内核模块中task_cgroup_path函数在5.14-6.8版本编译失败问题

问题背景 #

我在内核模块中使用task_cgroup_path函数，该最小可复现示例在4.18版本内核可正常编译，但在5.14到6.8版本内核下编译失败。

原代码 #

#include <linux/module.h>
//#include <linux/printk.h>
//#include <linux/time.h>
#include <linux/cgroup.h>

struct timespec64 ts;

int init_module(void) 
{ 
    char *cpath_buf;
    int cpath_ret;

    cpath_buf = kmalloc(PATH_MAX, GFP_KERNEL);
    ktime_get_real_ts64((struct timespec64 *)&ts);
    cpath_ret = task_cgroup_path(current, cpath_buf, PATH_MAX);
//        pr_info("Hello world (%lld/%ld).\n", ts.tv_sec, ts.tv_nsec);
       
        return 0; 
} 

void cleanup_module(void) 
{ 
        pr_info("Goodbye world 0.\n"); 
} 

MODULE_LICENSE("GPL");

编译错误信息 #

root@kerndev:~/tst_time# make
make -C /lib/modules/6.8.0-55-generic/build M=/root/tst_time  modules 
make[1]: Entering directory '/usr/src/linux-headers-6.8.0-55-generic'
warning: the compiler differs from the one used to build the kernel
  The kernel was built by: x86_64-linux-gnu-gcc-13 (Ubuntu 13.3.0-6ubuntu2~24.04) 13.3.0
  You are using:           gcc-13 (Ubuntu 13.3.0-6ubuntu2~24.04) 13.3.0
  CC [M]  /root/tst_time/m_timespec.o
/root/tst_time/m_timespec.c: In function ‘init_module’:
/root/tst_time/m_timespec.c:15:21: error: implicit declaration of function ‘task_cgroup_path’; did you mean ‘sock_cgroup_ptr’? [-Werror=implicit-function-declaration]
   15 |         cpath_ret = task_cgroup_path(current, cpath_buf, PATH_MAX);
      |                     ^~~~~~~~~~~~~~~~
      |                     sock_cgroup_ptr
cc1: some warnings being treated as errors
make[3]: *** [scripts/Makefile.build:243: /root/tst_time/m_timespec.o] Error 1
make[2]: *** [/usr/src/linux-headers-6.8.0-55-generic/Makefile:1925: /root/tst_time] Error 2
make[1]: *** [Makefile:240: __sub-make] Error 2
make[1]: Leaving directory '/usr/src/linux-headers-6.8.0-55-generic'
make: *** [Makefile:6: all] Error 2
root@kerndev:~/tst_time# uname -a
Linux kerndev 6.8.0-55-generic #57-Ubuntu SMP PREEMPT_DYNAMIC Wed Feb 12 23:42:21 UTC 2025 x86_64 x86_64 x86_64 GNU/Linux

解决方案 #

task_cgroup_path函数在5.14版本内核中被移除，官方替代接口为task_cgroup_path_from_root，以下是适配后的代码：

#include <linux/module.h>
#include <linux/cgroup.h>
#include <linux/sched.h>
#include <linux/slab.h>

struct timespec64 ts;

int init_module(void) 
{ 
    char *cpath_buf;
    int cpath_ret;

    cpath_buf = kmalloc(PATH_MAX, GFP_KERNEL);
    if (!cpath_buf) {
        pr_err("kmalloc failed for cgroup path buffer\n");
        return -ENOMEM;
    }

    // 使用task_cgroup_path_from_root替代原task_cgroup_path
    // 传入NULL表示使用默认根cgroup
    cpath_ret = task_cgroup_path_from_root(current, cpath_buf, PATH_MAX, NULL);
    if (cpath_ret > 0) {
        pr_info("Current task cgroup path: %.*s\n", cpath_ret, cpath_buf);
    } else if (cpath_ret < 0) {
        pr_err("Failed to get cgroup path, err: %d\n", cpath_ret);
    }

    kfree(cpath_buf);
    return 0; 
} 

void cleanup_module(void) 
{ 
    pr_info("Goodbye world 0.\n"); 
} 

MODULE_LICENSE("GPL");

关键说明 #

• task_cgroup_path_from_root新增了根cgroup参数，传入NULL即可实现原task_cgroup_path的功能；
• 新增了内存分配失败的错误检查，避免空指针访问；
• 函数返回值为正表示成功写入的字符数，负数表示错误码，需根据返回值做相应处理。

内容的提问来源于stack exchange，提问作者flav