On 3/25/21 6:10 AM, Arnaldo Carvalho de Melo wrote:
> Em Wed, Mar 24, 2021 at 11:53:16PM -0700, Yonghong Song escreveu:
>> For vmlinux built with clang thin-lto or lto for latest bpf-next,
>> there exist cross cu debuginfo type references. For example,
>>        compile unit 1:
>>           tag 10:  type A
>>        compile unit 2:
>>           ...
>>             refer to type A (tag 10 in compile unit 1)
>> I only checked a few but have seen type A may be a simple type
>> like "unsigned char" or a complex type like an array of base types.
>> I am using latest llvm trunk and bpf-next. I suspect llvm12 or
>> linus tree >= 5.12 rc2 should be able to exhibit the issue as well.
>> Both thin-lto and lto have the same issues.
>>
>> Current pahole cannot handle this. It will report types cannot
>> be found error. Bill Wendling has attempted to fix the issue
>> with [1] by permitting all tags/types are hashed to the same
>> hash table and then process cu's one by one. This does not
>> really work. The reason is that each cu resolves types locally
>> so for the above example we may have
>>    compile unit 1:
>>      type A : type_id = 10
>>    compile unit 2:
>>      refer to type A : type A will be resolved as type id = 10
>> But id 10 refers to compile unit 1, we will get either out
>> of bound type id or incorrect one.
>>
>> This patch set is a continuation of Bill's work. We still
>> increase the hashtable size and traverse all cu's before
>> recoding and finalization. But instead of creating one-to-one
>> mapping between debuginfo cu and pahole cu, we just create
>> one pahole cu, which should solve the above incorrect type
>> id issue.
>>
>> Patches #1 and #2 are refactoring the existing code
>> and Patch #3 added an option "merge_cus" to permit
>> merging all debuginfo cu's into one pahole cu.
>> For vmlinux built, it can be detected that if LTO or Thin-LTO
>> is enabled, "merge_cus" can be added into pahole
>> command line.
>>
>>    [1] https://www.spinics.net/lists/dwarves/msg00999.html
> 
> Thanks for working on this, I'll look at it today.

Thanks! In case that you want to test with the kernel, I attached a 
patch on top of bpf-next to use --merge_cus when building kernel and 
modules.

> 
> - Arnaldo
>