Re: [PATCH v2 16/19] gendwarfksyms: Add support for reserved structure fields

[Miroslav Benes <mbenes@suse.cz>]

Hi,

On Thu, 15 Aug 2024, Sami Tolvanen wrote:

> Distributions that want to maintain a stable kABI need the ability to
> add reserved fields to kernel data structures that they anticipate
> will be modified during the ABI support timeframe, either by LTS
> updates or backports.
> 
> With genksyms, developers would typically hide changes to the reserved
> fields from version calculation with #ifndef __GENKSYMS__, which would
> result in the symbol version not changing even though the actual type
> of the reserved field changes. When we process precompiled object
> files, this is again not an option.
> 
> To support stable symbol versions for reserved fields, change the
> union type processing to recognize field name prefixes, and if the
> union contains a field name that starts with __kabi_reserved, only use
> the type of that field for computing symbol versions. In other words,
> let's assume we have a structure where we want to reserve space for
> future changes:
> 
>   struct struct1 {
>     long a;
>     long __kabi_reserved_0; /* reserved for future use */
>   };
>   struct struct1 exported;
> 
> gendwarfksyms --debug produces the following output:
> 
>   variable structure_type struct1 {
>     member base_type long int byte_size(8) encoding(5) data_member_location(0),
>     member base_type long int byte_size(8) encoding(5) data_member_location(8),
>   } byte_size(16);
>   #SYMVER exported 0x67997f89
> 
> To take the reserved field into use, a distribution would replace it
> with a union, with one of the fields keeping the __kabi_reserved name
> prefix for the original type:
> 
>   struct struct1 {
>     long a;
>     union {
>       long __kabi_reserved_0;
>       struct {
>           int b;
>           int v;
>       };
>     };

yes, this is one of the approaches we use in SLES. We add kabi paddings 
to some structures in advance (see [1] as a random example) and then use 
it later if needed.

It is not the only approach. Much more often we do not have a padding and 
use alignment holes ([5]), addition of a new member to the end of a 
structure ([2] or [3]) and such "tricks" ([4] for a newly fully defined 
structure).

It is not feasible to add kabi paddings to all structures. We also have a 
different approach to kabi in terms of its coverage than RHEL does for 
example (as far as I know).

Not sure if it is interesting to upstream but I wanted to mention that it 
is not only about the ability to add reserved fields to kernel structures 
in practice.

Regards,
Miroslav

[1] https://github.com/SUSE/kernel-source/blob/SLE15-SP6/patches.suse/crypto-add-suse_kabi_padding.patch
[2] https://github.com/SUSE/kernel-source/blob/SLE15-SP6/patches.kabi/0001-iommu-Add-static-iommu_ops-release_domain.patch
[3] https://github.com/SUSE/kernel-source/blob/SLE15-SP6/patches.kabi/nfs-Block-on-write-congestion-kabi-fixup.patch.
[4] https://github.com/SUSE/kernel-source/blob/SLE15-SP6/patches.kabi/of-kabi-workaround.patch
[5] https://github.com/SUSE/kernel-source/blob/SLE15-SP6/patches.kabi/KVM-x86-Snapshot-if-a-vCPU-s-vendor-model-is-AMD-vs..patch