sysctl: fold sysctl_writes_strict checks into helper

The mode sysctl_writes_strict positional checks keep being copy and pasted
as we add new proc handlers.  Just add a helper to avoid code duplication.

Link: http://lkml.kernel.org/r/20170519033554.18592-4-mcgrof@kernel.org
Signed-off-by: Luis R. Rodriguez <mcgrof@kernel.org>
Suggested-by: Kees Cook <keescook@chromium.org>
Cc: Al Viro <viro@zeniv.linux.org.uk>
Cc: "Eric W. Biederman" <ebiederm@xmission.com>
Cc: Alexey Dobriyan <adobriyan@gmail.com>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>

1 files changed, 32 insertions, 24 deletions

diff --git a/kernel/sysctl.c b/kernel/sysctl.c
index 02725178694a..6f3bb1f099fa 100644
--- a/kernel/sysctl.c
+++ b/kernel/sysctl.c
@@ -1971,6 +1971,32 @@ static void warn_sysctl_write(struct ctl_table *table)
 }
 
 /**
+ * proc_first_pos_non_zero_ignore - check if firs position is allowed
+ * @ppos: file position
+ * @table: the sysctl table
+ *
+ * Returns true if the first position is non-zero and the sysctl_writes_strict
+ * mode indicates this is not allowed for numeric input types. String proc
+ * hadlers can ignore the return value.
+ */
+static bool proc_first_pos_non_zero_ignore(loff_t *ppos,
+                                           struct ctl_table *table)
+{
+        if (!*ppos)
+                return false;
+
+        switch (sysctl_writes_strict) {
+        case SYSCTL_WRITES_STRICT:
+                return true;
+        case SYSCTL_WRITES_WARN:
+                warn_sysctl_write(table);
+                return false;
+        default:
+                return false;
+        }
+}
+
+/**
  * proc_dostring - read a string sysctl
  * @table: the sysctl table
  * @write: %TRUE if this is a write to the sysctl file
@@ -1990,8 +2016,8 @@ static void warn_sysctl_write(struct ctl_table *table)
 int proc_dostring(struct ctl_table *table, int write,
                   void __user *buffer, size_t *lenp, loff_t *ppos)
 {
-        if (write && *ppos && sysctl_writes_strict == SYSCTL_WRITES_WARN)
-                warn_sysctl_write(table);
+        if (write)
+                proc_first_pos_non_zero_ignore(ppos, table);
 
         return _proc_do_string((char *)(table->data), table->maxlen, write,
                                (char __user *)buffer, lenp, ppos);
@@ -2193,17 +2219,8 @@ static int __do_proc_dointvec(void *tbl_data, struct ctl_table *table,
                 conv = do_proc_dointvec_conv;
 
         if (write) {
-                if (*ppos) {
-                        switch (sysctl_writes_strict) {
-                        case SYSCTL_WRITES_STRICT:
-                                goto out;
-                        case SYSCTL_WRITES_WARN:
-                                warn_sysctl_write(table);
-                                break;
-                        default:
-                                break;
-                        }
-                }
+                if (proc_first_pos_non_zero_ignore(ppos, table))
+                        goto out;
 
                 if (left > PAGE_SIZE - 1)
                         left = PAGE_SIZE - 1;
@@ -2468,17 +2485,8 @@ static int __do_proc_doulongvec_minmax(void *data, struct ctl_table *table, int
         left = *lenp;
 
         if (write) {
-                if (*ppos) {
-                        switch (sysctl_writes_strict) {
-                        case SYSCTL_WRITES_STRICT:
-                                goto out;
-                        case SYSCTL_WRITES_WARN:
-                                warn_sysctl_write(table);
-                                break;
-                        default:
-                                break;
-                        }
-                }
+                if (proc_first_pos_non_zero_ignore(ppos, table))
+                        goto out;
 
                 if (left > PAGE_SIZE - 1)
                         left = PAGE_SIZE - 1;