bpf: Introduce bpf_sysctl_{get,set}_new_value helpers

Add helpers to work with new value being written to sysctl by user
space.

bpf_sysctl_get_new_value() copies value being written to sysctl into
provided buffer.

bpf_sysctl_set_new_value() overrides new value being written by user
space with a one from provided buffer. Buffer should contain string
representation of the value, similar to what can be seen in /proc/sys/.

Both helpers can be used only on sysctl write.

File position matters and can be managed by an interface that will be
introduced separately. E.g. if user space calls sys_write to a file in
/proc/sys/ at file position = X, where X > 0, then the value set by
bpf_sysctl_set_new_value() will be written starting from X. If program
wants to override whole value with specified buffer, file position has
to be set to zero.

Documentation for the new helpers is provided in bpf.h UAPI.

Signed-off-by: Andrey Ignatov <rdna@fb.com>
Signed-off-by: Alexei Starovoitov <ast@kernel.org>

1 files changed, 17 insertions, 5 deletions

diff --git a/fs/proc/proc_sysctl.c b/fs/proc/proc_sysctl.c
index e01b02150340..023101c6f0d7 100644
--- a/fs/proc/proc_sysctl.c
+++ b/fs/proc/proc_sysctl.c
@@ -570,8 +570,8 @@ static ssize_t proc_sys_call_handler(struct file *filp, void __user *buf,
         struct inode *inode = file_inode(filp);
         struct ctl_table_header *head = grab_header(inode);
         struct ctl_table *table = PROC_I(inode)->sysctl_entry;
+        void *new_buf = NULL;
         ssize_t error;
-        size_t res;
 
         if (IS_ERR(head))
                 return PTR_ERR(head);
@@ -589,15 +589,27 @@ static ssize_t proc_sys_call_handler(struct file *filp, void __user *buf,
         if (!table->proc_handler)
                 goto out;
 
-        error = BPF_CGROUP_RUN_PROG_SYSCTL(head, table, write);
+        error = BPF_CGROUP_RUN_PROG_SYSCTL(head, table, write, buf, &count,
+                                           &new_buf);
         if (error)
                 goto out;
 
         /* careful: calling conventions are nasty here */
-        res = count;
-        error = table->proc_handler(table, write, buf, &res, ppos);
+        if (new_buf) {
+                mm_segment_t old_fs;
+
+                old_fs = get_fs();
+                set_fs(KERNEL_DS);
+                error = table->proc_handler(table, write, (void __user *)new_buf,
+                                            &count, ppos);
+                set_fs(old_fs);
+                kfree(new_buf);
+        } else {
+                error = table->proc_handler(table, write, buf, &count, ppos);
+        }
+
         if (!error)
-                error = res;
+                error = count;
 out:
         sysctl_head_finish(head);