Re: [PATCH dwarves] dwarf_loader: handle subprogram ret type with abstract_origin properly

[Yonghong Song <yhs@fb.com>]

On 4/2/21 7:57 AM, Arnaldo Carvalho de Melo wrote:
> Em Fri, Apr 02, 2021 at 11:04:18AM -0300, Arnaldo Carvalho de Melo escreveu:
>> Em Thu, Apr 01, 2021 at 05:00:46PM -0700, David Blaikie escreveu:
>>> On Thu, Apr 1, 2021 at 4:41 PM Yonghong Song <yhs@fb.com> wrote:
>>>> On 4/1/21 3:27 PM, David Blaikie wrote:
>>>>> Though people may come up with novel uses of DWARF features. What would
>>>>> happen if this constraint were violated/what's your motivation for
>>>>> asking (I don't quite understand the connection between test_progs
>>>>> failure description, and this question)
> 
>>>> I have some codes to check the tag associated with abstract_origin
>>>> for a subprogram must be a subprogram. Through experiment, I didn't
>>>> see a violation, so I wonder that I can get confirmation from you
>>>> and then I may delete that code.
> 
>>>> The test_progs failure exposed the bug, that is all.
> 
>>>> pahole cannot handle all weird usages of dwarf, so I think pahole
>>>> is fine only to support well-formed dwarf.
> 
>>> Sounds good. Thanks for the context!
> 
>> David, since you took the time to go thru the changes and to agree that
>> Yonghong's fix is good, can I add a:
> 
>> Acked-by: David Blaikie <dblaikie@gmail.com>
> 
>> to this patch?
> 
>> Maybe even a:
> 
>> Reviewed-by: David Blaikie <dblaikie@gmail.com>
> 
> What I have is at tmp.master, please take a look and check that
> everything is ok, the only think I wished to fix but I think can be left
> for later is in the tmp.master branch at:
> 
>   git://git.kernel.org/pub/scm/devel/pahole/pahole.git tmp.master

Thanks. I checked out the branch and did some testing with latest clang 
trunk (just pulled in).

With kernel LTO note support, I tested gcc non-lto, and llvm-lto mode, 
it works fine.

Without kernel LTO note support, I tested
    gcc non-lto  <=== ok
    llvm non-lto  <=== not ok
    llvm lto     <=== ok

Surprisingly llvm non-lto vmlinux had the same "tcp_slow_start" issue.
Some previous version of clang does not have this issue.
I double checked the dwarfdump and it is indeed has the same reason
for lto vmlinux. I checked abbrev section and there is no cross-cu
references.

That means we need to adapt this patch
    dwarf_loader: Handle subprogram ret type with abstract_origin properly
for non merging case as well.
The previous patch fixed lto subprogram abstract_origin issue,
I will submit a followup patch for this.

> 
> I did some testing for this ret type fix:
> 
> https://git.kernel.org/pub/scm/devel/pahole/pahole.git/commit/?h=tmp.master
> 
> And for the LTO ELF notes:
> 
> https://git.kernel.org/pub/scm/devel/pahole/pahole.git/commit/?h=tmp.master&id=7a79d2d7a573a863aa36fd06f540fe9fa824db4e
> 
> The only remaining thing, which I think can be left for 1.22 is:
> 
> [acme@five pahole]$ btfdiff vmlinux.clang.thin.LTO
> vmlinux.clang.thin.LTO           vmlinux.clang.thin.LTO+ELF_note
> [acme@five pahole]$ btfdiff vmlinux.clang.thin.LTO+ELF_note
> --- /tmp/btfdiff.dwarf.CtLJpQ	2021-04-02 11:55:09.658433186 -0300
> +++ /tmp/btfdiff.btf.d3L3vy	2021-04-02 11:55:09.925439277 -0300
> @@ -67255,7 +67255,7 @@ struct cpu_rmap {
>   	struct {
>   		u16                index;                /*    16     2 */
>   		u16                dist;                 /*    18     2 */
> -	} near[0]; /*    16     0 */
> +	} near[]; /*    16     0 */
> 
>   	/* size: 16, cachelines: 1, members: 5 */
>   	/* last cacheline: 16 bytes */
> @@ -101181,7 +101181,7 @@ struct linux_efi_memreserve {
>   	struct {
>   		phys_addr_t        base;                 /*    16     8 */
>   		phys_addr_t        size;                 /*    24     8 */
> -	} entry[0]; /*    16     0 */
> +	} entry[]; /*    16     0 */
> 
>   	/* size: 16, cachelines: 1, members: 4 */
>   	/* last cacheline: 16 bytes */
> @@ -113516,7 +113516,7 @@ struct netlink_policy_dump_state {
>   	struct {
>   		const struct nla_policy  * policy;       /*    16     8 */
>   		unsigned int       maxtype;              /*    24     4 */
> -	} policies[0]; /*    16     0 */
> +	} policies[]; /*    16     0 */
> 
>   	/* size: 16, cachelines: 1, members: 4 */
>   	/* sum members: 12, holes: 1, sum holes: 4 */
> [acme@five pahole]$
> 
> - Arnaldo
>