> From: Marat Khalili [mailto:[email protected]] > Sent: Wednesday, 18 March 2026 14.07 > > > > Handling immediate as unsigned is questionable, especially in the > > > BPF_IND case > > > it may produce incorrect results. > > > > In Classic BPF (cBPF), when the immediate "k" is negative (when cast > to signed integer), it is used > > for getting packet metadata (e.g. SKF_AD_VLAN_TAG gets the VLAN ID); > otherwise it is considered > > unsigned. > > Yes. Since we don't support it, we should probably consider these > offsets invalid.
+1 > > And in the BPF_IND case one might be tempted to load end of some packet > area in > the register and use negative offsets, we probably should handle it > correctly. In Classic BPF, negative "k" has special meaning for both BPF_ABS and BPF_IND. So we should consider it invalid for both cases. That prevents applications from using it the way you describe. And it will allow us to add BPF library support for Linux-compatible special meanings later, without breaking the ABI. > > > > To make things worse, `__rte_pktmbuf_read` is also buggy when > passed > > > very large > > > lengths (again, technically not ARM eBPF fault). > > > > Are you referring to the potential integer wraparound in the off+len > > rte_pktmbuf_pkt_len(m) > > comparison? > > [BZ1724] > > Or some other bug in __rte_pktmbuf_read()? > > > > [BZ1724]: https://bugs.dpdk.org/show_bug.cgi?id=1724 > > Technically that one manifested itself in rte_pktmbuf_read (without > underscores), but essentially the root cause is the same. In the eBPF > BPF_ABS > case we could potentially rely on `__rte_pktmbuf_read` for refusing to > accept > negative values converted to large integers, but due to overflows we > cannot. Agree. The off+len wraparound in rte_pktmbuf_read() and __rte_pktmbuf_read() is clearly a bug, and not intentional. Reading the function documentation supports the conclusion that the wraparound is a bug; the off and len parameters are unsigned, and the documentation says nothing about an ability to make them behave as signed.
