bpf: implement numbers iterator

Implement the first open-coded iterator type over a range of integers.

It's public API consists of:
  - bpf_iter_num_new() constructor, which accepts [start, end) range
    (that is, start is inclusive, end is exclusive).
  - bpf_iter_num_next() which will keep returning read-only pointer to int
    until the range is exhausted, at which point NULL will be returned.
    If bpf_iter_num_next() is kept calling after this, NULL will be
    persistently returned.
  - bpf_iter_num_destroy() destructor, which needs to be called at some
    point to clean up iterator state. BPF verifier enforces that iterator
    destructor is called at some point before BPF program exits.

Note that `start = end = X` is a valid combination to setup an empty
iterator. bpf_iter_num_new() will return 0 (success) for any such
combination.

If bpf_iter_num_new() detects invalid combination of input arguments, it
returns error, resets iterator state to, effectively, empty iterator, so
any subsequent call to bpf_iter_num_next() will keep returning NULL.

BPF verifier has no knowledge that returned integers are in the
[start, end) value range, as both `start` and `end` are not statically
known and enforced: they are runtime values.

While the implementation is pretty trivial, some care needs to be taken
to avoid overflows and underflows. Subsequent selftests will validate
correctness of [start, end) semantics, especially around extremes
(INT_MIN and INT_MAX).

Similarly to bpf_loop(), we enforce that no more than BPF_MAX_LOOPS can
be specified.

bpf_iter_num_{new,next,destroy}() is a logical evolution from bounded
BPF loops and bpf_loop() helper and is the basis for implementing
ergonomic BPF loops with no statically known or verified bounds.
Subsequent patches implement bpf_for() macro, demonstrating how this can
be wrapped into something that works and feels like a normal for() loop
in C language.

Signed-off-by: Andrii Nakryiko <andrii@kernel.org>
Link: https://lore.kernel.org/r/20230308184121.1165081-5-andrii@kernel.org
Signed-off-by: Alexei Starovoitov <ast@kernel.org>

1 files changed, 70 insertions, 0 deletions

diff --git a/kernel/bpf/bpf_iter.c b/kernel/bpf/bpf_iter.c
index 5dc307bdeaeb..96856f130cbf 100644
--- a/kernel/bpf/bpf_iter.c
+++ b/kernel/bpf/bpf_iter.c
@@ -776,3 +776,73 @@ const struct bpf_func_proto bpf_loop_proto = {
 	.arg3_type	= ARG_PTR_TO_STACK_OR_NULL,
 	.arg4_type	= ARG_ANYTHING,
 };
+
+struct bpf_iter_num_kern {
+	int cur; /* current value, inclusive */
+	int end; /* final value, exclusive */
+} __aligned(8);
+
+__diag_push();
+__diag_ignore_all("-Wmissing-prototypes",
+		  "Global functions as their definitions will be in vmlinux BTF");
+
+__bpf_kfunc int bpf_iter_num_new(struct bpf_iter_num *it, int start, int end)
+{
+	struct bpf_iter_num_kern *s = (void *)it;
+
+	BUILD_BUG_ON(sizeof(struct bpf_iter_num_kern) != sizeof(struct bpf_iter_num));
+	BUILD_BUG_ON(__alignof__(struct bpf_iter_num_kern) != __alignof__(struct bpf_iter_num));
+
+	BTF_TYPE_EMIT(struct btf_iter_num);
+
+	/* start == end is legit, it's an empty range and we'll just get NULL
+	 * on first (and any subsequent) bpf_iter_num_next() call
+	 */
+	if (start > end) {
+		s->cur = s->end = 0;
+		return -EINVAL;
+	}
+
+	/* avoid overflows, e.g., if start == INT_MIN and end == INT_MAX */
+	if ((s64)end - (s64)start > BPF_MAX_LOOPS) {
+		s->cur = s->end = 0;
+		return -E2BIG;
+	}
+
+	/* user will call bpf_iter_num_next() first,
+	 * which will set s->cur to exactly start value;
+	 * underflow shouldn't matter
+	 */
+	s->cur = start - 1;
+	s->end = end;
+
+	return 0;
+}
+
+__bpf_kfunc int *bpf_iter_num_next(struct bpf_iter_num* it)
+{
+	struct bpf_iter_num_kern *s = (void *)it;
+
+	/* check failed initialization or if we are done (same behavior);
+	 * need to be careful about overflow, so convert to s64 for checks,
+	 * e.g., if s->cur == s->end == INT_MAX, we can't just do
+	 * s->cur + 1 >= s->end
+	 */
+	if ((s64)(s->cur + 1) >= s->end) {
+		s->cur = s->end = 0;
+		return NULL;
+	}
+
+	s->cur++;
+
+	return &s->cur;
+}
+
+__bpf_kfunc void bpf_iter_num_destroy(struct bpf_iter_num *it)
+{
+	struct bpf_iter_num_kern *s = (void *)it;
+
+	s->cur = s->end = 0;
+}
+
+__diag_pop();