Date: Fri, 13 Jul 2001 11:38:22 +0930 From: Greg Lehey <grog@FreeBSD.org> To: y-carden@uniandes.edu.co Cc: FreeBSD Hackers <hackers@freebsd.org> Subject: Re: Some questions about kernel programming Message-ID: <20010713113822.V45037@wantadilla.lemis.com> In-Reply-To: <M2001071206580901828@Ayax.uniandes.edu.co>; from y-carden@uniandes.edu.co on Thu, Jul 12, 2001 at 06:58:09AM -0500 References: <M2001071206580901828@Ayax.uniandes.edu.co>
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On Thursday, 12 July 2001 at 6:58:09 -0500, y-carden@uniandes.edu.co wrote:
> Dear Friends
>
> I have some questions about kernel programming:
You'd be better off sending mail like this to -hackers. I've followed
up there.
> 1. Why I can call some system calls functions into the kernel but
> another not?, for example: I can call printf(), but I can't call
> socket().
You can't call system calls from the kernel. printf() is a library
call in userland; there's a different, but similar printf() in the
kernel.
> 2. Into kernel I can call the socket "low level" functions
> that this system calls invoke sosocket(), soconnect(), etc.
> but, How I do replace the send() system call? Perhaps, Can I call
> write() into kernel with same parameters?
> For example :
> /* res = send(skt, buf, buflen, 0); */
> res = write (skt, buf, buflen);
write() doesn't exist in the kernel. The simple answer is "you're
going to have to read what the send() syscall does and emulate it".
First, though, you need to answer the question "why do I want to do
this in the kernel?"
> 3. How I can copy a pointer string ( character array ) from user space to
> kernel space using copyin() without the following problem (I can't
> pass the length the explicitly from user land):
>
> struct MySystemCall_args {
> char * address;
> };
>
> int MySystemCall( p,uap)
> struct proc *p;
> register struct MySystemCall_args *uap;
> {
> char *the_address;
>
> printf(" ---> uap->address : %s\n", uap->address );
> printf(" ---> (strlen (uap->address) * sizeof(char)) : %d \n",
> (strlen (uap->address) * sizeof(char)) );
> copyin(uap->address, the_address, (strlen (uap->address) * sizeof(char))
> );
> printf("the_address: %s \n", the_address );
> printf("strlen (the_address): %d \n", strlen (the_address) );
>
> When this code run in mode kernel:
> ---> uap->address : 127.0.0.1
> ---> (strlen (uap->address) * sizeof(char)) : 9
> the_address : 127.0.0.1\M-"\M-Y\M-GX\M-p+\M-@@\M-_\M-*\M-@
> strlen (the_address): 20
>
> This crash the kernel later...
You've forgotten the terminating \0. Add one to the length.
Greg
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