selftests/bpf: Tests for is_scalar_branch_taken tnum logic

This patch adds tests for the new jeq and jne logic in
is_scalar_branch_taken. The following shows the first test failing
before the previous patch is applied. Once the previous patch is
applied, the verifier can use the tnum values to deduce that instruction
7 is dead code.

  0: call bpf_get_prandom_u32#7  ; R0_w=scalar()
  1: w0 = w0                     ; R0_w=scalar(smin=0,smax=umax=0xffffffff,var_off=(0x0; 0xffffffff))
  2: r0 >>= 30                   ; R0_w=scalar(smin=smin32=0,smax=umax=smax32=umax32=3,var_off=(0x0; 0x3))
  3: r0 <<= 30                   ; R0_w=scalar(smin=0,smax=umax=umax32=0xc0000000,smax32=0x40000000,var_off=(0x0; 0xc0000000))
  4: r1 = r0                     ; R0_w=scalar(id=1,smin=0,smax=umax=umax32=0xc0000000,smax32=0x40000000,var_off=(0x0; 0xc0000000)) R1_w=scalar(id=1,smin=0,smax=umax=umax32=0xc0000000,smax32=0x40000000,var_off=(0x0; 0xc0000000))
  5: r1 += 1024                  ; R1_w=scalar(smin=umin=umin32=1024,smax=umax=umax32=0xc0000400,smin32=0x80000400,smax32=0x40000400,var_off=(0x400; 0xc0000000))
  6: if r1 != r0 goto pc+1       ; R0_w=scalar(id=1,smin=umin=umin32=1024,smax=umax=umax32=0xc0000000,smin32=0x80000400,smax32=0x40000000,var_off=(0x400; 0xc0000000)) R1_w=scalar(smin=umin=umin32=1024,smax=umax=umax32=0xc0000000,smin32=0x80000400,smax32=0x40000400,var_off=(0x400; 0xc0000000))
  7: r10 = 0
  frame pointer is read only

Signed-off-by: Paul Chaignon <[email protected]>
Signed-off-by: Daniel Borkmann <[email protected]>
Acked-by: Eduard Zingerman <[email protected]>
Link: https://lore.kernel.org/bpf/550004f935e2553bdb2fb1f09cbde7d0452112d0.1755694148.git.paul.chaignon@gmail.com

Author: pchaigno

tools/testing/selftests/bpf/progs/verifier_bounds.c

@@ -1668,4 +1668,45 @@ l0_%=:	r0 = 0;				\
 	: __clobber_all);
 }
 
+SEC("socket")
+__description("dead jne branch due to disagreeing tnums")
+__success __log_level(2)
+__naked void jne_disagreeing_tnums(void *ctx)
+{
+	asm volatile("			\
+	call %[bpf_get_prandom_u32];	\
+	w0 = w0;			\
+	r0 >>= 30;			\
+	r0 <<= 30;			\
+	r1 = r0;			\
+	r1 += 1024;			\
+	if r1 != r0 goto +1;		\
+	r10 = 0;			\
+	exit;				\
+"	:
+	: __imm(bpf_get_prandom_u32)
+	: __clobber_all);
+}
+
+SEC("socket")
+__description("dead jeq branch due to disagreeing tnums")
+__success __log_level(2)
+__naked void jeq_disagreeing_tnums(void *ctx)
+{
+	asm volatile("			\
+	call %[bpf_get_prandom_u32];	\
+	w0 = w0;			\
+	r0 >>= 30;			\
+	r0 <<= 30;			\
+	r1 = r0;			\
+	r1 += 1024;			\
+	if r1 == r0 goto +1;		\
+	exit;				\
+	r10 = 0;			\
+	exit;				\
+"	:
+	: __imm(bpf_get_prandom_u32)
+	: __clobber_all);
+}
+
 char _license[] SEC("license") = "GPL";