Read linked list using sequence files

Question

I'm trying to implement linked list (or circular linked list as it is defined in <linux/list.h>) traversal in linux kernel using sequence files while I want to print data from all nodes to seq file and read them in user space application. I'm not a big fan of C programming so I don't understand how to achieve this. I know how to read/write to proc file using procfs (read/write) functions. Right now I'm using those, so when client application asks for data from linux kernel in proc read function I simply go through the list and store all data in buffer which I later copy to user. Problem is that this buffer may overflow, that's why I want to use seq files.

I understand basic concept of seq files, but I don't know how to exactly do list traversal and printing as I can't pass pointer to my node structure to iterating function.

This is struct which represents linked list's node:

struct my_data {
    unsigned int id;
    unsigned char action;
    struct list_head list;
};

These are my definitions for seq_file operations

static void *seq_start(struct seq_file *s, loff_t *pos)
{
    /* The entire data structure for this iterator is a single loff_t 
     * value holding the current position.
     */
    loff_t *spos = kmalloc(sizeof(loff_t), GFP_KERNEL);

    if (!spos)
        return NULL;

    *spos = *pos;

    return spos;
}

static void *seq_next(struct seq_file *s, void *v, loff_t *pos)
{
    return pos;
}

static void seq_stop(struct seq_file *s, void *v)
{
}

static int seq_show(struct seq_file *s, void *v)
{
    seq_printf(s, "print stuff\n");
    return 0;
}

Now to the point. I would really appreciate if someone enlighten me how to iterate over linked list using seq_next() function and how to access data stored in nodes in seq_show().

There are only about a hundred examples in the Linux kernel of how to use seq files to display the contents of linked list. fs/proc is a good start. — Jonathon Reinhart
– Jonathon Reinhart, Commented May 3, 2015 at 12:44
Have you read Documentation/filesystems/seq_file.txt? Particularly, pay attention to seq_list_*() functions. You can find those functions usage in kernel and use found code as example. For example, see next files: drivers/input/input.c, drivers/char/misc.c. — Sam Protsenko
– Sam Protsenko, Commented May 3, 2015 at 13:13
@SamProtsenko This is exactly what I was looking for, thanks for pointing in right direction — mezo
– mezo, Commented May 3, 2015 at 13:27

Nayan · Accepted Answer · 2020-07-16 16:46:51Z

2

struct list_head test_head;   //head of the link list


static void *seq_start(struct seq_file *s, loff_t *pos)
{
    return seq_list_start(&test_head, *pos);  
}

static void *seq_next(struct seq_file *s, void *v, loff_t *pos)
{
    return seq_list_next(v, &test_head, pos);
}

static int seq_show(struct seq_file *s, void *v)
{
    struct my_data *entry;
    entry = list_entry(v, struct my_data, _list);
    seq_printf(seq, "id = %u and action = %u\n", entry->id, entry->action);
    return 0;
}

seq_list_start and seq_list_next functions have been implemented in fs\seq_file.c

answered Jul 16, 2020 at 16:46

Nayan

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