Re: [PATCH bpf-next] selftests/bpf: add helper retval linked scalar pruning selftest

[Shung-Hsi Yu <shung-hsi.yu@suse.com>]

On Thu, Jun 11, 2026 at 09:55:55AM -0700, Alexei Starovoitov wrote:
> On Thu Jun 11, 2026 at 9:07 AM PDT, Zhenzhong Wu wrote:
> > Add a verifier runtime test for a branch pattern where a helper return
> > value and a related scalar stay live across the same control-flow
> > sequence. Rust/Aya-generated eBPF can naturally produce this shape when
> > a match on a helper status keeps data derived before the helper call
> > live across the same branches. Such code commonly uses the helper return
> > value in r0, where 0 means success, producing an r0 == 0 / r0 != 0
> > branch shape.
[...]
> > +SEC("tc")
> > +__description("helper retval linked scalar pruning")
> > +__success __retval(0)
> > +__naked void helper_retval_linked_scalar_pruning(void)
> > +{
> > +	asm volatile (
> > +	"r7 = *(u32 *)(r1 + %[__sk_buff_data_end]);"
> > +	"r5 = *(u32 *)(r1 + %[__sk_buff_data]);"
> > +	"r7 -= r5;"
> > +	"r2 = 0;"
> > +	"r3 = r10;"
> > +	"r3 += -8;"
> > +	"r4 = 1;"
> > +	"call %[bpf_skb_load_bytes];"
> > +	"r0 += 1;"
> > +	"r6 = 1;"
> > +	/* success path keeps r7 independent; failure path links r7 to r0. */
> > +	"if r0 == 1 goto l0_%=;"
> 
> this exercises linked registers with BPF_ADD_CONST logic.
> We already have such tests. Why do we need this one?
> How is it different?

BPF_ADD_CONST wasn't what was meant to be tested.

The main logic is r7.id == r0.id only happens on "if r0 == 1 goto l0_%="
fall through, and does not have such link otherwise. I only check tests
added in commit c0087d59e504 ("selftests/bpf: tests for per-insn
sync_linked_regs() precision tracking"), but it doesn't seem like such
conditional linking was tested. 

The other rational is that this seem like a common pattern that is
genereated from Rust-based BPF program.

> > +	/* success path keeps r7 independent; failure path links r7 to r0. */
> > +	"if r0 == 1 goto l0_%=;"
> > +	"r7 = r0;"
         ^^^^^^^ conditional scalar linking

> > +"l0_%=: if r0 != 1 goto l1_%=;"
> > +	"r7 <<= 32;"
> > +	"r7 >>= 32;"
> > +	"if r7 != %[test_data_len] goto l1_%=;"
> > +	"r0 = 0;"
> > +	"exit;"
> > +"l1_%=: r0 = r6;"
> > +	"exit;"
> > +	:
> > +	: __imm(bpf_skb_load_bytes),
> > +	  __imm_const(__sk_buff_data, offsetof(struct __sk_buff, data)),
> > +	  __imm_const(__sk_buff_data_end, offsetof(struct __sk_buff, data_end)),
> > +	  __imm_const(test_data_len, TEST_DATA_LEN)
> > +	: __clobber_all);
> > +}
[...]