linux-kernel - Re: [PATCH v3 3/9] selftests/sgx: Handle relocations in test enclave

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Message-ID: <8c950b3d-0650-1a73-b7f7-eaddff2bc876@cs.kuleuven.be>
Date:   Fri, 25 Aug 2023 15:27:35 +0200
From:   Jo Van Bulck <jo.vanbulck@...kuleuven.be>
To:     "Huang, Kai" <kai.huang@...el.com>,
        "linux-sgx@...r.kernel.org" <linux-sgx@...r.kernel.org>,
        "jarkko@...nel.org" <jarkko@...nel.org>,
        "linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>
Cc:     "dave.hansen@...ux.intel.com" <dave.hansen@...ux.intel.com>
Subject: Re: [PATCH v3 3/9] selftests/sgx: Handle relocations in test enclave

On 22.08.23 03:11, Huang, Kai wrote:>> +/*
>> + * Symbol placed at the start of the enclave image by the linker script.
>> + * Declare this extern symbol with visibility "hidden" to ensure the
>> + * compiler does not access it through the GOT.
>> + */
>> +extern const uint8_t __attribute__((visibility("hidden"))) __encl_base;
>> +static const uint64_t encl_base = (uint64_t)&__encl_base;
> 
> I had hard time to understand this.  The __encl_base is a symbol which is a
> fixed value set by the compiler/linker.  encl_base has the real storage in the
> .data section, but the value is also build-time fixed.  IIUC we need some code
> to explicitly override it, but I don't see where it's done.  Perhaps I missed
> something?

Thank you for catching this. Such initialization would indeed have to be 
explicitly overridden at runtime and I somehow overlooked this (it seems 
I left the line to actually run the tests commented out after 
compilation in my test script for all versions; this is now fixed). 
Apologies for the confusion, my bad! I've reverted this back to an 
explicit (uit64_t)&__encl_base cast in the next patch iteration to avoid 
such confusion.

>> +
>> +typedef void (*encl_op_t)(void *);
>> +const encl_op_t encl_op_array[ENCL_OP_MAX] = {
>> +	do_encl_op_put_to_buf,
>> +	do_encl_op_get_from_buf,
>> +	do_encl_op_put_to_addr,
>> +	do_encl_op_get_from_addr,
>> +	do_encl_op_nop,
>> +	do_encl_eaccept,
>> +	do_encl_emodpe,
>> +	do_encl_init_tcs_page,
>> +};
> 
> Any reason it cannot be 'static'?

I tested static indeed also works and will include this in the next 
iteration.

>> diff --git a/tools/testing/selftests/sgx/test_encl.lds b/tools/testing/selftests/sgx/test_encl.lds
>> index 62d37160f..b86c86060 100644
>> --- a/tools/testing/selftests/sgx/test_encl.lds
>> +++ b/tools/testing/selftests/sgx/test_encl.lds
>> @@ -32,6 +32,8 @@ SECTIONS
>>   		*(.note*)
>>   		*(.debug*)
>>   		*(.eh_frame*)
>> +		*(.dyn*)
>> +		*(.gnu.hash)
> 
> This looks can be in a separate patch, because it's not directly related to what
> you are trying to fix.
> 
> But I don't want to make things unnecessarily complicated for selftests, so fine
> to me if you still want to keep it.  But if you do, perhaps you can add some
> justification to the changelog saying something like: opportunistically discard
> ".dyn*" and ".gnu.hash" which the enclave loader cannot handle.  Anyway, still
> better to make a separate patch for such purpose IMHO.

Thanks, splitting this off in a separate commit for the next iteration.

Best,
Jo