On Fri, 26 Nov 1999, Arun Sharma wrote:
>
> You seem to have a misconception here. Lets take the example of sys_read
> (linux/fs/read_write.c). The argument buf is a userland absolute address
> (not relative to some base) of the form 0x8xxxxxxx, passed unmodified
> from the read system call in libc. You can try printing it out to
> confirm this.
>
> However, in traditional UNIX - for example FreeBSD, the syscall
> arguments get copied on to the kernel stack and a pointer to the
> "user area" is passed to the C routine that implements the system call.
>
> In both the cases, one needs to do copyin/copyout to transfer the data.
> All of this is contained in one file: /usr/include/asm/uaccess.h on
> Linux.
Thank you Arun for the reply.
This is what i understand, please correct me wherever i'm wrong.
Earlier, since we refer to "u area", we need to pass u->u_base to
copyin/copyout. But that is no more needed in newer versions and
linux also follows that. And hence there is no further need to keep that
absolute address in u area. Also is "struct task" the one
keeps informations related to u area.
Now i have a silly doubt. You say "0x8xxxxxxx", but what's the
significance of that '8' there.
Thanking you once again
sudheer
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