Re: [patch 06/11] syslets: core, documentation

[Davide Libenzi <davidel@xmailserver.org>]

On Tue, 13 Feb 2007, Davide Libenzi wrote:

> > > I can understand that chaining syscalls requires variable sharing, but 
> > > the majority of the parameters passed to syscalls are just direct 
> > > ones. Maybe a smart method that allows you to know if a parameter is a 
> > > direct one or a pointer to one? An "unsigned int pmap" where bit N is 
> > > 1 if param N is an indirection? Hmm?
> > 
> > adding such things tends to slow down atom parsing.
> 
> I really think it simplifies it. You simply *copy* the parameter (I'd say 
> that 70+% of cases falls inside here), and if the current "pmap" bit is 
> set, then you do all the indirection copy-from-userspace stuff.
> It also simplify userspace a lot, since you can now pass arrays and 
> structure pointers directly, w/out saving them in a temporary variable.

Very rough sketch below ...


---
struct syslet_uatom {
	unsigned long                           flags;
	unsigned int                            nr;
	unsigned short                          nparams;
	unsigned short                          pmap;
	long __user                             *ret_ptr;
	struct syslet_uatom     __user          *next;
	unsigned long           __user          args[6];
	void __user                             *private;
};

long copy_uatom(struct syslet_atom *atom, struct syslet_uatom __user *uatom)
{
	unsigned short i, pmap;
	unsigned long __user *arg_ptr;
	long ret = 0;

	if (!access_ok(VERIFY_WRITE, uatom, sizeof(*uatom)))
		return -EFAULT;

	ret = __get_user(atom->nr, &uatom->nr);
	ret |= __get_user(atom->nparams, &uatom->nparams);
	ret |= __get_user(pmap, &uatom->pmap);
	ret |= __get_user(atom->ret_ptr, &uatom->ret_ptr);
	ret |= __get_user(atom->flags, &uatom->flags);
	ret |= __get_user(atom->next, &uatom->next);
	if (unlikely(atom->nparams >= 6))
		return -EINVAL;
	for (i = 0; i < atom->nparams; i++, pmap >>= 1) {
		atom->args[i] = uatom->args[i];
		if (unlikely(pmap & 1)) {
			arg_ptr = (unsigned long __user *) atom->args[i];
			if (!access_ok(VERIFY_WRITE, arg_ptr, sizeof(*arg_ptr)))
				return -EFAULT;
			ret |= __get_user(atom->args[i], arg_ptr);
		}
	}

	return ret;
}

void init_utaom(struct syslet_uatom *ua, unsigned long flags, unsigned int nr,
		long *ret_ptr, struct syslet_uatom *next, void *private,
		int nparams, ...)
{
	int i, mode;
	va_list args;

	ua->flags = flags;
	ua->nr = nr;
	ua->ret_ptr = ret_ptr;
	ua->next = next;
	ua->private = private;
	ua->nparams = nparams;
	ua->pmap = 0;
	va_start(args, nparams);
	for (i = 0; i < nparams; i++) {
		mode = va_arg(args, int);
		ua->args[i] = va_arg(args, unsigned long);
		if (mode == UASYNC_INDIR)
			ua->pmap |= 1 << i;
	}
	va_end(args);
}


#define UASYNC_IMM 0
#define UASYNC_INDIR 1
#define UAPD(a) UASYNC_IMM, (unsigned long) (a)
#define UAPI(a) UASYNC_INDIR, (unsigned long) (a)


void foo(void)
{
	int fd;
	long res;
	struct stat stb;
	struct syslet_uatom ua;

	init_utaom(&ua, 0, __NR_fstat, &res, NULL, NULL, 2,
		   UAPI(&fd), UAPD(&stb));
	...
}
---



- Davide


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