Re: [PATCH bpf-next v2 3/3] selftests/bpf: Test the inlining of bpf_kptr_xchg()

[Eduard Zingerman <eddyz87@gmail.com>]

On Sat, 2023-12-23 at 18:40 +0800, Hou Tao wrote:
> From: Hou Tao <houtao1@huawei.com>
> 
> The test uses bpf_prog_get_info_by_fd() to obtain the xlated
> instructions of the program first. Since these instructions have
> already been rewritten by the verifier, the tests then checks whether
> the rewritten instructions are as expected.
> 
> Signed-off-by: Hou Tao <houtao1@huawei.com>

Thank you for adding this test, one nitpick below.

[...]

> +#define private(name) SEC(".bss." #name) __hidden __attribute__((aligned(8)))
> +private(kptr) struct bin_data __kptr * ptr;
> +
> +SEC("tc")
> +int kptr_xchg_inline(void *ctx)
> +{
> +	void *old;
> +
> +	old = bpf_kptr_xchg(&ptr, NULL);
> +	if (old)
> +		bpf_obj_drop(old);
> +
> +	return 0;
> +}

This is highly unlikely, but in theory nothing guarantees that LLVM
would generate code exactly as expected by pattern in test_kptr_xchg_inline().
It would be more fail-proof to use inline assembly and a naked
function instead of C code, e.g.:

SEC("tc")
__naked int kptr_xchg_inline(void)
{
	asm volatile (
		"r1 = %[ptr] ll;"
		"r2 = 0;"
		"call %[bpf_kptr_xchg];"
		"if r0 == 0 goto 1f;"
		"r1 = r0;"
		"r2 = 0;"
		"call %[bpf_obj_drop_impl];"
"1:"
		"r0 = 0;"
		"exit;"
		:
		: __imm_addr(ptr),
		  __imm(bpf_kptr_xchg),
		  __imm(bpf_obj_drop_impl)
		: __clobber_all
	);
}

/* BTF FUNC records are not generated for kfuncs referenced
 * from inline assembly. These records are necessary for
 * libbpf to link the program. The function below is a hack
 * to ensure that BTF FUNC records are generated.
 */
void __btf_root(void)
{
	bpf_obj_drop(NULL);
}

wdyt?