There are cases (caught by subsequent reg_bounds tests in selftests/bpf) where performing one round of __reg_deduce_bounds() doesn't propagate all the information from, say, s32 to u32 bounds and than from newly learned u32 bounds back to u64 and s64. So perform __reg_deduce_bounds() twice to make sure such derivations are propagated fully after reg_bounds_sync(). One such example is test `(s64)[0xffffffff00000001; 0] (u64)< 0xffffffff00000000` from selftest patch from this patch set. It demonstrates an intricate dance of u64 -> s64 -> u64 -> u32 bounds adjustments, which requires two rounds of __reg_deduce_bounds(). Here are corresponding refinement log from selftest, showing evolution of knowledge. REFINING (FALSE R1) (u64)SRC=[0xffffffff00000000; U64_MAX] (u64)DST_OLD=[0; U64_MAX] (u64)DST_NEW=[0xffffffff00000000; U64_MAX] REFINING (FALSE R1) (u64)SRC=[0xffffffff00000000; U64_MAX] (s64)DST_OLD=[0xffffffff00000001; 0] (s64)DST_NEW=[0xffffffff00000001; -1] REFINING (FALSE R1) (s64)SRC=[0xffffffff00000001; -1] (u64)DST_OLD=[0xffffffff00000000; U64_MAX] (u64)DST_NEW=[0xffffffff00000001; U64_MAX] REFINING (FALSE R1) (u64)SRC=[0xffffffff00000001; U64_MAX] (u32)DST_OLD=[0; U32_MAX] (u32)DST_NEW=[1; U32_MAX] R1 initially has smin/smax set to [0xffffffff00000001; -1], while umin/umax is unknown. After (u64)< comparison, in FALSE branch we gain knowledge that umin/umax is [0xffffffff00000000; U64_MAX]. That causes smin/smax to learn that zero can't happen and upper bound is -1. Then smin/smax is adjusted from umin/umax improving lower bound from 0xffffffff00000000 to 0xffffffff00000001. And then eventually umin32/umax32 bounds are drived from umin/umax and become [1; U32_MAX]. Selftest in the last patch is actually implementing a multi-round fixed-point convergence logic, but so far all the tests are handled by two rounds of reg_bounds_sync() on the verifier state, so we keep it simple for now. Signed-off-by: Andrii Nakryiko <andrii@xxxxxxxxxx> --- kernel/bpf/verifier.c | 1 + 1 file changed, 1 insertion(+) diff --git a/kernel/bpf/verifier.c b/kernel/bpf/verifier.c index d0d0a1a1b662..2991e9dd4475 100644 --- a/kernel/bpf/verifier.c +++ b/kernel/bpf/verifier.c @@ -2605,6 +2605,7 @@ static void reg_bounds_sync(struct bpf_reg_state *reg) __update_reg_bounds(reg); /* We might have learned something about the sign bit. */ __reg_deduce_bounds(reg); + __reg_deduce_bounds(reg); /* We might have learned some bits from the bounds. */ __reg_bound_offset(reg); /* Intersecting with the old var_off might have improved our bounds -- 2.34.1
