procfs{2, 3}: Change to use offset parameter

To make sure the behavior of the read and write operations are correct
with offset, update it each time. Also, since it is using the offset,
modify the part of read for removing unnecessary variable.

examples/procfs2.c

@@ -55,6 +55,7 @@ static ssize_t procfile_write(struct file *file, const char __user *buff,
         return -EFAULT;
 
     procfs_buffer[procfs_buffer_size & (PROCFS_MAX_SIZE - 1)] = '\0';
+    *off += procfs_buffer_size;
     pr_info("procfile write %s\n", procfs_buffer);
 
     return procfs_buffer_size;

examples/procfs3.c

@@ -8,12 +8,13 @@
 #include <linux/sched.h>
 #include <linux/uaccess.h>
 #include <linux/version.h>
+#include <linux/minmax.h>
 
 #if LINUX_VERSION_CODE >= KERNEL_VERSION(5, 6, 0)
 #define HAVE_PROC_OPS
 #endif
 
-#define PROCFS_MAX_SIZE 2048
+#define PROCFS_MAX_SIZE 2048UL
 #define PROCFS_ENTRY_FILENAME "buffer2k"
 
 static struct proc_dir_entry *our_proc_file;
@@ -23,17 +24,15 @@ static unsigned long procfs_buffer_size = 0;
 static ssize_t procfs_read(struct file *filp, char __user *buffer,
                            size_t length, loff_t *offset)
 {
-    static int finished = 0;
-
-    if (finished) {
+    if (*offset || procfs_buffer_size == 0) {
         pr_debug("procfs_read: END\n");
-        finished = 0;
+        *offset = 0;
         return 0;
     }
-    finished = 1;
-
+    procfs_buffer_size = min(procfs_buffer_size, length);
     if (copy_to_user(buffer, procfs_buffer, procfs_buffer_size))
         return -EFAULT;
+    *offset += procfs_buffer_size;
 
     pr_debug("procfs_read: read %lu bytes\n", procfs_buffer_size);
     return procfs_buffer_size;
@@ -41,12 +40,10 @@ static ssize_t procfs_read(struct file *filp, char __user *buffer,
 static ssize_t procfs_write(struct file *file, const char __user *buffer,
                             size_t len, loff_t *off)
 {
-    if (len > PROCFS_MAX_SIZE)
-        procfs_buffer_size = PROCFS_MAX_SIZE;
-    else
-        procfs_buffer_size = len;
+    procfs_buffer_size = min(PROCFS_MAX_SIZE, len);
     if (copy_from_user(procfs_buffer, buffer, procfs_buffer_size))
         return -EFAULT;
+    *off += procfs_buffer_size;
 
     pr_debug("procfs_write: write %lu bytes\n", procfs_buffer_size);
     return procfs_buffer_size;