在 Linux 内核中可写 seq_file

Question

我正在运行对 seq_file 进行一些实验，对此有些困惑。

我分析了seq_file.c and judging by seq_printf implementation the internal char *buf of the struct seq_file is used entirely to store a formatted string to copy to a user in seq_read. But there is seq_write function defined in in seq_file.c中常用函数的实现，可以写入缓冲区。

问题： 是否可以重用 struct seq_file 的内部缓冲区并将其用于写入来自用户还是仅用于数据格式化？

我目前使用另一个缓冲区来写入数据，struct seq_file 仅用于数据格式化：

static char buf[4096];    
static char *limit = buf;

void *pfsw_seq_start(struct seq_file *m, loff_t *pos){
    if(*pos >= limit - buf) {
        return NULL;
    }
    char *data = buf + *pos;
    *pos = limit - buf;
    return data;
}

void pfsw_seq_stop(struct seq_file *m, void *v){ }

void *pfsw_seq_next(struct seq_file *m, void *v, loff_t *pos){ return NULL; }

int pfsw_seq_show(struct seq_file *m, void *v){
    seq_printf(m, "Data: %s\n", (char *) v);
    return 0;
}

ssize_t pfsw_seq_write(struct file *filp, const char __user * user_data, size_t sz, loff_t *off){
    if(*off < 0 || *off > sizeof buf){
        return -EINVAL;
    }
    size_t space_left_from_off = sizeof buf - (size_t) *off;
    size_t bytes_to_write = space_left_from_off <= sz ? space_left_from_off : sz;
    if(copy_from_user(buf + *off, user_data, bytes_to_write)){
        return -EAGAIN;
    }
    *off += bytes_to_write;
    if(*off > limit - buf){
        limit = buf + *off;
    }
    return bytes_to_write;
}

所以我定义struct file_operations为

static const struct seq_operations seq_ops = {
    .start = pfsw_seq_start,
    .stop  = pfsw_seq_stop,
    .next  = pfsw_seq_next,
    .show  = pfsw_seq_show
};

int pfsw_seq_open(struct inode *ino, struct file *filp){ 
    return seq_open(filp, &seq_ops);
}

static const struct file_operations fops = {
    .open = pfsw_seq_open,
    .read = seq_read,
    .write = pfsw_seq_write,
    .release = seq_release,
};

Answer 1

您要找的帮手是simple_write_to_buffer。

给定缓冲区指针及其可用大小，帮助程序处理传递给 .write 函数的所有参数：

ssize_t pfsw_write(struct file *filp, const char __user * user_data, size_t sz, loff_t *off)
{
    // This performs the most work.
    ssize_t res = simple_write_to_buffer(buf, sizeof(buf), off, user_data, sz);

    // Optionally, you may update the buffer's actual size.
    // NOTE: In the simple form this doesn't work with **concurrent** writing.

    if ((res > 0) && *off > buf_size) {
      buf_size = *off;
    }

    return res;
}

simple_write_to_buffer 助手的 "pair" 是 simple_read_from_buffer。该助手可用于 .read 方法的实现：

ssize_t pfsw_read(struct file filp*, char __user * user_data, size_t sz, loff_t *off)
{
    // Here the helper does the whole work.
    //
    // 'buf_size' could be 'sizeof(buf)' in case of the buffer of the fixed size.
    // Alternatively, you need to take care about concurrency;
    // see the comments in the 'pfsw_write' function.
    return simple_read_from_buffer(buf, buf_size, off, user_data, sz);
}

子系统seq_file仅用于实现读取文件（.read函数）。

虽然 seq_read 是 .read 函数的实际实现，但 seq_write 只是一个 帮助程序 用于 struct seq_operations 中的 .show 函数。

有关 seq_file 和写入文件的相关问题，请参见。

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