bpf: Add file_pos field to bpf_sysctl ctx

Add file_pos field to bpf_sysctl context to read and write sysctl file
position at which sysctl is being accessed (read or written).

The field can be used to e.g. override whole sysctl value on write to
sysctl even when sys_write is called by user space with file_pos > 0. Or
BPF program may reject such accesses.

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

include/linux/filter.h

@@ -1188,6 +1188,9 @@ struct bpf_sysctl_kern {
 	size_t new_len;
 	int new_updated;
 	int write;
+	loff_t *ppos;
+	/* Temporary "register" for indirect stores to ppos. */
+	u64 tmp_reg;
 };
 
 #endif /* __LINUX_FILTER_H__ */