CODEWITHSUNDEEP
clinux-kernel
Stefan Kendall
Stefan Kendall

Reputation: 67802

Proper way to use copy_to_user?

I'm trying to define a system call that modifies the character buffer passed to it. Specifically, something like this:

...
asmlinkage int sys_mycall( char __user *buff, int len )
{
   char tmp[1000];
   copy_from_user(tmp, buff, len);
   /* change tmp here */
   copy_to_user( buff, &tmp, len );
}

Here, copy_to_user returns -1, and the buffer from the calling program is unchanged. What's happening?

Upvotes: 6

Views: 28905

Answers (2)

Alex
Alex

Reputation: 81

Remeber that tmp is already a pointer! Correct way to do it:

copy_to_user( buff, tmp, len );

Upvotes: 8

caf
caf

Reputation: 239011

That looks OK. It's possible that the buffer that userspace passed is mapped read-only - for example if it's in the text segment (eg. a string literal). By the way, this is probably what you want:

return copy_to_user(buff, &tmp, len) ? -EFAULT : 0;

Upvotes: 5

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