[PATCH] [3/11] SYSCTL: Add proc_rcu_string to manage sysctls using rcu strings




Add a helper to use the new rcu strings for managing access
to text sysctls. Conversions will be in follow-on patches.

An alternative would be to use seqlocks here, but RCU seemed
cleaner.

Signed-off-by: Andi Kleen <ak@xxxxxxxxxxxxxxx>

---
include/linux/sysctl.h | 2 +
kernel/sysctl.c | 66 +++++++++++++++++++++++++++++++++++++++++++++++++
kernel/sysctl_check.c | 1
3 files changed, 69 insertions(+)

Index: linux-2.6.33-rc1-ak/include/linux/sysctl.h
===================================================================
--- linux-2.6.33-rc1-ak.orig/include/linux/sysctl.h
+++ linux-2.6.33-rc1-ak/include/linux/sysctl.h
@@ -969,6 +969,8 @@ typedef int proc_handler (struct ctl_tab

extern int proc_dostring(struct ctl_table *, int,
void __user *, size_t *, loff_t *);
+extern int proc_rcu_string(struct ctl_table *, int,
+ void __user *, size_t *, loff_t *);
extern int proc_dointvec(struct ctl_table *, int,
void __user *, size_t *, loff_t *);
extern int proc_dointvec_minmax(struct ctl_table *, int,
Index: linux-2.6.33-rc1-ak/kernel/sysctl.c
===================================================================
--- linux-2.6.33-rc1-ak.orig/kernel/sysctl.c
+++ linux-2.6.33-rc1-ak/kernel/sysctl.c
@@ -50,6 +50,7 @@
#include <linux/ftrace.h>
#include <linux/slow-work.h>
#include <linux/perf_event.h>
+#include <linux/rcustring.h>

#include <asm/uaccess.h>
#include <asm/processor.h>
@@ -2016,6 +2017,60 @@ static int _proc_do_string(void* data, i
}

/**
+ * proc_rcu_string - sysctl string with rcu protection
+ * @table: the sysctl table
+ * @write: %TRUE if this is a write to the sysctl file
+ * @buffer: the user buffer
+ * @lenp: the size of the user buffer
+ * @ppos: file position
+ *
+ * Handle a string sysctl similar to proc_dostring.
+ * The main difference is that the data pointer in the table
+ * points to a pointer to a string. The string should be initially
+ * pointing to a statically allocated (as a C object, not on the heap)
+ * default. When it is replaced old uses will be protected by
+ * RCU. The reader should use rcu_read_lock()/unlock() or
+ * access_rcu_string().
+ */
+int proc_rcu_string(struct ctl_table *table, int write,
+ void __user *buffer, size_t *lenp, loff_t *ppos)
+{
+ int ret;
+
+ if (write) {
+ /* protect writers against each other */
+ static DEFINE_MUTEX(rcu_string_mutex);
+ char *old;
+ char *new;
+
+ new = alloc_rcu_string(table->maxlen, GFP_KERNEL);
+ if (!new)
+ return -ENOMEM;
+ mutex_lock(&rcu_string_mutex);
+ old = *(char **)(table->data);
+ strcpy(new, old);
+ ret = _proc_do_string(new, table->maxlen, write, buffer, lenp, ppos);
+ rcu_assign_pointer(*(char **)(table->data), new);
+ /*
+ * For the first initialization allow constant strings.
+ */
+ if (!kernel_address((unsigned long)old))
+ free_rcu_string(old);
+ mutex_unlock(&rcu_string_mutex);
+ } else {
+ char *str;
+
+ str = access_rcu_string(*(char **)(table->data), table->maxlen,
+ GFP_KERNEL);
+ if (!str)
+ return -ENOMEM;
+ ret = _proc_do_string(str, table->maxlen, write, buffer, lenp, ppos);
+ kfree(str);
+ }
+ return ret;
+}
+
+/**
* proc_dostring - read a string sysctl
* @table: the sysctl table
* @write: %TRUE if this is a write to the sysctl file
@@ -2030,6 +2085,10 @@ static int _proc_do_string(void* data, i
* and a newline '\n' is added. It is truncated if the buffer is
* not large enough.
*
+ * WARNING: this should be only used for read only strings
+ * or when you have a wrapper with special locking. Otherwise
+ * use proc_rcu_string to avoid races with the consumer.
+ *
* Returns 0 on success.
*/
int proc_dostring(struct ctl_table *table, int write,
@@ -2614,6 +2673,12 @@ int proc_dostring(struct ctl_table *tabl
return -ENOSYS;
}

+int proc_rcu_string(struct ctl_table *table, int write,
+ void __user *buffer, size_t *lenp, loff_t *ppos)
+{
+ return -ENOSYS;
+}
+
int proc_dointvec(struct ctl_table *table, int write,
void __user *buffer, size_t *lenp, loff_t *ppos)
{
@@ -2670,6 +2735,7 @@ EXPORT_SYMBOL(proc_dointvec_minmax);
EXPORT_SYMBOL(proc_dointvec_userhz_jiffies);
EXPORT_SYMBOL(proc_dointvec_ms_jiffies);
EXPORT_SYMBOL(proc_dostring);
+EXPORT_SYMBOL(proc_rcu_string);
EXPORT_SYMBOL(proc_doulongvec_minmax);
EXPORT_SYMBOL(proc_doulongvec_ms_jiffies_minmax);
EXPORT_SYMBOL(register_sysctl_table);
Index: linux-2.6.33-rc1-ak/kernel/sysctl_check.c
===================================================================
--- linux-2.6.33-rc1-ak.orig/kernel/sysctl_check.c
+++ linux-2.6.33-rc1-ak/kernel/sysctl_check.c
@@ -131,6 +131,7 @@ int sysctl_check_table(struct nsproxy *n
set_fail(&fail, table, "Directory with extra2");
} else {
if ((table->proc_handler == proc_dostring) ||
+ (table->proc_handler == proc_rcu_string) ||
(table->proc_handler == proc_dointvec) ||
(table->proc_handler == proc_dointvec_minmax) ||
(table->proc_handler == proc_dointvec_jiffies) ||
--
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