On Wed, Oct 22, 2025 at 12:46 PM Eduard Zingerman <[email protected]> wrote: > > On Wed, 2025-10-22 at 11:14 -0700, Yonghong Song wrote: > > > > On 10/22/25 9:44 AM, KaFai Wan wrote: > > > When conditional jumps are performed on the same register (e.g., r0 <= r0, > > > r0 > r0, r0 < r0) where the register holds a scalar with range, the > > > verifier > > > incorrectly attempts to adjust the register's min/max bounds. This leads > > > to > > > invalid range bounds and triggers a BUG warning: > > > > > > verifier bug: REG INVARIANTS VIOLATION (true_reg1): range bounds > > > violation u64=[0x1, 0x0] s64=[0x1, 0x0] u32=[0x1, 0x0] s32=[0x1, 0x0] > > > var_off=(0x0, 0x0) > > > WARNING: CPU: 0 PID: 93 at kernel/bpf/verifier.c:2731 > > > reg_bounds_sanity_check+0x163/0x220 > > > Modules linked in: > > > CPU: 0 UID: 0 PID: 93 Comm: repro-x-3 Tainted: G W > > > 6.18.0-rc1-ge7586577b75f-dirty #218 PREEMPT(full) > > > Tainted: [W]=WARN > > > Hardware name: QEMU Ubuntu 24.04 PC (i440FX + PIIX, 1996), BIOS > > > 1.16.3-debian-1.16.3-2 04/01/2014 > > > RIP: 0010:reg_bounds_sanity_check+0x163/0x220 > > > Call Trace: > > > <TASK> > > > reg_set_min_max.part.0+0x1b1/0x360 > > > check_cond_jmp_op+0x1195/0x1a60 > > > do_check_common+0x33ac/0x33c0 > > > ... > > > > > > The issue occurs in reg_set_min_max() function where bounds adjustment > > > logic > > > is applied even when both registers being compared are the same. > > > Comparing a > > > register with itself should not change its bounds since the comparison > > > result > > > is always known (e.g., r0 == r0 is always true, r0 < r0 is always false). > > > > > > Fix this by adding an early return in reg_set_min_max() when false_reg1 > > > and > > > false_reg2 point to the same register, skipping the unnecessary bounds > > > adjustment that leads to the verifier bug. > > > > > > Reported-by: Kaiyan Mei <[email protected]> > > > Reported-by: Yinhao Hu <[email protected]> > > > Closes: > > > https://lore.kernel.org/all/[email protected]/ > > > Fixes: 0df1a55afa83 ("bpf: Warn on internal verifier errors") > > > Signed-off-by: KaFai Wan <[email protected]> > > > --- > > > kernel/bpf/verifier.c | 4 ++++ > > > 1 file changed, 4 insertions(+) > > > > > > diff --git a/kernel/bpf/verifier.c b/kernel/bpf/verifier.c > > > index 6d175849e57a..420ad512d1af 100644 > > > --- a/kernel/bpf/verifier.c > > > +++ b/kernel/bpf/verifier.c > > > @@ -16429,6 +16429,10 @@ static int reg_set_min_max(struct > > > bpf_verifier_env *env, > > > if (false_reg1->type != SCALAR_VALUE || false_reg2->type != > > > SCALAR_VALUE) > > > return 0; > > > > > > + /* If conditional jumps on the same register, skip the adjustment */ > > > + if (false_reg1 == false_reg2) > > > + return 0; > > > > Your change looks good. But this is a special case and it should not > > happen for any compiler generated code. So could you investigate > > why regs_refine_cond_op() does not work? Since false_reg1 and false_reg2 > > is the same, so register refinement should keep the same. Probably > > some minor change in regs_refine_cond_op(...) should work? > > > > > + > > > /* fallthrough (FALSE) branch */ > > > regs_refine_cond_op(false_reg1, false_reg2, rev_opcode(opcode), > > > is_jmp32); > > > reg_bounds_sync(false_reg1); > > I think regs_refine_cond_op() is not written in a way to handle same > registers passed as reg1 and reg2. E.g. in this particular case the > condition is reformulated as "r0 < r0", and then the following branch > is taken: > > static void regs_refine_cond_op(struct bpf_reg_state *reg1, struct > bpf_reg_state *reg2, > u8 opcode, bool is_jmp32) > { > ... > case BPF_JLT: // condition is rephrased as r0 < r0 > if (is_jmp32) { > ... > } else { > reg1->umax_value = min(reg1->umax_value, > reg2->umax_value - 1); > reg2->umin_value = max(reg1->umin_value + 1, > reg2->umin_value); > } > break; > ... > } > > Note that intent is to adjust umax of the LHS (reg1) register and umin > of the RHS (reg2) register. But here it ends up adjusting the same register. > > (a) before refinement: u64=[0x0, 0x80000000] s64=[0x0, 0x80000000] u32=[0x0, > 0x80000000] s32=[0x80000000, 0x0] > (b) after refinement: u64=[0x1, 0x7fffffff] s64=[0x0, 0x80000000] u32=[0x0, > 0x80000000] s32=[0x80000000, 0x0] > (c) after sync : u64=[0x1, 0x0] s64=[0x1, 0x0] u32=[0x1, 0x0] s32=[0x1, > 0x0] > > At (b) the u64 range translated to s32 is > 0, while s32 range is <= 0, > hence the invariant violation. > > I think it's better to move the reg1 == reg2 check inside > regs_refine_cond_op(), or to handle this case in is_branch_taken().
hmm. bu then regs_refine_cond_op() will skip it, yet reg_set_min_max() will still be doing pointless work with reg_bounds_sync() and sanity check. The current patch makes more sense to me.
