Re: [PATCH v12 6/8] selftests/bpf: Fixing powerpc JIT disassembly failure

> 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