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linux-kernel - Linux 内核序列“show”例程的返回值

问题描述

我正在尝试了解 Linux 内核中的序列文件。恕我直言，他们是一个被误解的野兽，考虑到这一点，我从网上编译了一个现成的内核模块。为了便于访问，我在此处复制（修改后的）代码：

#include <linux/module.h>
#include <linux/moduleparam.h>
#include <linux/init.h>
#include <linux/kernel.h>
#include <linux/proc_fs.h>
#include <linux/fs.h>
#include <linux/seq_file.h>
#include <linux/slab.h>

static int limit = 10; //default value, it can be changed here or
module_param(limit, int, S_IRUGO); //transfered as a module parameter

static int* even_ptr; //we will work with dynamic memory

/**
 * start
 */
static void *ct_seq_start(struct seq_file *s, loff_t *pos) {
     printk(KERN_INFO "Entering start(), pos = %Ld, seq-file pos = %lu.\n", *pos, s->count);

     if ((*pos) >= limit) {     // are we done?
         printk(KERN_INFO "Apparently, we're done.\n");
         return NULL;
     }
     //Allocate an integer to hold our increasing even value
     even_ptr = kmalloc(sizeof(int), GFP_KERNEL);

     if (!even_ptr)     // fatal kernel allocation error
         return NULL;

     printk(KERN_INFO "In start(), even_ptr = 0x%pX.\n", even_ptr);
     *even_ptr = (*pos)*2;
     return even_ptr;
}

/**
 * show
 */
static int ct_seq_show(struct seq_file *s, void *v) {
     printk(KERN_INFO "In show(), even = %d.\n", *(int*)v);
     seq_printf(s, "The current value of the even number is %d\n", *(int*)v);
     // return (*(int *)v); // <-- Instead of a customary 'zero' I return the value of 'v' which is the actual even number that we are printing to the kernel log as well as the seq file
     return 0;
}

/**
 * next
 */
static void *ct_seq_next(struct seq_file *s, void *v, loff_t *pos) {
     printk(KERN_INFO "In next(), v = 0x%pX, *(int*)v=%d, pos=%Ld, seq-file pos=%lu.\n", v, *(int*)v, *pos, s->count);

     (*pos)++;              //increase my position counter
     if (*pos >= limit)     //are we done?
          return NULL;

     *(int*)v += 2;         //to the next even value

     return v;
 }

/**
 * stop
 */
static void ct_seq_stop(struct seq_file *s, void *v) {
     printk(KERN_INFO "Entering stop().\n");

     if (v)
         printk(KERN_INFO "v is %pX.\n", v);
     else
         printk(KERN_INFO "v is null.\n");

     printk(KERN_INFO "In stop(), even_ptr = %pX.\n", even_ptr);

     if (even_ptr) {
         printk(KERN_INFO "Freeing and clearing even_ptr.\n");
         kfree(even_ptr);
         even_ptr = NULL;
     } else
         printk(KERN_INFO "even_ptr is already null.\n");
}

/**
 * This structure gathers functions which control the sequential reading
 */
static struct seq_operations ct_seq_ops = {
     .start = ct_seq_start,
     .next  = ct_seq_next,
     .stop  = ct_seq_stop,
     .show  = ct_seq_show
};

/**
 * This function is called when a file from /proc is opened
 */
static int ct_open(struct inode *inode, struct file *file) {
     return seq_open(file, &ct_seq_ops);
};

/**
 * This structure gathers functions for a /proc-file operations
 */
static struct file_operations ct_file_ops = {
     .owner   = THIS_MODULE,
     .open    = ct_open,
     .read    = seq_read,
     .llseek  = seq_lseek,
     .release = seq_release
};

/**
 * This function is called when this module is loaded into the kernel
 */
static int __init ct_init(void) {
     proc_create("evens", 0, NULL, &ct_file_ops);
     return 0;
}

/**
 * This function is called when this module is removed from the kernel
 */
static void __exit ct_exit(void) {
     remove_proc_entry("evens", NULL);
}

module_init(ct_init);
module_exit(ct_exit);

MODULE_LICENSE("GPL");

我的内核日志显示：

Jun 30 19:10:22 mintab kernel: [ 8213.539959] Entering start(), pos = 0, seq-file pos = 0.
Jun 30 19:10:22 mintab kernel: [ 8213.539960] In start(), even_ptr = 0xffff9241772a8000.
Jun 30 19:10:22 mintab kernel: [ 8213.539960] In show(), even = 0.
Jun 30 19:10:22 mintab kernel: [ 8213.539962] In next(), v = 0xffff9241772a8000, *(int*)v=0, pos=0, seq-file pos=42.
Jun 30 19:10:22 mintab kernel: [ 8213.539962] In show(), even = 2.
Jun 30 19:10:22 mintab kernel: [ 8213.539963] In next(), v = 0xffff9241772a8000, *(int*)v=2, pos=1, seq-file pos=42.
Jun 30 19:10:22 mintab kernel: [ 8213.539963] In show(), even = 4.
Jun 30 19:10:22 mintab kernel: [ 8213.539964] In next(), v = 0xffff9241772a8000, *(int*)v=4, pos=2, seq-file pos=42.
Jun 30 19:10:22 mintab kernel: [ 8213.539964] In show(), even = 6.
Jun 30 19:10:22 mintab kernel: [ 8213.539965] In next(), v = 0xffff9241772a8000, *(int*)v=6, pos=3, seq-file pos=42.
Jun 30 19:10:22 mintab kernel: [ 8213.539965] In show(), even = 8.
Jun 30 19:10:22 mintab kernel: [ 8213.539966] In next(), v = 0xffff9241772a8000, *(int*)v=8, pos=4, seq-file pos=42.
Jun 30 19:10:22 mintab kernel: [ 8213.539966] In show(), even = 10.
Jun 30 19:10:22 mintab kernel: [ 8213.539967] In next(), v = 0xffff9241772a8000, *(int*)v=10, pos=5, seq-file pos=42.
Jun 30 19:10:22 mintab kernel: [ 8213.539967] In show(), even = 12.
Jun 30 19:10:22 mintab kernel: [ 8213.539968] In next(), v = 0xffff9241772a8000, *(int*)v=12, pos=6, seq-file pos=42.
Jun 30 19:10:22 mintab kernel: [ 8213.539969] In show(), even = 14.
Jun 30 19:10:22 mintab kernel: [ 8213.539969] In next(), v = 0xffff9241772a8000, *(int*)v=14, pos=7, seq-file pos=42.
Jun 30 19:10:22 mintab kernel: [ 8213.539970] In show(), even = 16.
Jun 30 19:10:22 mintab kernel: [ 8213.539970] In next(), v = 0xffff9241772a8000, *(int*)v=16, pos=8, seq-file pos=42.
Jun 30 19:10:22 mintab kernel: [ 8213.539971] In show(), even = 18.
Jun 30 19:10:22 mintab kernel: [ 8213.539971] In next(), v = 0xffff9241772a8000, *(int*)v=18, pos=9, seq-file pos=42.
Jun 30 19:10:22 mintab kernel: [ 8213.539972] Entering stop().
Jun 30 19:10:22 mintab kernel: [ 8213.539972] v is null.
Jun 30 19:10:22 mintab kernel: [ 8213.539972] In stop(), even_ptr = ffff9241772a8000.
Jun 30 19:10:22 mintab kernel: [ 8213.539973] Freeing and clearing even_ptr.
Jun 30 19:10:22 mintab kernel: [ 8213.539996] Entering start(), pos = 10, seq-file pos = 0.
Jun 30 19:10:22 mintab kernel: [ 8213.539996] Apparently, we're done.
Jun 30 19:10:22 mintab kernel: [ 8213.539997] Entering stop().
Jun 30 19:10:22 mintab kernel: [ 8213.539997] v is null.
Jun 30 19:10:22 mintab kernel: [ 8213.539997] In stop(), even_ptr =           (null).
Jun 30 19:10:22 mintab kernel: [ 8213.539998] even_ptr is already null.

并且proc文件只给出以下输出然后停止： 偶数的当前值为0

我假设“显示”例程必须返回 0 才能被认为对底层 seq 文件生态系统没有错误。但是，一个小的正整数不会表示相同吗？还是他们以其他方式使用？

UPDATE1：正如我们的朋友 Tsyvarev 的回答所提到的，明确要求从节目例程中返回 0，因为其他任何事情都被视为失败，因此相应地恢复。话虽如此，任何人都可以解释为什么我们在用尽所有数字后再次进入 start 并且v是 finally NULL？我特别想知道我们为什么要这样做？

Jul  1 11:16:59 mintab kernel: [31765.915542] v is null.
Jul  1 11:16:59 mintab kernel: [31765.915542] In stop(), even_ptr = ffff9c7df619b340.
Jul  1 11:16:59 mintab kernel: [31765.915542] Freeing and clearing even_ptr.
Jul  1 11:16:59 mintab kernel: [31765.915551] Entering start(), pos = 200, seq-file pos = 0.
Jul  1 11:16:59 mintab kernel: [31765.915551] Apparently, we're done.
Jul  1 11:16:59 mintab kernel: [31765.915552] Entering stop().
Jul  1 11:16:59 mintab kernel: [31765.915552] v is null.
Jul  1 11:16:59 mintab kernel: [31765.915553] In stop(), even_ptr =           (null).

此外，欢迎提供更多链接到更详细和（希望）更简单的序列文件机制解释。

标签： linux-kernelkernel-modulesequencefile

解决方案

顺序文件在内核文档中描述，位于Documentation/filesystems/seq_file.txt。根据文档，show函数应该返回：

  • 0成功
  • -ERR（负错误代码）失败
  • SEQ_SKIP 丢弃当前项目的任何输出

当前实现将任何正返回值show视为 SEQ_SKIP：

// (from traverse() function in fs/seq_file.c)
// ...
error = m->op->show(m, p);

if (error < 0) // <-- negative return value is an error indicator
    break;
if (unlikely(error)) { // <-- this is for positive return values
    error = 0; // <-- as if 'show' returns 0...
    m->count = 0; // <-- ... but with output dropped
}

因此，如果您想报告show成功，只需返回 0。

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