Re: [RFC,PATCH 1/2] seccomp_filters: system call filtering usingBPF

[Oleg Nesterov]

On 01/11, Will Drewry wrote:
>
> +__weak u8 *seccomp_get_regs(u8 *scratch, size_t *available)
> +{
> +	/* regset is usually returned based on task personality, not current
> +	 * system call convention.  This behavior makes it unsafe to execute
> +	 * BPF programs over regviews if is_compat_task or the personality
> +	 * have changed since the program was installed.
> +	 */
> +	const struct user_regset_view *view = task_user_regset_view(current);
> +	const struct user_regset *regset = &view->regsets[0];
> +	size_t scratch_size = *available;
> +	if (regset->core_note_type != NT_PRSTATUS) {
> +		/* The architecture should override this method for speed. */
> +		regset = find_prstatus(view);
> +		if (!regset)
> +			return NULL;
> +	}
> +	*available = regset->n * regset->size;
> +	/* Make sure the scratch space isn't exceeded. */
> +	if (*available > scratch_size)
> +		*available = scratch_size;
> +	if (regset->get(current, regset, 0, *available, scratch, NULL))
> +		return NULL;
> +	return scratch;
> +}
> +
> +/**
> + * seccomp_test_filters - tests 'current' against the given syscall
> + * @syscall: number of the system call to test
> + *
> + * Returns 0 on ok and non-zero on error/failure.
> + */
> +int seccomp_test_filters(int syscall)
> +{
> +	struct seccomp_filter *filter;
> +	u8 regs_tmp[sizeof(struct user_regs_struct)], *regs;
> +	size_t regs_size = sizeof(struct user_regs_struct);
> +	int ret = -EACCES;
> +
> +	filter = current->seccomp.filter; /* uses task ref */
> +	if (!filter)
> +		goto out;
> +
> +	/* All filters in the list are required to share the same system call
> +	 * convention so only the first filter is ever checked.
> +	 */
> +	if (seccomp_check_personality(filter))
> +		goto out;
> +
> +	/* Grab the user_regs_struct.  Normally, regs == &regs_tmp, but
> +	 * that is not mandatory.  E.g., it may return a point to
> +	 * task_pt_regs(current).  NULL checking is mandatory.
> +	 */
> +	regs = seccomp_get_regs(regs_tmp, &regs_size);

Stupid question. I am sure you know what are you doing ;) and I know
nothing about !x86 arches.

But could you explain why it is designed to use user_regs_struct ?
Why we can't simply use task_pt_regs() and avoid the (costly) regsets?

Just curious.

Oleg.

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