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

[Hari Bathini <hbathini@linux.ibm.com>]

On 06/08/26 10:49 pm, Hari Bathini wrote:
> 
> 
> On 05/08/26 11:36 am, Saket Kumar Bhaskar wrote:
>> From: Abhishek Dubey <adubey@linux.ibm.com>
>>
>> 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.

With that..

Reviewed-by: Hari Bathini <hbathini@linux.ibm.com>

> 
>> Signed-off-by: Abhishek Dubey <adubey@linux.ibm.com>
>> Signed-off-by: Saket Kumar Bhaskar <skb99@linux.ibm.com>
>> Tested-by: Yeswanth Krishna Tellakula <yeswanth@linux.ibm.com>
>> ---
>>   .../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