Re: [PATCH] LoongArch: ldconfig: Ignore EF_LARCH_OBJABI_V1 in shared objects

[Carlos O'Donell <carlos@redhat.com>]

On 3/27/23 12:54, Adhemerval Zanella Netto wrote:
> 
> 
> On 27/03/23 11:57, Xi Ruoyao wrote:
>> On Mon, 2023-03-27 at 09:44 -0400, Carlos O'Donell wrote:
>>> On 3/26/23 07:13, Xi Ruoyao via Libc-alpha wrote:
>>>> Binutils 2.40 sets EF_LARCH_OBJABI_V1 for shared objects:
>>>>
>>>>     $ ld --version | head -n1
>>>>     GNU ld (GNU Binutils) 2.40
>>>>     $ echo 'int dummy;' > dummy.c
>>>>     $ cc dummy.c -shared
>>>>     $ readelf -h a.out | grep Flags
>>>>     Flags:                             0x43, DOUBLE-FLOAT, OBJ-v1
>>>>
>>>> We need to ignore it in ldconfig or ldconfig will consider all shared
>>>> objects linked by Binutils 2.40 "unsupported".  Maybe we should stop
>>>> setting EF_LARCH_OBJABI_V1 for shared objects, but Binutils 2.40 is
>>>> already released and we cannot change it.
>>>> ---
>>>>  sysdeps/unix/sysv/linux/loongarch/readelflib.c | 2 +-
>>>>  1 file changed, 1 insertion(+), 1 deletion(-)
>>>>
>>>> diff --git a/sysdeps/unix/sysv/linux/loongarch/readelflib.c b/sysdeps/unix/sysv/linux/loongarch/readelflib.c
>>>> index bcaff86b36..ceba355959 100644
>>>> --- a/sysdeps/unix/sysv/linux/loongarch/readelflib.c
>>>> +++ b/sysdeps/unix/sysv/linux/loongarch/readelflib.c
>>>> @@ -40,7 +40,7 @@ process_elf_file (const char *file_name, const char *lib, int *flag,
>>>>  
>>>>    ret = process_elf64_file (file_name, lib, flag, isa_level, soname,
>>>>                                 file_contents, file_length);
>>>> -  flags = elf64_header->e_flags;
>>>> +  flags = elf64_header->e_flags & ~EF_LARCH_OBJABI_V1;
>>>
>>> Are such objects ABI compatible?
>>
>> This flag was designed for indicating the relocation type usage in a .o
>> file:

OK, so the final form of the shared objects is compatible, this is only about static
link compatibility and non-shared relocations.

I would suggest reviewing ".gnu_attribute" for an extension to Ehdr->ef_flags that
other architectures use, like ppc64le, to do such compatibility checking and error
detection.

>> If EF_LARCH_OBJABI_V1 is set, it's allowed to use relocation types with
>> ID in [64, 100], but it's prohibited to use relocation types with ID in
>> [22, 46].
>>
>> If EF_LARCH_OBJABI_V1 is not set, it's allowed to use relocation types
>> with ID in [22, 46], but it's prohibited to use relocation types with ID
>> in [64, 100].
>>
>> A linker may only support the .o files with EF_LARCH_OBJABI_V1 unset
>> (for example, GNU ld 2.38), or only support the .o files with
>> EF_LARCH_OBJABI_V1 set (for example, LLD under review at
>> https://reviews.llvm.org/D138135), or support both (for example, GNU ld
>> 2.40).
>>
>> If a linker supports both, it's OK to link a EF_LARCH_OBJABI_V1 .o file
>> together with a non-EF_LARCH_OBJABI_V1 .o file into an executable or
>> shared object.
>>
>> But none of relocation type ID in [22, 46] or [64, 100] is runtime
>> relocation.  I. e. those relocation types should not show up in a shared
>> object at all (if one shows up, the linker is buggy).  So
>> EF_LARCH_OBJABI_V1 makes no difference for shared objects. 
> 
> Since the boat is already sailed with 2.40, I think the proposed patch
> is fine (it would be better if you indeed fix it on 2.41).  I would 
> just like to add a comment on why this is required:
> 
>   /* The EF_LARCH_OBJABI_V1 flag indicate which set of static relocations
>      the object might use and it only considered during static linking, 
>      it does not reflect in runtime relocations.  However some binutils 
>      version might set it on dynamic shared object, so clear it to avoid 
>      see the SO as unsupported.  */ 

Agreed.

commit 07dd75589ecbedec5162a5645d57f8bd093a45db
Author: caiyinyu <caiyinyu@loongson.cn>
Date:   Tue Mar 28 09:19:53 2023 +0800

    LoongArch: ldconfig: Add comments for using EF_LARCH_OBJABI_V1
    
    We added Adhemerval Zanella's comment to explain the reason for
    using EF_LARCH_OBJABI_V1.

And the comment is now in the sources.

This is better, but I have a post-commit review comment here about the overall
usage of ELF flags.

While it is possible to use Ehdr->e_flags for such purposes as marking static
relocation compatibility, you also have the broader use of ".gnu_attribute"
via:

GNU Object Attributes:
https://sourceware.org/binutils/docs/as/GNU-Object-Attributes.html

Which allows a richer set of non-flag-based attributes to be used for object
files and for static link diagnostics.
 -- 
Cheers,
Carlos.