CVE-2026-53090: bpf: Fix ld_{abs,ind} failure path analysis in subprogs

In the Linux kernel, the following vulnerability has been resolved:

bpf: Fix ld{abs,ind} failure path analysis in subprogs

Usage of ld{abs,ind} instructions got extended into subprogs some time ago via commit 09b28d76eac4 ("bpf: Add abnormal return checks."). These are only allowed in subprograms when the latter are BTF annotated and have scalar return types.

The code generator in bpfgenldabs() has an abnormal exit path (r0=0 + exit) from legacy cBPF times. While the enforcement is on scalar return types, the verifier must also simulate the path of abnormal exit if the packet data load via ld{abs,ind} failed.

This is currently not the case. Fix it by having the verifier simulate both success and failure paths, and extend it in similar ways as we do for tail calls. The success path (r0=unknown, continue to next insn) is pushed onto stack for later validation and the r0=0 and return to the caller is done on the fall-through side.