[PATCH bpf-next v3 5/6] selftests/bpf: Cover invariant violation case from syzbot

Paul Chaignon
2026-04-02T15:11:44Z

This patch adds a selftest for the change in the previous patch. The
selftest is derived from a syzbot reproducer from [1] (among the 22
reproducers on that page, only 4 still reproduced on latest bpf tree,
all being small variants of the same invariant violation).

The test case failure without the previous patch is shown below.

0: R1=ctx() R10=fp0
0: (85) call bpf_get_prandom_u32#7 ; R0=scalar()
1: (bf) r5 = r0 ; R0=scalar(id=1) R5=scalar(id=1)
2: (57) r5 &= -4 ; R5=scalar(smax=0x7ffffffffffffffc,umax=0xfffffffffffffffc,smax32=0x7ffffffc,umax32=0xfffffffc,var_off=(0x0; 0xfffffffffffffffc))
3: (bf) r7 = r0 ; R0=scalar(id=1) R7=scalar(id=1)
4: (57) r7 &= 1 ; R7=scalar(smin=smin32=0,smax=umax=smax32=umax32=1,var_off=(0x0; 0x1))
5: (07) r7 += -43 ; R7=scalar(smin=smin32=-43,smax=smax32=-42,umin=0xffffffffffffffd5,umax=0xffffffffffffffd6,umin32=0xffffffd5,umax32=0xffffffd6,var_off=(0xffffffffffffffd4; 0x3))
6: (5e) if w5 != w7 goto pc+1
verifier bug: REG INVARIANTS VIOLATION (false_reg1): range bounds violation u64=[0xffffffd5, 0xffffffffffffffd4] s64=[0x80000000ffffffd5, 0x7fffffffffffffd4] u32=[0xffffffd5, 0xffffffd4] s32=[0xffffffd5, 0xffffffd4] var_off=(0xffffffd4, 0xffffffff00000000)

R5 and R7 are prepared such that their tnums intersection results in a
known constant but that constant isn't within R7's u32 bounds.
is_branch_taken isn't able to detect this case today, so the verifier
walks the impossible fallthrough branch. After regs_refine_cond_op and
reg_bounds_sync refine R5 on the assumption that the branch is taken,
the impossibility becomes apparent and results in an invariant violation
for R5: umin32 is greater than umax32.

The previous patch fixes this by using regs_refine_cond_op and
reg_bounds_sync in is_branch_taken to detect the impossible branch. The
fallthrough branch is therefore correctly detected as dead code.

Link: https://syzkaller.appspot.com/bug?extid=c950cc277150935cc0b5 [1]
Signed-off-by: Paul Chaignon <paul.chaignon@gmail.com>
Acked-by: Mykyta Yatsenko <yatsenko@meta.com>
---
.../selftests/bpf/progs/verifier_bounds.c | 29 +++++++++++++++++++
1 file changed, 29 insertions(+)

diff --git a/tools/testing/selftests/bpf/progs/verifier_bounds.c b/tools/testing/selftests/bpf/progs/verifier_bounds.c
index bb20f0f06f05..df7c62a12318 100644
--- a/tools/testing/selftests/bpf/progs/verifier_bounds.c
+++ b/tools/testing/selftests/bpf/progs/verifier_bounds.c
@@ -2165,4 +2165,33 @@ l0_%=: r0 = 0; \
: __clobber_all);
}

+/*
+ * Last jump can be detected as always taken because the intersection of R5 and
+ * R7 32bit tnums produces a constant that isn't within R7's s32 bounds.
+ */
+SEC("socket")
+__description("dead branch: tnums give impossible constant if equal")
+__success
+__flag(BPF_F_TEST_REG_INVARIANTS)
+__naked void tnums_equal_impossible_constant(void *ctx)
+{
+ asm volatile(" \
+ call %[bpf_get_prandom_u32]; \
+ r5 = r0; \
+ /* Set r5's var_off32 to (0; 0xfffffffc) */ \
+ r5 &= 0xfffffffffffffffc; \
+ r7 = r0; \
+ /* Set r7's var_off32 to (0x0; 0x1) */ \
+ r7 &= 0x1; \
+ /* Now, s32=[-43; -42], var_off32=(0xffffffd4; 0x3) */ \
+ r7 += -43; \
+ /* On fallthrough, var_off32=-44, not in s32 */ \
+ if w5 != w7 goto +1; \
+ r10 = 0; \
+ exit; \
+" :
+ : __imm(bpf_get_prandom_u32)
+ : __clobber_all);
+}
+
char _license[] SEC("license") = "GPL";
--
2.43.0