On Mon, 2025-11-03 at 14:31 +0800, KaFai Wan wrote: > When conditional jumps are performed on the same scalar register > (e.g., r0 <= r0, r0 > r0, r0 < r0), the BPF verifier incorrectly > attempts to adjust the register's min/max bounds. This leads to > invalid range bounds and triggers a BUG warning. > > The problematic BPF program: > 0: call bpf_get_prandom_u32 > 1: w8 = 0x80000000 > 2: r0 &= r8 > 3: if r0 > r0 goto <exit> > > The instruction 3 triggers kernel warning: > 3: if r0 > r0 goto <exit> > true_reg1: range bounds violation u64=[0x1, 0x0] s64=[0x1, 0x0] u32=[0x1, > 0x0] s32=[0x1, 0x0] var_off=(0x0, 0x0) > true_reg2: const tnum out of sync with range bounds u64=[0x0, > 0xffffffffffffffff] s64=[0x8000000000000000, 0x7fffffffffffffff] > var_off=(0x0, 0x0) > > Comparing a register with itself should not change its bounds and > for most comparison operations, comparing a register with itself has > a known result (e.g., r0 == r0 is always true, r0 < r0 is always false). > > Fix this by: > 1. Enhance is_scalar_branch_taken() to properly handle branch direction > computation for same register comparisons across all BPF jump operations > 2. Adds early return in reg_set_min_max() to avoid bounds adjustment > for unknown branch directions (e.g., BPF_JSET) on the same register > > The fix ensures that unnecessary bounds adjustments are skipped, preventing > the verifier bug while maintaining correct branch direction analysis. > > Reported-by: Kaiyan Mei <[email protected]> > Reported-by: Yinhao Hu <[email protected]> > Closes: > https://lore.kernel.org/all/[email protected]/ > Signed-off-by: KaFai Wan <[email protected]> > ---
Acked-by: Eduard Zingerman <[email protected]> [...]
