[PATCH bpf-next v5 1/6] bpf: Correct the overflow check comment in bpf_iter_num_next()

[Puranjay Mohan <puranjay@kernel.org>]

The comment on the s->cur + 1 >= s->end check claims the (s64) cast is
needed to avoid overflow when s->cur == s->end == INT_MAX. It isn't:
s->cur + 1 is computed in int and wraps before the cast, so the cast
changes nothing (INT_MAX + 1 compares the same either way).

The wraparound is the point. bpf_iter_num_new() sets s->cur = start - 1,
which wraps to INT_MAX for start == INT_MIN, and the wrapping s->cur + 1
brings it back to start. (s64)s->cur + 1 would instead break iterators
starting at INT_MIN.

Drop the cast and reword the comment. No functional change; the wrap is
well-defined under -fno-strict-overflow.

Signed-off-by: Puranjay Mohan <puranjay@kernel.org>
---
 kernel/bpf/bpf_iter.c | 9 ++++-----
 1 file changed, 4 insertions(+), 5 deletions(-)

diff --git a/kernel/bpf/bpf_iter.c b/kernel/bpf/bpf_iter.c
index f5eaeb2493d4a..b235e117e206a 100644
--- a/kernel/bpf/bpf_iter.c
+++ b/kernel/bpf/bpf_iter.c
@@ -802,12 +802,11 @@ __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
+	/*
+	 * s->cur < s->end while iterating, else s->cur == s->end == 0; the signed
+	 * s->cur + 1 >= s->end holds even when s->cur + 1 wraps (start == INT_MIN).
 	 */
-	if ((s64)(s->cur + 1) >= s->end) {
+	if (s->cur + 1 >= s->end) {
 		s->cur = s->end = 0;
 		return NULL;
 	}
-- 
2.53.0-Meta