selftests/bpf: Add tests for delta tracking when src_reg == dst_reg

Extend the verifier_linked_scalars BPF selftest with a rX += rX test
such that the div-by-zero path is rejected in the fixed case.

  # LDLIBS=-static PKG_CONFIG='pkg-config --static' ./vmtest.sh -- ./test_progs -t verifier_linked_scalars
  [...]
  ./test_progs -t verifier_linked_scalars
  #612/1   verifier_linked_scalars/scalars: find linked scalars:OK
  #612/2   verifier_linked_scalars/sync_linked_regs_preserves_id:OK
  #612/3   verifier_linked_scalars/scalars_neg:OK
  #612/4   verifier_linked_scalars/scalars_neg_sub:OK
  #612/5   verifier_linked_scalars/scalars_neg_alu32_add:OK
  #612/6   verifier_linked_scalars/scalars_neg_alu32_sub:OK
  #612/7   verifier_linked_scalars/scalars_pos:OK
  #612/8   verifier_linked_scalars/scalars_sub_neg_imm:OK
  #612/9   verifier_linked_scalars/scalars_double_add:OK
  #612/10  verifier_linked_scalars/scalars_sync_delta_overflow:OK
  #612/11  verifier_linked_scalars/scalars_sync_delta_overflow_large_range:OK
  #612/12  verifier_linked_scalars/scalars_alu32_big_offset:OK
  #612/13  verifier_linked_scalars/scalars_alu32_basic:OK
  #612/14  verifier_linked_scalars/scalars_alu32_wrap:OK
  #612/15  verifier_linked_scalars/scalars_alu32_zext_linked_reg:OK
  #612/16  verifier_linked_scalars/scalars_alu32_alu64_cross_type:OK
  #612/17  verifier_linked_scalars/scalars_alu32_alu64_regsafe_pruning:OK
  #612/18  verifier_linked_scalars/alu32_negative_offset:OK
  #612/19  verifier_linked_scalars/spurious_precision_marks:OK
  #612/20  verifier_linked_scalars/scalars_self_add_clears_id:OK
  #612/21  verifier_linked_scalars/scalars_self_add_alu32_clears_id:OK
  #612     verifier_linked_scalars:OK
  Summary: 1/21 PASSED, 0 SKIPPED, 0 FAILED

Signed-off-by: Daniel Borkmann <daniel@iogearbox.net>
Link: https://lore.kernel.org/r/20260407192421.508817-3-daniel@iogearbox.net
Signed-off-by: Alexei Starovoitov <ast@kernel.org>

1 files changed, 57 insertions, 0 deletions

diff --git a/tools/testing/selftests/bpf/progs/verifier_linked_scalars.c b/tools/testing/selftests/bpf/progs/verifier_linked_scalars.c
index f4f8a055af8a..09146e5db061 100644
--- a/tools/testing/selftests/bpf/progs/verifier_linked_scalars.c
+++ b/tools/testing/selftests/bpf/progs/verifier_linked_scalars.c
@@ -535,4 +535,61 @@ int spurious_precision_marks(void *ctx)
 	return 0;
 }
 
+/*
+ * Test that r += r (self-add, src_reg == dst_reg) clears the scalar ID
+ * so that sync_linked_regs() does not propagate an incorrect delta.
+ */
+SEC("socket")
+__failure
+__msg("div by zero")
+__naked void scalars_self_add_clears_id(void)
+{
+	asm volatile ("						\
+	call %[bpf_get_prandom_u32];				\
+	r6 = r0;		/* r6 unknown, id A */		\
+	r7 = r6;		/* r7 linked to r6, id A */	\
+	call %[bpf_get_prandom_u32];				\
+	r8 = r0;		/* r8 unknown, id B */		\
+	r9 = r8;		/* r9 linked to r8, id B */	\
+	if r7 != 1 goto l_exit_%=;				\
+	/* r7 == 1; sync propagates: r6 = 1 (known, id A) */	\
+	r6 += r6;		/* r6 = 2; should clear id */	\
+	if r7 == r9 goto l_exit_%=;				\
+	/* Bug: r6 synced to r7(1)+delta(2)=3; Fix: r6 = 2 */	\
+	if r6 == 3 goto l_exit_%=;				\
+	r0 /= 0;						\
+l_exit_%=:							\
+	r0 = 0;							\
+	exit;							\
+"	:
+	: __imm(bpf_get_prandom_u32)
+	: __clobber_all);
+}
+
+/* Same as above but with alu32 such that w6 += w6 also clears id. */
+SEC("socket")
+__failure
+__msg("div by zero")
+__naked void scalars_self_add_alu32_clears_id(void)
+{
+	asm volatile ("						\
+	call %[bpf_get_prandom_u32];				\
+	w6 = w0;						\
+	w7 = w6;						\
+	call %[bpf_get_prandom_u32];				\
+	w8 = w0;						\
+	w9 = w8;						\
+	if w7 != 1 goto l_exit_%=;				\
+	w6 += w6;						\
+	if w7 == w9 goto l_exit_%=;				\
+	if w6 == 3 goto l_exit_%=;				\
+	r0 /= 0;						\
+l_exit_%=:							\
+	r0 = 0;							\
+	exit;							\
+"	:
+	: __imm(bpf_get_prandom_u32)
+	: __clobber_all);
+}
+
 char _license[] SEC("license") = "GPL";