On Mon, 2025-10-27 at 13:09 -0700, Eduard Zingerman wrote:
> On Sat, 2025-10-25 at 13:30 +0800, 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: 92 at kernel/bpf/verifier.c:2731
> > reg_bounds_sanity_check+0x163/0x220
> > 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+0xf7/0x1d0
> > check_cond_jmp_op+0x57b/0x1730
> > ? print_bpf_insn+0x3d5/0xa50
> > do_check_common+0x33ac/0x33c0
> > ...
> >
> > The root cause is in regs_refine_cond_op() where BPF_JLT/BPF_JSLT operations
> > adjust both min/max bounds on the same register, causing invalid bounds.
> >
> > Since comparing a register with itself should not change its bounds (the
> > comparison result is always known: r0 == r0 is always true, r0 < r0 is
> > always false), the bounds adjustment is unnecessary.
> >
> > Fix this by:
> > 1. Enhance is_branch_taken() and is_scalar_branch_taken() to properly
> > handle branch direction computation for same register comparisons
> > across all BPF jump operations
> > 2. For unknown branch directions (e.g., BPF_JSET), add early return in
> > reg_set_min_max() to avoid bounds adjustment 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]/
> > Fixes: 0df1a55afa83 ("bpf: Warn on internal verifier errors")
> > Signed-off-by: KaFai Wan <[email protected]>
> > ---
> > kernel/bpf/verifier.c | 32 ++++++++++++++++++++++++++++++++
> > 1 file changed, 32 insertions(+)
> >
> > diff --git a/kernel/bpf/verifier.c b/kernel/bpf/verifier.c
> > index 6d175849e57a..653fa96ed0df 100644
> > --- a/kernel/bpf/verifier.c
> > +++ b/kernel/bpf/verifier.c
> > @@ -16037,6 +16037,12 @@ static int is_scalar_branch_taken(struct
> > bpf_reg_state *reg1, struct
> > bpf_reg_sta
> > }
> > break;
> > case BPF_JSET:
> > + if (reg1 == reg2) {
> > + if (tnum_is_const(t1))
> > + return t1.value != 0;
> > + else
> > + return (smin1 <= 0 && smax1 >= 0) ? -1 : 1;
> > + }
>
> I think this logic is fine, but it needs tests for multiple cases.
>
ok, I'll add tests for that.
> > if (!is_reg_const(reg2, is_jmp32)) {
> > swap(reg1, reg2);
> > swap(t1, t2);
> > @@ -16172,6 +16178,25 @@ 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:
> > + return 1;
> > + case BPF_JGT:
> > + case BPF_JLT:
> > + case BPF_JSGT:
> > + case BPF_JSLT:
> > + case BPF_JNE:
> > + return 0;
> > + default:
> > + break;
> > + }
> > + }
> > +
>
> I think Alexei was against my suggestion to put it in
> is_branch_taken() and preferred is_scalar_branch_taken() instead.
>
Hmm, I misunderstood that. If put in is_scalar_branch_taken() then just for
scalar cases,
just confirm that.
> > if (reg_is_pkt_pointer_any(reg1) && reg_is_pkt_pointer_any(reg2) &&
> > !is_jmp32)
> > return is_pkt_ptr_branch_taken(reg1, reg2, opcode);
> >
> > @@ -16429,6 +16454,13 @@ static int reg_set_min_max(struct bpf_verifier_env
> > *env,
> > if (false_reg1->type != SCALAR_VALUE || false_reg2->type !=
> > SCALAR_VALUE)
> > return 0;
> >
> > + /* We compute branch direction for same registers in is_branch_taken()
> > and
> > + * is_scalar_branch_taken(). For unknown branch directions (e.g.,
> > BPF_JSET)
> > + * on the same registers, we don't need to adjusts the min/max values.
> > + */
> > + if (false_reg1 == false_reg2)
> > + return 0;
> > +
> > /* fallthrough (FALSE) branch */
> > regs_refine_cond_op(false_reg1, false_reg2, rev_opcode(opcode),
> > is_jmp32);
> > reg_bounds_sync(false_reg1);
--
Thanks,
KaFai
