[PATCH bpf] bpf: Fix precedence bug in convert_bpf_ld_abs alignment check

Daniel Borkmann
2026-04-16T12:27:28Z

Fix an operator precedence issue in convert_bpf_ld_abs() where the
expression offset + ip_align % size evaluates as offset + (ip_align % size)
due to % having higher precedence than +. That latter evaluation does
not make any sense. The intended check is (offset + ip_align) % size == 0
to verify that the packet load offset is properly aligned for direct
access.

With NET_IP_ALIGN == 2, the bug causes the inline fast-path for direct
packet loads to almost never be taken on !CONFIG_HAVE_EFFICIENT_UNALIGNED_ACCESS
platforms. This forces nearly all cBPF BPF_LD_ABS packet loads through
the bpf_skb_load_helper slow path on the affected archs.

Fixes: e0cea7ce988c ("bpf: implement ld_abs/ld_ind in native bpf")
Signed-off-by: Daniel Borkmann <daniel@iogearbox.net>
---
net/core/filter.c | 2 +-
1 file changed, 1 insertion(+), 1 deletion(-)

diff --git a/net/core/filter.c b/net/core/filter.c
index a10cdcb7103e..35f5ee7af64e 100644
--- a/net/core/filter.c
+++ b/net/core/filter.c
@@ -508,7 +508,7 @@ static bool convert_bpf_ld_abs(struct sock_filter *fp, struct bpf_insn **insnp)
((unaligned_ok && offset >= 0) ||
(!unaligned_ok && offset >= 0 &&
offset + ip_align >= 0 &&
- offset + ip_align % size == 0))) {
+ (offset + ip_align) % size == 0))) {
bool ldx_off_ok = offset <= S16_MAX;

*insn++ = BPF_MOV64_REG(BPF_REG_TMP, BPF_REG_H);
--
2.43.0