Read write files within a Linux kernel module

Question

I know all the discussions about why one should not read write files from kernel  instead how to use  proc or netlink to do that  I want to read write anyway  I have also read  Driving Me Nuts - Things You Never Should Do in the Kernel   However  the problem is that 2 6 30 does not export sys read    Rather it s wrapped in SYSCALL DEFINE3  So if I use it in my module  I get the following warnings   WARNING   sys read   xxx ko  undefined  WARNING   sys open   xxx ko  undefined    Obviously insmod cannot load the module because linking does not happen correctly   Questions    How to read write within kernel after 2 6 22  where sys read   sys open   are not exported   In general  how to use system calls wrapped in macro SYSCALL DEFINEn   from within the kernel

User · Answer

Since version 4 14 of Linux kernel  vfs read and vfs write functions are no longer exported for use in modules  Instead  functions exclusively for kernel s file access are provided     Read the file from the kernel space  ssize t kernel read struct file  file  void  buf  size t count  loff t  pos      Write the file from the kernel space  ssize t kernel write struct file  file  const void  buf  size t count              loff t  pos     Also  filp open no longer accepts user-space string  so it can be used for kernel access directly  without dance with set fs

User · Answer

You should be aware that you should avoid file I O from within Linux kernel when possible  The main idea is to go  one level deeper  and call VFS level functions instead of the syscall handler directly   Includes    include  lt linux fs h gt   include  lt asm segment h gt   include  lt asm uaccess h gt   include  lt linux buffer head h gt    Opening a file  similar to open    struct file  file open const char  path  int flags  int rights         struct file  filp   NULL      mm segment t oldfs      int err   0       oldfs   get fs        set fs get ds         filp   filp open path  flags  rights       set fs oldfs       if  IS ERR filp             err   PTR ERR filp           return NULL            return filp      Close a file  similar to close    void file close struct file  file         filp close file  NULL       Reading data from a file  similar to pread    int file read struct file  file  unsigned long long offset  unsigned char  data  unsigned int size         mm segment t oldfs      int ret       oldfs   get fs        set fs get ds          ret   vfs read file  data  size   amp offset        set fs oldfs       return ret         Writing data to a file  similar to pwrite    int file write struct file  file  unsigned long long offset  unsigned char  data  unsigned int size         mm segment t oldfs      int ret       oldfs   get fs        set fs get ds          ret   vfs write file  data  size   amp offset        set fs oldfs       return ret      Syncing changes a file  similar to fsync    int file sync struct file  file         vfs fsync file  0       return 0       Edit  Originally  I proposed using file fsync  which is gone in newer kernel versions  Thanks to the poor guy suggesting the change  but whose change was rejected  The edit was rejected before I could review it

[c] Read/write files within a Linux kernel module

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