Re: [PATCH v10 3/8] selftest/bpf: Fixing powerpc JIT disassembly failure

On 05/08/26 11:36 am, Saket Kumar Bhaskar wrote:
> From: Abhishek Dubey <[email protected]>
> 
> 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.

> 
> Fixes: b991fc520700 ("selftests/bpf: utility function to get program disassembly after jit")

The fixes tag on this patch doesn't really help. Please drop it.

> Signed-off-by: Abhishek Dubey <[email protected]>
> Signed-off-by: Saket Kumar Bhaskar <[email protected]>
> Tested-by: Yeswanth Krishna Tellakula <[email protected]>
> ---
>   .../selftests/bpf/jit_disasm_helpers.c        | 25 ++++++++++++++++++-
>   1 file changed, 24 insertions(+), 1 deletion(-)
> 
> diff --git a/tools/testing/selftests/bpf/jit_disasm_helpers.c b/tools/testing/selftests/bpf/jit_disasm_helpers.c
> index 3558fe10e28c..c4aa1d69d3bb 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);
> +	}
> +
>   	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];
>   	}

- Hari