Re: [patch 05/11] syslets: core code

[Pavel Machek <pavel@ucw.cz>]

Hi!

> > The upcall will setup a frame, execute the clet (where jump/conditions and 
> > userspace variable changes happen in machine code - gcc is pretty good in 
> > taking care of that for us) on its return, come back through a 
> > sys_async_return, and go back to userspace.
> 
> So, for example, this is the setup code for the current API (and that's a 
> really simple one - immagine going wacko with loops and userspace varaible 
> changes):
> 
> 
> static struct req *alloc_req(void)
> {
>         /*
>          * Constants can be picked up by syslets via static variables:
>          */
>         static long O_RDONLY_var = O_RDONLY;
>         static long FILE_BUF_SIZE_var = FILE_BUF_SIZE;
>                 
>         struct req *req;
>                  
>         if (freelist) {
>                 req = freelist;
>                 freelist = freelist->next_free;
>                 req->next_free = NULL;
>                 return req;
>         }
>                         
>         req = calloc(1, sizeof(struct req));
>  
>         /*
>          * This is the first atom in the syslet, it opens the file:
>          *
>          *  req->fd = open(req->filename, O_RDONLY);
>          *
>          * It is linked to the next read() atom.
>          */
>         req->filename_p = req->filename;
>         init_atom(req, &req->open_file, __NR_sys_open,
>                   &req->filename_p, &O_RDONLY_var, NULL, NULL, NULL, NULL,
>                   &req->fd, SYSLET_STOP_ON_NEGATIVE, &req->read_file);
>         
>         /*
>          * This second read() atom is linked back to itself, it skips to
>          * the next one on stop:
>          */
>         req->file_buf_ptr = req->file_buf;
>         init_atom(req, &req->read_file, __NR_sys_read,
>                   &req->fd, &req->file_buf_ptr, &FILE_BUF_SIZE_var,
>                   NULL, NULL, NULL, NULL,
>                   SYSLET_STOP_ON_NON_POSITIVE | SYSLET_SKIP_TO_NEXT_ON_STOP,
>                   &req->read_file);
>                 
>         /*
>          * This close() atom has NULL as next, this finishes the syslet:
>          */
>         init_atom(req, &req->close_file, __NR_sys_close,
>                   &req->fd, NULL, NULL, NULL, NULL, NULL, NULL, 0, NULL);
>                 
>         return req;
> }
> 
> 
> Here's how your clet would look like:
> 
> static long main_sync_loop(ctx *c)
> {
>         int fd;
>         char file_buf[FILE_BUF_SIZE+1];
>         
>         if ((fd = open(c->filename, O_RDONLY)) == -1)
>                 return -1;
>         while (read(fd, file_buf, FILE_BUF_SIZE) > 0)
>                 ;
>         close(fd);
>         return 0;
> }
> 
> 
> Kinda easier to code isn't it? And the cost of the upcall to schedule the 
> clet is widely amortized by the multple syscalls you're going to do inside 
> your clet.

I do not get it. What if clet includes

int *a = 0; *a = 1; /* enjoy your oops, stupid kernel? */

I.e. how do you make sure kernel is protected from malicious clets?

-- 
(english) http://www.livejournal.com/~pavelmachek
(cesky, pictures) http://atrey.karlin.mff.cuni.cz/~pavel/picture/horses/blog.html
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