On Wed, Oct 22, 2025 at 1:30 PM Eduard Zingerman <[email protected]> wrote: > > On Wed, 2025-10-22 at 13:12 -0700, Alexei Starovoitov wrote: > > 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. > > Well, if we want to avoid useless work, we need something like: > > @@ -16173,6 +16173,25 @@ static int is_pkt_ptr_branch_taken(struct > bpf_reg_state *dst_reg, > static int is_branch_taken(struct bpf_reg_state *reg1, struct bpf_reg_state > *reg2, > u8 opcode, bool is_jmp32) > { > + if (reg1 == reg2) { > + switch (opcode) { > + case BPF_JGE: > + case BPF_JLE: > + case BPF_JSGE: > + case BPF_JSLE: > + case BPF_JEQ: > + case BPF_JSET: > + return 1; > + case BPF_JGT: > + case BPF_JLT: > + case BPF_JSGT: > + case BPF_JSLT: > + case BPF_JNE: > + return 0; > + default: > + return -1; > + } > + } > > But that's too much code for an artificial case. > Idk, either way is fine with me.
Makes sense to me.
