned int flags, struct nameidata *nd)

Index: linux-2.6-cvs/fs/open.c
===================================================================
RCS file: /home/adsharma/disk2/cvs/linux-2.5/fs/open.c,v retrieving revision 1.71

diff -u -r1.71 open.c
@@ -584,7 +584,7 @@
 		goto dput_and_out;
 
 	set_fs_root(current->fs, nd.mnt, nd.dentry);
-	set_fs_altroot();
+	set_fs_altroot(NULL);
 	error = 0;
 dput_and_out:
 	path_release(&nd);

Index: linux-2.6-cvs/include/linux/fs_struct.h
===================================================================
RCS file: /home/adsharma/disk2/cvs/linux-2.5/include/linux/fs_struct.h,v retrieving revision 1.7

diff -u -r1.7 fs_struct.h
@@ -19,7 +19,7 @@
 }
 
 extern void exit_fs(struct task_struct *);
-extern void set_fs_altroot(void);
+extern int set_fs_altroot(const char __user *);
 extern void set_fs_root(struct fs_struct *, struct vfsmount *, struct dentry *);
 extern void set_fs_pwd(struct fs_struct *, struct vfsmount *, struct dentry *);
 extern struct fs_struct *copy_fs_struct(struct fs_struct *);

Index: linux-2.6-cvs/kernel/exec_domain.c
===================================================================
RCS file: /home/adsharma/disk2/cvs/linux-2.5/kernel/exec_domain.c,v retrieving revision 1.18

diff -u -r1.18 exec_domain.c
@@ -167,7 +167,7 @@
 	current->personality = personality;
 	oep = current_thread_info()->exec_domain;
 	current_thread_info()->exec_domain = ep;
-	set_fs_altroot();
+	set_fs_altroot(NULL);
 
 	module_put(oep->module);
 	return 0;

I think there is a memory leak. It needs a "putname()" call somewhere to match the

+ emul = getname(altroot);

If we are going this route, the discussion needs to happen on LKML as this is not an ia64 only kind of patch. But I'm still not sure that I understand what's wrong with the existing method of calling sys_personality() that we've been using for the past few years. Christoph ... can you expand a little on "we said no for qemu, so it's no to you too".

-Tony

I'd still like to see some number on how much smaller a userland emulation is. Best using qumu because that's opensource and we simply don't care about intel's propritary stuff. Else your patch looks pretty okay - but I'd need to go through linux-kernel and hooking up to all architectures.

[ cc'ing LKML - since this really belongs there ]

Original thread on linux-ia64 about supporting alternate root for user level emulators:

http://marc.theaimsgroup.com/?t=109708875700003&r=1&w=2

I've reattached the patch here for new readers.

I've prototyped a generic userland solution that covers just open and stat system calls (for completeness, all path walk related syscalls need to be covered) using the LD_PRELOAD approach.

I saw a 16% degradation in system time on this benchmark:

find /usr/src/linux -name '*.[chS]' | xargs grep fsck

mainly due to the doubling of the number of calls to open. Also, there was a slight increase in user time as well, due to malloc/free overhead.

shar file with the benchmark and system call interposer attached.

-Arun

Register the ia32 exec domain even though CONFIG_IA32_SUPPORT and CONFIG_COMPAT are turned off. This is necessary to support the alternate root feature for programs running with PER_LINUX32.

@@ -11,6 +11,8 @@
 
 #ifndef __ASSEMBLY__
 
+extern int register_ia32_exec_domain(void);
+
 # ifdef CONFIG_IA32_SUPPORT
 
 extern void ia32_cpu_init (void);

@@ -765,3 +765,19 @@
 }
 
 EXPORT_SYMBOL(machine_power_off);
+
+struct exec_domain ia32_exec_domain;
+
+int __init
+register_ia32_exec_domain()
+{
+	ia32_exec_domain.name = "Linux/x86";
+	ia32_exec_domain.handler = NULL;
+	ia32_exec_domain.pers_low = PER_LINUX32;
+	ia32_exec_domain.pers_high = PER_LINUX32;
+	ia32_exec_domain.signal_map = default_exec_domain.signal_map;
+	ia32_exec_domain.signal_invmap = default_exec_domain.signal_invmap;
+	return register_exec_domain(&ia32_exec_domain);
+}
+
+__initcall(register_ia32_exec_domain);

@@ -29,7 +29,6 @@
 
 extern void die_if_kernel (char *str, struct pt_regs *regs, long err);
 
-struct exec_domain ia32_exec_domain;
 struct page *ia32_shared_page[NR_CPUS];
 unsigned long *ia32_boot_gdt;
 unsigned long *cpu_gdt_table[NR_CPUS];

@@ -211,14 +210,6 @@
 static int __init
 ia32_init (void)
 {
-	ia32_exec_domain.name = "Linux/x86";
-	ia32_exec_domain.handler = NULL;
-	ia32_exec_domain.pers_low = PER_LINUX32;
-	ia32_exec_domain.pers_high = PER_LINUX32;
-	ia32_exec_domain.signal_map = default_exec_domain.signal_map;
-	ia32_exec_domain.signal_invmap = default_exec_domain.signal_invmap;
-	register_exec_domain(&ia32_exec_domain);
-
 #if PAGE_SHIFT > IA32_PAGE_SHIFT
 	{
 		extern kmem_cache_t *partial_page_cachep;

#!/bin/sh
# This is a shell archive (produced by GNU sharutils 4.2.1). # To extract the files from this archive, save it to some FILE, remove

# everything before the `!/bin/sh' line above, then type `sh FILE'.
#
# Made on 2004-10-11 13:43 PDT by <[email protected]>.
# Source directory was `/home/adsharma'.
#
# Existing files will *not* be overwritten unless `-c' is specified.
#
# This shar contains:
# length mode       name
# ------ ---------- ------------------------------------------
#    157 -rw-r--r-- perf/Makefile
#   1227 -rw-r--r-- perf/namei.c
#     56 -rwxr-xr-x perf/test1.sh
#     88 -rwxr-xr-x perf/test.sh
#     85 -rw-r--r-- perf/results.txt
#
save_IFS="${IFS}"
IFS="${IFS}:"
gettext_dir=FAILED
locale_dir=FAILED
first_param="$1"
for dir in $PATH
do
  if test "$gettext_dir" = FAILED && test -f $dir/gettext \
     && ($dir/gettext --version >/dev/null 2>&1)
  then
    set `$dir/gettext --version 2>&1`
    if test "$3" = GNU
    then
      gettext_dir=$dir
    fi
  fi
  if test "$locale_dir" = FAILED && test -f $dir/shar \
     && ($dir/shar --print-text-domain-dir >/dev/null 2>&1)
  then
    locale_dir=`$dir/shar --print-text-domain-dir`
  fi
done
IFS="$save_IFS"
if test "$locale_dir" = FAILED || test "$gettext_dir" = FAILED
then
  echo=echo
else
  TEXTDOMAINDIR=$locale_dir
  export TEXTDOMAINDIR
  TEXTDOMAIN=sharutils
  export TEXTDOMAIN
  echo="$gettext_dir/gettext -s"
fi
if touch -am -t 200112312359.59 $$.touch >/dev/null 2>&1 && test ! -f 200112312359.59 -a -f $$.touch; then
  shar_touch='touch -am -t $1$2$3$4$5$6.$7 "$8"'
elif touch -am 123123592001.59 $$.touch >/dev/null 2>&1 && test ! -f 123123592001.59 -a ! -f 123123592001.5 -a -f $$.touch; then
  shar_touch='touch -am $3$4$5$6$1$2.$7 "$8"'
elif touch -am 1231235901 $$.touch >/dev/null 2>&1 && test ! -f 1231235901 -a -f $$.touch; then
  shar_touch='touch -am $3$4$5$6$2 "$8"'
else
  shar_touch=:
  echo
  $echo 'WARNING: not restoring timestamps.  Consider getting and'
  $echo "installing GNU \`touch', distributed in GNU File Utilities..."
  echo
fi
rm -f 200112312359.59 123123592001.59 123123592001.5 1231235901 $$.touch
#
if mkdir _sh21367; then
  $echo 'x -' 'creating lock directory'
else
  $echo 'failed to create lock directory'
  exit 1
fi
# ============= perf/Makefile ==============
if test ! -d 'perf'; then
  $echo 'x -' 'creating directory' 'perf'
  mkdir 'perf'
fi
if test -f 'perf/Makefile' && test "$first_param" != -c; then
  $echo 'x -' SKIPPING 'perf/Makefile' '(file already exists)'
else
  $echo 'x -' extracting 'perf/Makefile' '(text)'
  sed 's/^X//' << 'SHAR_EOF' > 'perf/Makefile' &&
CFLAGS		=	-D_GNU_SOURCE
X
all: libaltroot.so
X
libaltroot.so: namei.o
X	gcc -shared -g -fpic namei.o -o libaltroot.so -ldl
X
clean:
X	$(RM) namei.o libaltroot.so
SHAR_EOF
  (set 20 04 10 11 13 43 04 'perf/Makefile'; eval "$shar_touch") &&
  chmod 0644 'perf/Makefile' ||
  $echo 'restore of' 'perf/Makefile' 'failed'
  if ( md5sum --help 2>&1 | grep 'sage: md5sum \[' ) >/dev/null 2>&1 \
  && ( md5sum --version 2>&1 | grep -v 'textutils 1.12' ) >/dev/null; then
    md5sum -c << SHAR_EOF >/dev/null 2>&1 \
    || $echo 'perf/Makefile:' 'MD5 check failed'
c04da369bdfbb9e9a73346b393ac99a0  perf/Makefile
SHAR_EOF
  else
    shar_count="`LC_ALL= LC_CTYPE= LANG= wc -c < 'perf/Makefile'`"
    test 157 -eq "$shar_count" ||
    $echo 'perf/Makefile:' 'original size' '157,' 'current size' "$shar_count!"
  fi
fi
# ============= perf/namei.c ==============
if test -f 'perf/namei.c' && test "$first_param" != -c; then
  $echo 'x -' SKIPPING 'perf/namei.c' '(file already exists)'
else
  $echo 'x -' extracting 'perf/namei.c' '(text)'
  sed 's/^X//' << 'SHAR_EOF' > 'perf/namei.c' &&
#include <stdio.h>
#include <stdlib.h>
#include <signal.h>
#include <dlfcn.h>
#include <sys/types.h>
#include <sys/stat.h>
#include <linux/limits.h>
#include <fcntl.h>
X
#define EMULATE 1
X
int open(const char *pathname, int flags, ...)
{
X    static int (*func) (const char *, int, mode_t);
X    int ret;
X    mode_t mode;
X    char *buf;
X  
X    if (!func)
X	func = (int (*)()) dlsym(RTLD_NEXT, "open");
X
#ifdef EMULATE
X    if (*pathname == '/') {
X    	buf = malloc(PATH_MAX);
X	strncpy(buf, "/emul/ia32-linux", PATH_MAX);
X	strncat(buf, pathname, PATH_MAX-16);
X    	ret = func(buf, flags, mode);
X	if (ret != -1) {
X		free(buf);
X		return ret;
X	}
X	free(buf);
X    }
#endif
X
X    ret = func(pathname, flags, mode);
X
X    return ret;
}
X
int stat(const char *pathname, struct stat *buf1)
{
X    static int (*func) (const char *, struct stat *);
X    int ret;
X    char *buf;
X
X    if (!func)
X	func = (int (*)()) dlsym(RTLD_NEXT, "stat");
X
#ifdef EMULATE
X    if (*pathname == '/') {
X    	buf = malloc(PATH_MAX);
X	strncpy(buf, "/emul/ia32-linux", PATH_MAX);
X	strncat(buf, pathname, PATH_MAX-16);
X    	ret = func(buf, buf1);
X	if (ret != -1) {
X		free(buf);
X		return ret;
X	}
X	free(buf);
X    }
#endif
X
X    ret = func(pathname, buf1);
X
X    return ret;
}
SHAR_EOF
  (set 20 04 10 11 13 28 34 'perf/namei.c'; eval "$shar_touch") &&
  chmod 0644 'perf/namei.c' ||
  $echo 'restore of' 'perf/namei.c' 'failed'
  if ( md5sum --help 2>&1 | grep 'sage: md5sum \[' ) >/dev/null 2>&1 \
  && ( md5sum --version 2>&1 | grep -v 'textutils 1.12' ) >/dev/null; then
    md5sum -c << SHAR_EOF >/dev/null 2>&1 \
    || $echo 'perf/namei.c:' 'MD5 check failed'
968c756b5664cc2125b842b1750e40a8  perf/namei.c
SHAR_EOF
  else
    shar_count="`LC_ALL= LC_CTYPE= LANG= wc -c < 'perf/namei.c'`"
    test 1227 -eq "$shar_count" ||
    $echo 'perf/namei.c:' 'original size' '1227,' 'current size' "$shar_count!"
  fi
fi
# ============= perf/test1.sh ==============
if test -f 'perf/test1.sh' && test "$first_param" != -c; then
  $echo 'x -' SKIPPING 'perf/test1.sh' '(file already exists)'
else
  $echo 'x -' extracting 'perf/test1.sh' '(text)'
  sed 's/^X//' << 'SHAR_EOF' > 'perf/test1.sh' &&
#!/bin/sh
X
find `pwd` -name '*.[chS]' | xargs grep fsck
SHAR_EOF
  (set 20 04 10 11 13 41 25 'perf/test1.sh'; eval "$shar_touch") &&
  chmod 0755 'perf/test1.sh' ||
  $echo 'restore of' 'perf/test1.sh' 'failed'
  if ( md5sum --help 2>&1 | grep 'sage: md5sum \[' ) >/dev/null 2>&1 \
  && ( md5sum --version 2>&1 | grep -v 'textutils 1.12' ) >/dev/null; then
    md5sum -c << SHAR_EOF >/dev/null 2>&1 \
    || $echo 'perf/test1.sh:' 'MD5 check failed'
bd830f72d5ade56e00a8c5c4f0e4c5fe  perf/test1.sh
SHAR_EOF
  else
    shar_count="`LC_ALL= LC_CTYPE= LANG= wc -c < 'perf/test1.sh'`"
    test 56 -eq "$shar_count" ||
    $echo 'perf/test1.sh:' 'original size' '56,' 'current size' "$shar_count!"
  fi
fi
# ============= perf/test.sh ==============
if test -f 'perf/test.sh' && test "$first_param" != -c; then
  $echo 'x -' SKIPPING 'perf/test.sh' '(file already exists)'
else
  $echo 'x -' extracting 'perf/test.sh' '(text)'
  sed 's/^X//' << 'SHAR_EOF' > 'perf/test.sh' &&
#!/bin/sh
X
find `pwd` -name '*.[chS]' | LD_PRELOAD=~/perf/libaltroot.so xargs grep fsck
SHAR_EOF
  (set 20 04 10 11 13 41 25 'perf/test.sh'; eval "$shar_touch") &&
  chmod 0755 'perf/test.sh' ||
  $echo 'restore of' 'perf/test.sh' 'failed'
  if ( md5sum --help 2>&1 | grep 'sage: md5sum \[' ) >/dev/null 2>&1 \
  && ( md5sum --version 2>&1 | grep -v 'textutils 1.12' ) >/dev/null; then
    md5sum -c << SHAR_EOF >/dev/null 2>&1 \
    || $echo 'perf/test.sh:' 'MD5 check failed'
005c01ad066d70c7be266f1bec9074f6  perf/test.sh
SHAR_EOF
  else
    shar_count="`LC_ALL= LC_CTYPE= LANG= wc -c < 'perf/test.sh'`"
    test 88 -eq "$shar_count" ||
    $echo 'perf/test.sh:' 'original size' '88,' 'current size' "$shar_count!"
  fi
fi
# ============= perf/results.txt ==============
if test -f 'perf/results.txt' && test "$first_param" != -c; then
  $echo 'x -' SKIPPING 'perf/results.txt' '(file already exists)'
else
  $echo 'x -' extracting 'perf/results.txt' '(text)'
  sed 's/^X//' << 'SHAR_EOF' > 'perf/results.txt' &&
test1.sh:
X
real 14.49
user 14.09
sys 0.37
X
test.sh:
X
real 14.68
user 14.13
sys 0.43
X
SHAR_EOF
  (set 20 04 10 11 13 42 04 'perf/results.txt'; eval "$shar_touch") &&
  chmod 0644 'perf/results.txt' ||
  $echo 'restore of' 'perf/results.txt' 'failed'
  if ( md5sum --help 2>&1 | grep 'sage: md5sum \[' ) >/dev/null 2>&1 \
  && ( md5sum --version 2>&1 | grep -v 'textutils 1.12' ) >/dev/null; then
    md5sum -c << SHAR_EOF >/dev/null 2>&1 \
    || $echo 'perf/results.txt:' 'MD5 check failed'
db123bdf3d15d40c363be0924ccb2b6d  perf/results.txt
SHAR_EOF
  else
    shar_count="`LC_ALL= LC_CTYPE= LANG= wc -c < 'perf/results.txt'`"
    test 85 -eq "$shar_count" ||
    $echo 'perf/results.txt:' 'original size' '85,' 'current size' "$shar_count!"
  fi
fi
rm -fr _sh21367
exit 0

The patch is entirely bogus. This isn't at all ia64-specific, and doesn't live in arch/ia64. It's just as applicable on _all_ systems where we may want to do CPU or OS emulation.

If you make it generic so that qemu can use it for emulating i386 even on machines like ppc64, perhaps it would be saner.

dwmw2

There is already an architecture independent solution to the problem - set_fs_altroot(). The only problem is that it's not easily accessible from userland. Accessing it via set_personality() works only if there are multiple exec domains.

I can't see how anyone would accept a patch that adds linux-i386 exec domain in generic code.

Christoph doesn't like the idea of adding exec-domains just for this purpose and has suggested adding a new system call to set the altroot. A prototype patch to do this already exists. I will be cleaning it up and
posting it to LKML later this week. The main purpose of moving the
discussion to LKML was to see how receptive people were to the proposed new system call.

So far I haven't heard anyone complaining :)

-Arun

PS: Prototype patch with known bugs attached.

Index: linux-2.6-cvs/arch/ia64/kernel/entry.S
===================================================================
RCS file: /home/adsharma/disk2/cvs/linux-2.5/arch/ia64/kernel/entry.S,v retrieving revision 1.49

diff -u -r1.49 entry.S
@@ -1527,7 +1527,7 @@
 	data8 sys_mq_getsetattr
 	data8 sys_ni_syscall			// reserved for kexec_load
 	data8 sys_ni_syscall
-	data8 sys_ni_syscall			// 1270
+	data8 sys_altroot			// 1270
 	data8 sys_ni_syscall
 	data8 sys_ni_syscall
 	data8 sys_ni_syscall

Index: linux-2.6-cvs/fs/namei.c
===================================================================
RCS file: /home/adsharma/disk2/cvs/linux-2.5/fs/namei.c,v retrieving revision 1.110

diff -u -r1.110 namei.c
@@ -897,7 +897,7 @@
 	return 1;
 }
 
-void set_fs_altroot(void)
+int set_fs_altroot(const char __user *altroot)
 {
 	char *emul = __emul_prefix();
 	struct nameidata nd;

@@ -905,12 +905,20 @@
 	struct dentry *dentry = NULL, *olddentry;
 	int err;
 
+	if (altroot) {
+		emul = getname(altroot);
+		if (IS_ERR(emul))
+			return PTR_ERR(emul);
+	}
+
 	if (!emul)
 		goto set_it;
 	err = path_lookup(emul, LOOKUP_FOLLOW|LOOKUP_DIRECTORY|LOOKUP_NOALT, &nd);
 	if (!err) {
 		mnt = nd.mnt;
 		dentry = nd.dentry;
+	} else {
+		return err;
 	}
 set_it:
 	write_lock(&current->fs->lock);

@@ -923,6 +931,33 @@
 		dput(olddentry);
 		mntput(oldmnt);
 	}
+	return 0;
+}
+
+asmlinkage int sys_altroot(const char __user * altroot)
+{
+	if (atomic_read(&current->fs->count) != 1) {
+		struct fs_struct *fsp, *ofsp;
+
+		fsp = copy_fs_struct(current->fs);
+		if (fsp == NULL) {
+			return -ENOMEM;
+		}
+
+		task_lock(current);
+		ofsp = current->fs;
+		current->fs = fsp;
+		task_unlock(current);
+
+		put_fs_struct(ofsp);
+	}
+
+	/*
+	 * At that point we are guaranteed to be the sole owner of
+	 * current->fs.
+	 */
+
+	return set_fs_altroot(altroot);
 }
 
 int fastcall path_lookup(const char *name, unsigned int flags, struct nameidata *nd)

Index: linux-2.6-cvs/fs/open.c
===================================================================
RCS file: /home/adsharma/disk2/cvs/linux-2.5/fs/open.c,v retrieving revision 1.71

diff -u -r1.71 open.c
@@ -584,7 +584,7 @@
 		goto dput_and_out;
 
 	set_fs_root(current->fs, nd.mnt, nd.dentry);
-	set_fs_altroot();
+	set_fs_altroot(NULL);
 	error = 0;
 dput_and_out:
 	path_release(&nd);

Index: linux-2.6-cvs/include/linux/fs_struct.h
===================================================================
RCS file: /home/adsharma/disk2/cvs/linux-2.5/include/linux/fs_struct.h,v retrieving revision 1.7

diff -u -r1.7 fs_struct.h
@@ -19,7 +19,7 @@
 }
 
 extern void exit_fs(struct task_struct *);
-extern void set_fs_altroot(void);
+extern int set_fs_altroot(const char __user *);
 extern void set_fs_root(struct fs_struct *, struct vfsmount *, struct dentry *);
 extern void set_fs_pwd(struct fs_struct *, struct vfsmount *, struct dentry *);
 extern struct fs_struct *copy_fs_struct(struct fs_struct *);

Index: linux-2.6-cvs/kernel/exec_domain.c
===================================================================
RCS file: /home/adsharma/disk2/cvs/linux-2.5/kernel/exec_domain.c,v retrieving revision 1.18

diff -u -r1.18 exec_domain.c
@@ -167,7 +167,7 @@
 	current->personality = personality;
 	oep = current_thread_info()->exec_domain;
 	current_thread_info()->exec_domain = ep;
-	set_fs_altroot();
+	set_fs_altroot(NULL);
 
 	module_put(oep->module);
 	return 0;

Attached is the promised patch. It addresses Christoph's comments and fixes the bug Tony found as well.

-Arun

Add a new system call sys_altroot. This allows using the altroot feature on systems where there is only one exec domain.

diff -Nraup a/arch/ia64/kernel/entry.S b/arch/ia64/kernel/entry.S
@@ -1527,7 +1527,7 @@ sys_call_table:
 	data8 sys_mq_getsetattr
 	data8 sys_ni_syscall			// reserved for kexec_load
 	data8 sys_ni_syscall
-	data8 sys_ni_syscall			// 1270
+	data8 sys_setaltroot			// 1270
 	data8 sys_ni_syscall
 	data8 sys_ni_syscall
 	data8 sys_ni_syscall

diff -Nraup a/include/linux/syscalls.h b/include/linux/syscalls.h
@@ -489,6 +489,7 @@ asmlinkage long sys_nfsservctl(int cmd,
 				void __user *res);
 asmlinkage long sys_syslog(int type, char __user *buf, int len);
 asmlinkage long sys_uselib(const char __user *library);
+asmlinkage long sys_setaltroot(const char __user *altroot);
 asmlinkage long sys_ni_syscall(void);
 
 #endif

Arun> Attached is the promised patch. It addresses Christoph's Arun> comments and fixes the bug Tony found as well.

I like it. Once it's in (surely it will go in, no? ;-), we can change asm-ia64/namei.h to define a non-NULL __emul_prefix() only if CONFIG_IA32_SUPPORT is defined. One less hardcoded path to worry about!

	--david

It certainly looks saner, yes. If this is the way we're going, then the new syscall needs to be added on all other architectures, and I'd suggest that we should see a real open-source user of it too before it's merged. Does anyone have the qemu patches?

dwmw2

This is addressed in the most recent patch posted to the mailing list.

-Arun