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Error Handling & Error Codes

Error Handling

• The Linux kernel coding standard document recommends using the goto statement of the C programming language for error handling. (Only for error handling)

  This is just a recommendation!

• Error handling structure using if...else statement:

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  1
  if (try_something_1 != Err)
  2
  {
  3
      if (try_something_2 != Err)
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      {
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          if (try_something_3 != Err)
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          {
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          }
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          else
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          {
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              undo_try_something_2;
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              undo_try_something_1;
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          }
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      }
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      else
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      {
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          undo_try_something_1;
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      }
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  }
  

  You must UNDO the previous successful operations when an error is encountered along the way because there might be some resources opened or held by those operations.

• Error handling structure using goto statement:

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  1
  if (try_something_1 == Err)
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      goto err1;
  3
  if (try_something_2 == Err)
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      goto err2;
  5
  if (try_something_3 == Err)
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      goto err3;
  7
  
  8
  err3:
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      undo_try_something_2;
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  err2:
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      undo_try_something_1;
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  err1:
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      return ret;
  

  Looks cleaner than when using if...else statement.

• if...else and goto statements can also be used in combination.

Error Handling of Pointers During Kernel Function Return

• Some kernel macros can be used to deal with return of error pointers by kernel functions.

  • The reason why the kernel functions do not simply return NULL upon fail is because NULL does not give any idea what went well. Instead, the kernel functions are generally designed to return a pointer that can be interpreted to a meaningful value that tells the user what went wrong.

• The following are most frequently used macros (defined in include/linux/err.h) help understand what caused the kernel function fail.

  • IS_ERR()

  • PTR_ERR()

  • ERR_PTR()

Error Codes and Descriptions

• Your device driver (or kernel module) should be designed to return an appropriate error code when an error happens.

• The user space global variable errno will be set to that error code from the kernel space so that the user space code is able to understand what has gone wrong in the kernel space.

• Error codes are defined in the file include/uapi/asm-generic/errno-base.h

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  1
  /* SPDX-License-Identifier: GPL-2.0 WITH Linux-syscall-note */
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  #ifndef _ASM_GENERIC_ERRNO_BASE_H
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  #define _ASM_GENERIC_ERRNO_BASE_H
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  #define EPERM        1  /* Operation not permitted */
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  #define ENOENT       2  /* No such file or directory */
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  #define ESRCH        3  /* No such process */
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  #define EINTR        4  /* Interrupted system call */
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  #define EIO          5  /* I/O error */
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  #define ENXIO        6  /* No such device or address */
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  #define E2BIG        7  /* Argument list too long */
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  #define ENOEXEC      8  /* Exec format error */
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  #define EBADF        9  /* Bad file number */
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  #define ECHILD      10  /* No child processes */
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  #define EAGAIN      11  /* Try again */
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  #define ENOMEM      12  /* Out of memory */
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  #define EACCES      13  /* Permission denied */
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  #define EFAULT      14  /* Bad address */
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  #define ENOTBLK     15  /* Block device required */
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  #define EBUSY       16  /* Device or resource busy */
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  #define EEXIST      17  /* File exists */
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  #define EXDEV       18  /* Cross-device link */
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  #define ENODEV      19  /* No such device */
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  #define ENOTDIR     20  /* Not a directory */
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  #define EISDIR      21  /* Is a directory */
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  #define EINVAL      22  /* Invalid argument */
  27
  #define ENFILE      23  /* File table overflow */
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  #define EMFILE      24  /* Too many open files */
  29
  #define ENOTTY      25  /* Not a typewriter */
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  #define ETXTBSY     26  /* Text file busy */
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  #define EFBIG       27  /* File too large */
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  #define ENOSPC      28  /* No space left on device */
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  #define ESPIPE      29  /* Illegal seek */
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  #define EROFS       30  /* Read-only file system */
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  #define EMLINK      31  /* Too many links */
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  #define EPIPE       32  /* Broken pipe */
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  #define EDOM        33  /* Math argument out of domain of func */
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  #define ERANGE      34  /* Math result not representable */
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  #endif