Re: [PATCH bpf-next 2/4] selftests/bpf: utility function to get program disassembly after jit

[Yonghong Song <yonghong.song@linux.dev>]

On 8/8/24 6:05 PM, Eduard Zingerman wrote:
>      int get_jited_program_text(int fd, char *text, size_t text_sz)
>
> Loads and disassembles jited instructions for program pointed to by fd.
> Much like 'bpftool prog dump jited ...'.
>
> The code and makefile changes are inspired by jit_disasm.c from bpftool.
> Use llvm libraries to disassemble BPF program instead of libbfd to avoid
> issues with disassembly output stability pointed out in [1].
>
> Selftests makefile uses Makefile.feature to detect if LLVM libraries
> are available. If that is not the case selftests build proceeds but
> the function returns -ENOTSUP at runtime.
>
> [1] commit eb9d1acf634b ("bpftool: Add LLVM as default library for disassembling JIT-ed programs")
>
> Signed-off-by: Eduard Zingerman <eddyz87@gmail.com>
> ---
>   tools/testing/selftests/bpf/.gitignore        |   1 +
>   tools/testing/selftests/bpf/Makefile          |  51 +++-
>   .../selftests/bpf/jit_disasm_helpers.c        | 228 ++++++++++++++++++
>   .../selftests/bpf/jit_disasm_helpers.h        |  10 +
>   4 files changed, 288 insertions(+), 2 deletions(-)
>   create mode 100644 tools/testing/selftests/bpf/jit_disasm_helpers.c
>   create mode 100644 tools/testing/selftests/bpf/jit_disasm_helpers.h
>
[...]
> +static int disasm_one_func(FILE *text_out, uint8_t *image, __u32 len)
> +{
> +	char *label, *colon, *triple = NULL;
> +	LLVMDisasmContextRef ctx = NULL;
> +	struct local_labels labels = {};
> +	__u32 *label_pc, pc;
> +	int i, cnt, err = 0;
> +	char buf[256];
> +
> +	triple = LLVMGetDefaultTargetTriple();
> +	ctx = LLVMCreateDisasm(triple, &labels, 0, NULL, lookup_symbol);
> +	if (!ASSERT_OK_PTR(ctx, "LLVMCreateDisasm")) {
> +		err = -EINVAL;
> +		goto out;
> +	}
> +
> +	cnt = LLVMSetDisasmOptions(ctx, LLVMDisassembler_Option_PrintImmHex);
> +	if (!ASSERT_EQ(cnt, 1, "LLVMSetDisasmOptions")) {
> +		err = -EINVAL;
> +		goto out;
> +	}
> +
> +	/* discover labels */
> +	labels.prog_len = len;
> +	pc = 0;
> +	while (pc < len) {
> +		cnt = disasm_insn(ctx, image, len, pc, buf, 1);
> +		if (cnt < 0) {
> +			err = cnt;
> +			goto out;
> +		}
> +		pc += cnt;
> +	}
> +	qsort(labels.pcs, labels.cnt, sizeof(*labels.pcs), cmp_u32);
> +	/* GCC can't figure max bound for i and thus reports possible truncation */
> +#pragma GCC diagnostic push
> +#pragma GCC diagnostic ignored "-Wformat-truncation"
> +	for (i = 0; i < labels.cnt; ++i)
> +		snprintf(labels.names[i], sizeof(labels.names[i]), "L%d", i);
> +#pragma GCC diagnostic pop

"-Wformat-truncation" is only available for llvm >= 18. One of my build with llvm15
has the following warning/error:

jit_disasm_helpers.c:113:32: error: unknown warning group '-Wformat-truncation', ignored [-Werror,-Wunknown-warning-option]
#pragma GCC diagnostic ignored "-Wformat-truncation"

Maybe you want to guard with proper clang version?
Not sure on gcc side when "-Wformat-truncation" is supported.

> +
> +	/* now print with labels */
> +	labels.print_phase = true;
> +	pc = 0;
> +	while (pc < len) {
> +		cnt = disasm_insn(ctx, image, len, pc, buf, sizeof(buf));
> +		if (cnt < 0) {
> +			err = cnt;
> +			goto out;
> +		}
> +		label_pc = bsearch(&pc, labels.pcs, labels.cnt, sizeof(*labels.pcs), cmp_u32);
> +		label = "";
> +		colon = "";
> +		if (label_pc) {
> +			label = labels.names[label_pc - labels.pcs];
> +			colon = ":";
> +		}
> +		fprintf(text_out, "%x:\t", pc);
> +		for (i = 0; i < cnt; ++i)
> +			fprintf(text_out, "%02x ", image[pc + i]);
> +		for (i = cnt * 3; i < 12 * 3; ++i)
> +			fputc(' ', text_out);
> +		fprintf(text_out, "%s%s%s\n", label, colon, buf);
> +		pc += cnt;
> +	}
> +
> +out:
> +	if (triple)
> +		LLVMDisposeMessage(triple);
> +	if (ctx)
> +		LLVMDisasmDispose(ctx);
> +	return err;
> +}
> +
[...]