> selftests/bpf: Fixing powerpc JIT disassembly failure
>
> Ensure that the trampoline stubs JITed at the tail of the
> epilogue do not expose the dummy trampoline address stored
> in the last 8 bytes(64-bit) and last 4 bytes(32-bit)
> to the disassembly flow. Prevent the disassembler from
> ingesting this memory address, as it may occasionally decode
> into a seemingly valid but incorrect instruction. Fix this
> issue by truncating the last 8/4 bytes from JITed buffers
> before supplying them for disassembly.
This isn't a bug, but could the subject use the imperative ('Fix ...'
instead of 'Fixing ...'), and could the in-code comment and the
changelog agree on why the trailing bytes are dropped - is the hazard
that they cannot be decoded, or that they can be decoded into a
plausible-looking wrong instruction?
> diff --git a/tools/testing/selftests/bpf/jit_disasm_helpers.c
> b/tools/testing/selftests/bpf/jit_disasm_helpers.c
> index 3558fe10e28c..b291d934dd1a 100644
> --- a/tools/testing/selftests/bpf/jit_disasm_helpers.c
> +++ b/tools/testing/selftests/bpf/jit_disasm_helpers.c
> @@ -179,9 +179,11 @@ int get_jited_program_text(int fd, char *text, size_t
> text_sz)
> struct bpf_prog_info info = {};
> __u32 info_len = sizeof(info);
> __u32 jited_funcs, len, pc;
> + __u32 trunc_len = 0, disasm_len;
> __u32 *func_lens = NULL;
> FILE *text_out = NULL;
> uint8_t *image = NULL;
> + char *triple = NULL;
> int i, err = 0;
>
> if (!llvm_initialized) {
[ ... ]
> @@ -225,9 +227,30 @@ int get_jited_program_text(int fd, char *text, size_t
> text_sz)
> if (!ASSERT_OK(err, "bpf_prog_get_info_by_fd #2"))
> goto out;
>
> + /*
> + * last 8 bytes contains dummy_trampoline address in JIT
> + * output on 64-bit and last 4 bytes on 32-bit powerpc,
> + * which can't disassemble to a valid instruction.
> + */
> + triple = LLVMGetDefaultTargetTriple();
> + if (triple) {
> + if (strstr(triple, "powerpc64") || strstr(triple, "ppc64"))
> + trunc_len = 8;
> + else if (strstr(triple, "powerpc") || strstr(triple, "ppc"))
> + trunc_len = 4;
> + LLVMDisposeMessage(triple);
> + }
This isn't a bug, but would #if defined(__powerpc64__) / #elif
defined(__powerpc__) work here, matching how get_current_arch() in
test_loader.c already detects the arch, and avoiding a second
LLVMGetDefaultTargetTriple()/LLVMDisposeMessage() pair alongside the
one in disasm_one_func()?
> +
> for (pc = 0, i = 0; i < jited_funcs; ++i) {
> fprintf(text_out, "func #%d:\n", i);
> - disasm_one_func(text_out, image + pc, func_lens[i]);
> +
> + /*
> + * Disabled JIT have zero func_lens, hence underflow
> + */
> + disasm_len = func_lens[i] > trunc_len ?
> + func_lens[i] - trunc_len : 0;
> + disasm_one_func(text_out, image + pc, disasm_len);
> +
> fprintf(text_out, "\n");
> pc += func_lens[i];
> }
This isn't a bug, but since 32-bit powerpc has no __arch_* tag and
get_current_arch() has no case for it yet, would it be clearer to add
the 4-byte arm together with the 32-bit enablement rather than ahead of
it?
---
AI reviewed your patch. Please fix the bug or email reply why it's not a bug.
See: https://github.com/kernel-patches/vmtest/blob/master/ci/claude/README.md
CI run summary: https://github.com/kernel-patches/bpf/actions/runs/32100929603
