On Fri, 2025-10-31 at 09:37 -0700, Alexei Starovoitov wrote: > On Fri, Oct 31, 2025 at 8:44 AM KaFai Wan <[email protected]> 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]> > > --- > > kernel/bpf/verifier.c | 33 +++++++++++++++++++++++++++++++++ > > 1 file changed, 33 insertions(+) > > > > diff --git a/kernel/bpf/verifier.c b/kernel/bpf/verifier.c > > index 542e23fb19c7..a571263f4ebe 100644 > > --- a/kernel/bpf/verifier.c > > +++ b/kernel/bpf/verifier.c > > @@ -15995,6 +15995,8 @@ static int is_scalar_branch_taken(struct > > bpf_reg_state *reg1, struct > > bpf_reg_sta > > > > switch (opcode) { > > case BPF_JEQ: > > + if (reg1 == reg2) > > + return 1; > > /* constants, umin/umax and smin/smax checks would be > > * redundant in this case because they all should match > > */ > > @@ -16021,6 +16023,8 @@ static int is_scalar_branch_taken(struct > > bpf_reg_state *reg1, struct > > bpf_reg_sta > > } > > break; > > case BPF_JNE: > > + if (reg1 == reg2) > > + return 0; > > /* constants, umin/umax and smin/smax checks would be > > * redundant in this case because they all should match > > */ > > @@ -16047,6 +16051,12 @@ 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; > > + } > > if (!is_reg_const(reg2, is_jmp32)) { > > swap(reg1, reg2); > > swap(t1, t2); > > @@ -16059,48 +16069,64 @@ static int is_scalar_branch_taken(struct > > bpf_reg_state *reg1, struct > > bpf_reg_sta > > return 0; > > break; > > case BPF_JGT: > > + if (reg1 == reg2) > > + return 0; > > if (umin1 > umax2) > > return 1; > > else if (umax1 <= umin2) > > return 0; > > break; > > case BPF_JSGT: > > + if (reg1 == reg2) > > + return 0; > > This is uglier than the previous version. > reg1 == reg2 is a syzbot territory. > We shouldn't uglify the code everywhere because of it. > ok, will update in v4. > pw-bot: cr
-- Thanks, KaFai
