a936941881
Distributions that want to maintain a stable kABI need the ability
to make ABI compatible changes to kernel data structures without
affecting symbol versions, either because of LTS updates or backports.
With genksyms, developers would typically hide these changes from
version calculation with #ifndef __GENKSYMS__, which would result
in the symbol version not changing even though the actual type has
changed. When we process precompiled object files, this isn't an
option.
Change union processing to recognize field name prefixes that allow
the user to ignore the union completely during symbol versioning with
a __kabi_ignored prefix in a field name, or to replace the type of a
placeholder field using a __kabi_reserved field name prefix.
For example, assume we want to add a new field to an existing
alignment hole in a data structure, and ignore the new field when
calculating symbol versions:
struct struct1 {
int a;
/* a 4-byte alignment hole */
unsigned long b;
};
To add `int n` to the alignment hole, we can add a union that includes
a __kabi_ignored field that causes gendwarfksyms to ignore the entire
union:
struct struct1 {
int a;
union {
char __kabi_ignored_0;
int n;
};
unsigned long b;
};
With --stable, both structs produce the same symbol version.
Alternatively, when a distribution expects future modification to a
data structure, they can explicitly add reserved fields:
struct struct2 {
long a;
long __kabi_reserved_0; /* reserved for future use */
};
To take the field into use, we can again replace it with a union, with
one of the fields keeping the __kabi_reserved name prefix to indicate
the original type:
struct struct2 {
long a;
union {
long __kabi_reserved_0;
struct {
int b;
int v;
};
};
Here gendwarfksyms --stable replaces the union with the type of the
placeholder field when calculating versions.
Signed-off-by: Sami Tolvanen <samitolvanen@google.com>
Reviewed-by: Petr Pavlu <petr.pavlu@suse.com>
Signed-off-by: Masahiro Yamada <masahiroy@kernel.org>
30 lines
447 B
C
30 lines
447 B
C
// SPDX-License-Identifier: GPL-2.0
|
|
/*
|
|
* kabi_ex.c
|
|
*
|
|
* Copyright (C) 2024 Google LLC
|
|
*
|
|
* Examples for kABI stability features with --stable. See kabi_ex.h
|
|
* for details.
|
|
*/
|
|
|
|
#include "kabi_ex.h"
|
|
|
|
struct s e0;
|
|
enum e e1;
|
|
|
|
struct ex0a ex0a;
|
|
struct ex0b ex0b;
|
|
struct ex0c ex0c;
|
|
|
|
struct ex1a ex1a;
|
|
struct ex1b ex1b;
|
|
struct ex1c ex1c;
|
|
|
|
struct ex2a ex2a;
|
|
struct ex2b ex2b;
|
|
struct ex2c ex2c;
|
|
|
|
struct ex3a ex3a;
|
|
struct ex3b ex3b;
|
|
struct ex3c ex3c;
|