Constant F_TEST_RND_HI32 [src]

BPF_F_TEST_RND_HI32 is used in BPF_PROG_LOAD command for testing purpose. Verifier does sub-register def/use analysis and identifies instructions whose def only matters for low 32-bit, high 32-bit is never referenced later through implicit zero extension. Therefore verifier notifies JIT back-ends that it is safe to ignore clearing high 32-bit for these instructions. This saves some back-ends a lot of code-gen. However such optimization is not necessary on some arches, for example x86_64, arm64 etc, whose JIT back-ends hence hasn't used verifier's analysis result. But, we really want to have a way to be able to verify the correctness of the described optimization on x86_64 on which testsuites are frequently exercised. So, this flag is introduced. Once it is set, verifier will randomize high 32-bit for those instructions who has been identified as safe to ignore them. Then, if verifier is not doing correct analysis, such randomization will regress tests to expose bugs.