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SubjectRe: dynamic sysctl registration (pre2.0).4
Hi,

On Fri, 17 May 96 0:13:36 EDT, Tom Dyas <tdyas@eden.rutgers.edu> said:

> The following patch adds support in the sysctl system for dynamically
> adding and removing sysctl entries from existing sysctl tables.

Umm, take another look at the code --- linux has supported this since
1.3.59. :)

> It is fundamental redesign of how sysctl's are stored.

No such redesign is necessary!!!

> They are now stored as linked lists much as the procfs directory
> entries are stored.

The existing code already stores them in linked lists. However, the
way we do it is to maintain a linked list of separate entire table
hierarchies, so that whole sets of sysctl entries may be registered
and deregistered at once.

> This support is needed because modules such as binfmt_java may want
> to register sysctl's in existing tables. The current system does not
> allow this at all.

Have a look at the "register_sysctl_table" and
"unregister_sysctl_table" functions. The register function takes a
sysctl hierarchy as argument, and adds it to the list of available
tables. Any sysctl() request will be matched against each table on
the list until a match is found.

The register function also inserts all of the entries on the list into
the /proc/sys filesystem. However, there is a problem with this in
current kernels --- we don't cleanly handle the case where a directory
in a new table overlaps an existing directory. Patch 1 below fixes
this, as well as fixing a memory leak when we deregister sysctl tables
and adding some necessary auxiliary functions to the ksyms exported
symbol table.

Patch 2 below updates binfmt_java to add /proc/sys/kernel/java*
entries dynamically when inserted as a module; it's a good example of
how to use this feature.

> Some minor typos in include/asm-i386/unistd.h are corrected. Just some
> extra underscores in the syscall number macros which prevented me from
> using _syscall1 to make a sysctl syscall function the first time
> around.

This is also wrong. The underscores are there for a very good reason.

BSD-4.4 specifies the sysctl() function as

int sysctl (int *name, int nlen,
void *oldval, size_t *oldlenp,
void *newval, size_t newlen);

However, the i386 unistd.h doesn't allow us to pass more than 5
arguments into a system call. To overcome this, we define the actual
system call interface differently:

int sys_sysctl(struct __sysctl_args *args);

In the libc (or wherever else we want to link in the system call), we
define two functions: sysctl(), which is a library function, takes the
full argument list and packs the args into a struct. This struct is
is then passed into the second function, _sysctl(), which is the real
kernel function. The extra underscore in unistd.h is there to allow
us access to _sysctl(), which is in the kernel, rather than sysctl(),
which is not.

The actual syscall interface should look something like:

>>>>
#include <linux/unistd.h>
#include <linux/types.h>
#include <linux/sysctl.h>

_syscall1(int, _sysctl, struct __sysctl_args *, args);

int sysctl (int *name, int nlen, void *oldval, size_t *oldlenp,
void *newval, size_t newlen)
{
struct __sysctl_args args = {name, nlen, oldval, oldlenp,
newval, newlen};
return _sysctl(&args);
}
<<<<

The patches below are against 1.99.4. With the new kernel, I can do:

>>>>
[root@dax modules]# ls /proc/sys/kernel
domainname file-nr inode-max osrelease panic version
file-max hostname inode-nr ostype securelevel
[root@dax modules]# insmod binfmt_java.o
[root@dax modules]# ls /proc/sys/kernel/
domainname inode-max osrelease version
file-max inode-nr ostype
file-nr java-appletviewer panic
hostname java-interpreter securelevel
[root@dax modules]# rmmod binfmt_java
[root@dax modules]# ls /proc/sys/kernel/
domainname file-nr inode-max osrelease panic version
file-max hostname inode-nr ostype securelevel
[root@dax modules]#
<<<<

Cheers,
Stephen.
--
Stephen Tweedie <sct@dcs.ed.ac.uk>
Department of Computer Science, Edinburgh University, Scotland.

Patch 1: fixup sysctl
----------------------------------------------------------------
Index: linux/kernel/sysctl.c
===================================================================
RCS file: /home/rcs/CVS/linux/kernel/sysctl.c,v
retrieving revision 1.6
diff -u -r1.6 sysctl.c
--- sysctl.c 1996/05/15 13:13:32 1.6
+++ sysctl.c 1996/05/17 22:10:59
@@ -384,6 +386,7 @@
#ifdef CONFIG_PROC_FS
unregister_proc_table(table->ctl_table, &proc_sys_root);
#endif
+ kfree(table);
}

/*
@@ -396,8 +399,10 @@
static void register_proc_table(ctl_table * table, struct proc_dir_entry *root)
{
struct proc_dir_entry *de;
+ int exists;

for (; table->ctl_name; table++) {
+ exists = 0;
/* Can't do anything without a proc name. */
if (!table->procname)
continue;
@@ -426,12 +431,30 @@
}
/* Otherwise it's a subdir */
else {
- de->ops = &proc_dir_inode_operations;
- de->nlink++;
- de->mode |= S_IFDIR;
+ /* First things first --- does the subdirectory already
+ exist? */
+ struct proc_dir_entry *tmp;
+
+ for (tmp = root->subdir; tmp; tmp = tmp->next) {
+ if (de->namelen == tmp->namelen &&
+ !strncmp(de->name, tmp->name, tmp->namelen)
+ ) {
+ exists = 1;
+ break;
+ }
+ }
+ if (exists) {
+ kfree(de);
+ de = tmp;
+ } else {
+ de->ops = &proc_dir_inode_operations;
+ de->nlink++;
+ de->mode |= S_IFDIR;
+ }
}
table->de = de;
- proc_register_dynamic(root, de);
+ if (!exists)
+ proc_register_dynamic(root, de);
if (de->mode & S_IFDIR )
register_proc_table(table->child, de);
}
@@ -449,6 +472,10 @@
continue;
}
unregister_proc_table(table->child, de);
+ /* If this part of the /proc tree still contains
+ entries, don't delete it! */
+ if (de->subdir)
+ continue;
}
proc_unregister(root, de->low_ino);
kfree(de);
Index: linux/kernel/ksyms.c
===================================================================
RCS file: /home/rcs/CVS/linux/kernel/ksyms.c,v
retrieving revision 1.7
diff -u -r1.7 ksyms.c
--- ksyms.c 1996/05/15 13:13:31 1.7
+++ ksyms.c 1996/05/17 19:44:10
@@ -230,6 +230,11 @@
/* sysctl table registration */
X(register_sysctl_table),
X(unregister_sysctl_table),
+ X(proc_dostring),
+ X(proc_dointvec),
+ X(proc_dointvec_minmax),
+ X(sysctl_string),
+ X(sysctl_intvec),

/* interrupt handling */
X(request_irq),
----------------------------------------------------------------
Patch 2: add dynamic /proc/sys/kernel/java*
----------------------------------------------------------------
Index: linux/fs/binfmt_java.c
===================================================================
RCS file: /home/rcs/CVS/linux/fs/binfmt_java.c,v
retrieving revision 1.1
diff -u -r1.1 binfmt_java.c
--- binfmt_java.c 1996/05/15 13:12:19 1.1
+++ binfmt_java.c 1996/05/17 21:04:17
@@ -19,6 +19,26 @@
char binfmt_java_interpreter[65] = _PATH_JAVA;
char binfmt_java_appletviewer[65] = _PATH_APPLET;

+#ifdef MODULE
+#include <linux/sysctl.h>
+static struct ctl_table_header *java_sysctl_header;
+static ctl_table java_root_table[];
+static ctl_table java_kernel_table[];
+
+static ctl_table java_root_table[] = {
+ {CTL_KERN, "kernel", NULL, 0, 0555, java_kern_table},
+ {0}
+};
+
+static ctl_table java_kern_table[] = {
+ {KERN_JAVA_INTERPRETER, "java-interpreter", binfmt_java_interpreter,
+ 64, 0644, NULL, &proc_dostring, &sysctl_string },
+ {KERN_JAVA_APPLETVIEWER, "java-appletviewer", binfmt_java_appletviewer,
+ 64, 0644, NULL, &proc_dostring, &sysctl_string },
+ {0}
+};
+#endif
+
static int do_load_script(struct linux_binprm *bprm,struct pt_regs *regs)
{
char *cp, *interp, *i_name;
@@ -179,11 +199,13 @@
#ifdef MODULE
int init_module(void)
{
+ java_sysctl_header = register_sysctl_table(java_root_table, 0);
return init_java_binfmt();
}

void cleanup_module( void) {
printk(KERN_INFO "Removing JAVA Binary support...\n");
+ unregister_sysctl_table(java_sysctl_header);
unregister_binfmt(&java_format);
unregister_binfmt(&applet_format);
}
----------------------------------------------------------------

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