Re: [PATCH bpf-next 3/3] selftests/bpf: Add some tests related to 'may_goto 0' insns

[Eduard Zingerman <eddyz87@gmail.com>]

On Wed, 2025-01-15 at 21:51 -0800, Yonghong Song wrote:
> Add both asm-based and C-based tests which have 'may_goto 0' insns.
> 
> For the following code in C-based test,
>    int i, tmp[3];
>    for (i = 0; i < 3 && can_loop; i++)
>        tmp[i] = 0;
> 
> The clang compiler (clang 19 and 20) generates
>    may_goto 2
>    may_goto 1
>    may_goto 0
>    r1 = 0
>    r2 = 0
>    r3 = 0
> 
> The above asm codes are due to llvm pass SROAPass. This ensures the
> successful verification since tmp[0-2] are initialized.  Otherwise,
> the code without SROAPass like
>    may_goto 5
>    r1 = 0
>    may_goto 3
>    r2 = 0
>    may_goto 1
>    r3 = 0
> will have verification failure.
> 
> Although from the source code C-based test should have verification
> failure, clang compiler optimization generates code with successful
> verification. If gcc generates different asm codes than clang, the
> following code can be used for gcc:
>    int i, tmp[3];
>    for (i = 0; i < 3; i++)
>        tmp[i] = 0;
> 
> Signed-off-by: Yonghong Song <yonghong.song@linux.dev>
> ---

Acked-by: Eduard Zingerman <eddyz87@gmail.com>

[...]

> +SEC("raw_tp")
> +__description("may_goto batch with offsets 2/1/0")
> +__arch_x86_64
> +__xlated("0: r0 = 1")
> +__xlated("1: exit")
> +__success
> +__naked void may_goto_batch_1(void)
> +{
> +	asm volatile (
> +	".8byte %[may_goto1];"
> +	".8byte %[may_goto2];"
> +	".8byte %[may_goto3];"
> +	"r0 = 1;"
> +	".8byte %[may_goto1];"
> +	".8byte %[may_goto2];"
> +	".8byte %[may_goto3];"
> +	"exit;"
> +	:
> +	: __imm_insn(may_goto1, BPF_RAW_INSN(BPF_JMP | BPF_JCOND, 0, 0, 2 /* offset */, 0)),
> +	  __imm_insn(may_goto2, BPF_RAW_INSN(BPF_JMP | BPF_JCOND, 0, 0, 1 /* offset */, 0)),
> +	  __imm_insn(may_goto3, BPF_RAW_INSN(BPF_JMP | BPF_JCOND, 0, 0, 0 /* offset */, 0))

Rant: may_goto3 that does +0 jump is a bit confusing :)

> +	: __clobber_all);
> +}

[...]