Re: [PATCH bpf-next 2/3] selftests/bpf: Add tests for linked register tracking with negative offsets

[Eduard Zingerman <eddyz87@gmail.com>]

On Wed, 2026-01-07 at 12:39 -0800, Puranjay Mohan wrote:

[...]

> +/*
> + * Test that sync_linked_regs() correctly handles large offset differences.
> + * r1.off = S32_MIN, r2.off = 1, delta = S32_MIN - 1 requires 64-bit math.
> + */
> +SEC("socket")
> +__description("scalars: linked regs sync with large delta (S32_MIN offset)")
> +__success
> +__naked void scalars_sync_delta_overflow(void)
> +{
> +	asm volatile ("					\
> +	call %[bpf_get_prandom_u32];			\
> +	r0 &= 0xff;					\
> +	r1 = r0;					\
> +	r2 = r0;					\
> +	r1 += %[s32_min];				\
> +	r2 += 1;					\
> +	if r2 s< 100 goto l2_overflow;			\

If I remove the above check the test case still passes.
What is the purpose of the test?
If the purpose is to check that S32_MIN can be used as delta for
BPF_ADD operations, then constants in the second comparison should be
picked in a way for comparison to be unpredictable w/o first comparison.

> +	if r1 s< 0 goto l2_overflow;			\

> +	r0 /= 0;					\
> +l2_overflow:						\
> +	r0 = 0;						\
> +	exit;						\
> +"	:
> +	: __imm(bpf_get_prandom_u32),
> +	  [s32_min]"i"((int)(-2147483647 - 1))
> +	: __clobber_all);
> +}
> +
> +/*
> + * Another large delta case: r1.off = S32_MAX, r2.off = -1.
> + * delta = S32_MAX - (-1) = S32_MAX + 1 requires 64-bit math.
> + */
> +SEC("socket")
> +__description("scalars: linked regs sync with large delta (S32_MAX offset)")
> +__success
> +__naked void scalars_sync_delta_overflow_large_range(void)
> +{
> +	asm volatile ("					\
> +	call %[bpf_get_prandom_u32];			\
> +	r0 &= 0xff;					\
> +	r1 = r0;					\
> +	r2 = r0;					\
> +	r1 += %[s32_max];				\
> +	r2 += -1;					\
> +	if r2 s< 0 goto l2_large;			\

Same issue here.

> +	if r1 s>= 0 goto l2_large;			\
> +	r0 /= 0;					\
> +l2_large:						\
> +	r0 = 0;						\
> +	exit;						\
> +"	:
> +	: __imm(bpf_get_prandom_u32),
> +	  [s32_max]"i"((int)2147483647)
                       ^^^^^^^^^^^^^^^
Out of curiosity, did you write this by hand or was it generated?

> +	: __clobber_all);
> +}
> +
>  char _license[] SEC("license") = "GPL";