Re: [PATCH v3 1/9] fuzz: Make fork_fuzz.ld compatible with LLVM's LLD

[Daniele Buono <dbuono@linux.vnet.ibm.com>]

Thanks Alex,
do you think you could also give it a try linking with LLD?

just add --extra-ldflags="-fuse-ld=lld"

I do see some small differences when moving from BFD ro LLD, but they 
should not be of importance. The position of the data.fuzz* is kept.

size -A on qemu-fuzz-i386, LTO DISABLED:

BFD
section                  size       addr
[...]
.got                    10704   29849128
.data                 1160800   29859840
__sancov_pcs          3362992   31020640
.data.fuzz_start       210187   34385920
.data.fuzz_ordered     211456   34596352
.bss                  9659608   34807808
.comment                  225          0
[...]

BFD
section                  size       addr
[...]
.got                      816   27824632
.got.plt                 9992   27825448
.data                 1160808   27839536
.data.fuzz_start       210187   29003776
.data.fuzz_ordered     211456   29214208
.data.fuzz_end              0   29425664
.tm_clone_table             0   29425664
__sancov_pcs          3362992   29425664
.bss                  9659624   32788672

I tried running the fuzzer and didn't seem to have any issues, but I
haven't tried a test-build with OSS-Fuzz. Is there a info somewhere
on how to do that?

Thanks,
Daniele

On 11/6/2020 9:50 AM, Alexander Bulekov wrote:
> On 201105 1718, Daniele Buono wrote:
>> LLVM's linker, LLD, supports the keyword "INSERT AFTER", starting with
>> version 11.
>> However, when multiple sections are defined in the same "INSERT AFTER",
>> they are added in a reversed order, compared to BFD's LD.
>>
>> This patch makes fork_fuzz.ld generic enough to work with both linkers.
>> Each section now has its own "INSERT AFTER" keyword, so proper ordering is
>> defined between the sections added.
>>
> 
> Hi Daniele,
> Good to know that LLVM now has support for "INSERT AFTER" :)
> 
> I compared the resulting symbols between __FUZZ_COUNTERS_{START,END}
> (after linking with BFD) before/after this patch, and they look good. I
> also ran a test-build with OSS-Fuzz container and confirmed that the
> resulting binary also had proper symbols.
> 
> Reviewed-by: Alexander Bulekov <alxndr@bu.edu>
> Tested-by: Alexander Bulekov <alxndr@bu.edu>
> 
> Thanks
> 
>> Signed-off-by: Daniele Buono <dbuono@linux.vnet.ibm.com>
>> ---
>>   tests/qtest/fuzz/fork_fuzz.ld | 12 +++++++++++-
>>   1 file changed, 11 insertions(+), 1 deletion(-)
>>
>> diff --git a/tests/qtest/fuzz/fork_fuzz.ld b/tests/qtest/fuzz/fork_fuzz.ld
>> index bfb667ed06..cfb88b7fdb 100644
>> --- a/tests/qtest/fuzz/fork_fuzz.ld
>> +++ b/tests/qtest/fuzz/fork_fuzz.ld
>> @@ -16,6 +16,11 @@ SECTIONS
>>         /* Lowest stack counter */
>>         *(__sancov_lowest_stack);
>>     }
>> +}
>> +INSERT AFTER .data;
>> +
>> +SECTIONS
>> +{
>>     .data.fuzz_ordered :
>>     {
>>         /*
>> @@ -34,6 +39,11 @@ SECTIONS
>>          */
>>          *(.bss._ZN6fuzzer3TPCE);
>>     }
>> +}
>> +INSERT AFTER .data.fuzz_start;
>> +
>> +SECTIONS
>> +{
>>     .data.fuzz_end : ALIGN(4K)
>>     {
>>         __FUZZ_COUNTERS_END = .;
>> @@ -43,4 +53,4 @@ SECTIONS
>>    * Don't overwrite the SECTIONS in the default linker script. Instead insert the
>>    * above into the default script
>>    */
>> -INSERT AFTER .data;
>> +INSERT AFTER .data.fuzz_ordered;
>> -- 
>> 2.17.1
>>
>