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

Yonghong Song
2024-08-13T16:05:30Z

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;
> +}
> + …