Saner seq_file example adapted from kernel doc

kernel_module/seq_file.c

@@ -1,12 +1,27 @@
 /*
-Adapted from: http://allskyee.blogspot.co.uk/2011/12/proc-seqfile-write.html
+Adapted from: Documentation/filesystems/seq_file.txt
+but we limit the count to the max module parameter.
 
 Writting trivial read fops is repetitive and error prone.
 
 The seq_file API makes the process much easier for those trivial cases.
 
-There is not write version however, as writes are more complex:
+This example is behaves just like a file that contains:
+
+	0
+	1
+	2
+
+However, we only store a single integer in memory
+and calculate the file on the fly in an iterator fashion.
+
+There is not write version, as writes are more complex:
 https://stackoverflow.com/questions/30710517/how-to-implement-a-writable-proc-file-by-using-seq-file-in-a-driver-module
+
+Bibliography:
+
+- Documentation/filesystems/seq_file.txt
+- https://stackoverflow.com/questions/25399112/how-to-use-a-seq-file-in-linux-modules
 */
 
 #include <asm/uaccess.h> /* copy_from_user, copy_to_user */
@@ -16,43 +31,70 @@ https://stackoverflow.com/questions/30710517/how-to-implement-a-writable-proc-fi
 #include <linux/module.h>
 #include <linux/printk.h> /* pr_info */
 #include <linux/seq_file.h> /* seq_read, seq_lseek, single_release */
+#include <linux/slab.h>
 #include <uapi/linux/stat.h> /* S_IRUSR */
 
 MODULE_LICENSE("GPL");
 
+static int max = 2;
+module_param(max, int, S_IRUSR | S_IWUSR);
+
 static struct dentry *debugfs_file;
 
-static void * next(struct seq_file *s, void *v, loff_t *pos)
+/* Called at the beginning of every read.
+ *
+ * The return value is passsed to the first show.
+ * It normally represents the current position of the iterator.
+ * It could be any struct, but we use just a single integer here.
+ *
+ * NULL return means stop should be called next, and so the read will be empty..
+ * This happens for example for an ftell that goes beyond the file size.
+ */
+static void *start(struct seq_file *s, loff_t *pos)
 {
-	pr_info("next\n");
-    (*(unsigned long *)v)++;
-    (*pos)++;
-    return *pos < 3 ? v : NULL;
-}
-
-static void * start(struct seq_file *s, loff_t *pos)
-{
-    static unsigned long counter = 0;
+	loff_t *spos;
 
 	pr_info("start pos = %llx\n", (unsigned long long)*pos);
-    if (*pos == 0) {
-        return &counter;
-	} else {
-        *pos = 0;
-        return NULL;
-    }
+ 	spos = kmalloc(sizeof(loff_t), GFP_KERNEL);
+	if (!spos || *pos >= max)
+		return NULL;
+	*spos = *pos;
+	return spos;
 }
 
-static int show(struct seq_file *s, void *v)
+/* The return value is passed to next show.
+ * If NULL, stop is called next instead of show, and read ends.
+ *
+ * Can get called multiple times, until enough data is returned for the read.
+ */
+static void *next(struct seq_file *s, void *v, loff_t *pos)
 {
-	pr_info("show\n");
-    seq_printf(s, "%lx\n", *(unsigned long *)v);
-    return 0;
+	loff_t *spos;
+
+	spos = v;
+	pr_info("next pos = %llx\n", (unsigned long long)*pos);
+	if (*pos >= max)
+		return NULL;
+	*pos = ++*spos;
+	return spos;
 }
 
+/* Called at the end of every read. */
 static void stop(struct seq_file *s, void *v)
 {
 	pr_info("stop\n");
+	kfree(v);
+}
+
+/* Return 0 means success, SEQ_SKIP ignores previous prints, negative for error. */
+static int show(struct seq_file *s, void *v)
+{
+	loff_t *spos;
+
+	spos = v;
+	pr_info("show pos = %llx\n", (unsigned long long)*spos);
+	seq_printf(s, "%llx\n", (long long unsigned)*spos);
+	return 0;
 }
 
 static struct seq_operations my_seq_ops = {
@@ -64,6 +106,7 @@ static struct seq_operations my_seq_ops = {
 
 static int open(struct inode *inode, struct file *file)
 {
+	pr_info("open\n");
     return seq_open(file, &my_seq_ops);
 }
 
@@ -77,8 +120,13 @@ static struct file_operations fops = {
 
 static int myinit(void)
 {
-	debugfs_file = debugfs_create_file("lkmc_seq_file", S_IRUSR | S_IWUSR, NULL, NULL, &fops);
-	return 0;
+	debugfs_file = debugfs_create_file(
+		"lkmc_seq_file", S_IRUSR | S_IWUSR, NULL, NULL, &fops);
+	if (debugfs_file) {
+		return 0;
+	} else {
+		return -EINVAL;
+	}
 }
 
 static void myexit(void)

kernel_module/seq_file_single.c

@@ -1,5 +1,11 @@
 /*
-For single reads, single_open is an even more convenient version of seq_file.
+If you have the entire read output upfront, single_open
+is an even more convenient version of seq_file.
+
+This example behaves like a file that contains:
+
+	ab
+	cd
 */
 
 #include <asm/uaccess.h> /* copy_from_user, copy_to_user */
@@ -17,7 +23,7 @@ static struct dentry *debugfs_file;
 
 static int show(struct seq_file *m, void *v)
 {
-	seq_printf(m, "abcd\n");
+	seq_printf(m, "ab\ncd\n");
 	return 0;
 }
 
@@ -36,8 +42,13 @@ static const struct file_operations fops = {
 
 static int myinit(void)
 {
-	debugfs_file = debugfs_create_file("lkmc_seq_file", S_IRUSR | S_IWUSR, NULL, NULL, &fops);
-	return 0;
+	debugfs_file = debugfs_create_file(
+		"lkmc_seq_file_single", S_IRUSR | S_IWUSR, NULL, NULL, &fops);
+	if (debugfs_file) {
+		return 0;
+	} else {
+		return -EINVAL;
+	}
 }
 
 static void myexit(void)

rootfs_overlay/seq_file.sh

@@ -11,9 +11,17 @@ cat 'lkmc_seq_file'
 # => 2
 
 cat 'lkmc_seq_file'
-# => 3
-# => 4
-# => 5
+# => 0
+# => 1
+# => 2
 
-# TODO understand, why does this print nothing?
-dd if='lkmc_seq_file' bs=1 count=2 skip=2
+dd if='lkmc_seq_file' bs=1 count=2 skip=0 status=none
+# => 0
+dd if='lkmc_seq_file' bs=1 count=4 skip=0 status=none
+# => 0
+# => 1
+dd if='lkmc_seq_file' bs=1 count=2 skip=2 status=none
+# => 1
+dd if='lkmc_seq_file' bs=4 count=1 skip=0 status=none
+# => 0
+# => 1

rootfs_overlay/seq_file_single.sh

@@ -3,6 +3,8 @@ set -ex
 insmod /seq_file_single.ko
 cd /sys/kernel/debug
 cat 'lkmc_seq_file_single'
-# => abcd
-dd if='lkmc_seq_file_single' bs=1 skip=2
+# => ab
 # => cd
+dd if='lkmc_seq_file_single' bs=1 count=3 skip=1
+# => b
+# => c