Invoking readlink system call from user space

[sandyrmk]

While intercepting read system call, I'm trying to get find the filename for a given file descriptor and print it. For this, I'm trying to invoke the readlink system call from user space...

/*global declaration*/
asmlinkage long (*original_readlink)(const char __user *,char __user *, int bufsiz);

/*Inside my_sys_read */
int len=0;
char readfilepath[100];
len = original_readlink("/proc/1735/fd/4",readfilepath,99);

/* In my init()*/
original_readlink=(void *)syscall_table[__NR_readlink];

But when I invoke this readlink function, Im getting a return value of -14. It is supposed to return -1 for an error and 0 for a success.. But I get -14. Do any of you have an idea on this? If so please could you help me...

[Rubberman]

What function does original_readlink() point to. It is a pointer to a function, but you don't show what function it has been assigned. FWIW, the readlink() function is a user-space function that does what you want. Read the man page: man 2 readlink

[sandyrmk]

Thank you for your response...

I initially tried using readlink function directly, but it threw up a compile-time error,
error: implicit declaration of function ‘readlink’

I included the header <asm/unistd.h>

My unistd.h has only the macro defined for readlink system call...

#define __NR_readlink 1035
__SYSCALL(__NR_readlink, sys_readlink)

Please let me know how can I invoke readlink in this case...

[Rubberman]

What happens if you only #include <unistd.h>? That should work. If it doesn't, then your compiler installation may be broken...