[PATCH v5][makedumpfile 6/9] Add makedumpfile extensions support

[HAGIO KAZUHITO(萩尾 一仁)]

On 2026/06/16 11:17, Tao Liu wrote:
> On Tue, Jun 16, 2026 at 1:39 PM HAGIO KAZUHITO(萩尾　一仁)
> <k-hagio-ab at nec.com> wrote:
>>
>> On 2026/06/16 8:03, Tao Liu wrote:
>>> Hi Stephen & Kazu,
>>>
>>> On Tue, Jun 16, 2026 at 5:12 AM Stephen Brennan
>>> <stephen.s.brennan at oracle.com> wrote:
>>>>
>>>> HAGIO KAZUHITO(萩尾　一仁)   <k-hagio-ab at nec.com> writes:
>>>>> On 2026/04/14 19:26, Tao Liu wrote:
>>>>>
>>>>>> +static bool init_kallsyms_btf(void)
>>>>>> +{
>>>>>> +    int count;
>>>>>> +    bool ret = false;
>>>>>> +    /* We will load module's btf/kallsyms on demand */
>>>>>> +    bool init_ksyms_module = false;
>>>>>> +    bool init_ktypes_module = false;
>>>>>> +
>>>>>> +    if (check_ksyms_require_modname("vmlinux", &count)) {
>>>>>
>>>>> Thank you for the explanation [1].
>>>>>
>>>>> so which code path adds "vmlinux" to the mods array before this line?
>>>>> I could not find it.
>>>>
>>>> Hello Kazu,
>>>>
>>>> In kallsyms.c from patch 2, there is the line:
>>>>     INIT_MOD_SYM(vmlinux, _stext);
>>>> This adds a struct ksym_info to the main makedumpfile executable's
>>>> array, which requests symbol "_stext" from vmlinux.
>>>>
>>>> In kallsyms.c "init_kernel_kallsyms()", again from patch 2, the first
>>>> step is to call:
>>>>      register_ksym_section((char *)__start_init_ksyms,
>>>>                            (char *)__stop_init_ksyms))
>>>> Which is a function generated by the macro REGISTER_SECTION(). The
>>>> implementation will use add_ksym_modname() whens it encounters this
>>>> symbol, ensuring that "vmlinux" is part of the modname array.
>>>
>>> Thanks for the detailed info, yes, it is exactly how "vmlinux" is
>>> added to the array.
>>
>> hmm, maybe I still misread it though..
>>
>> The init_kernel_kallsyms() is called _after_ check_ksyms_require_modname().
>> so I've meant that if no extension adds "vmlinux" to the modname array,
>> no one calls it.  is it ok?
>>
>>           if (check_ksyms_require_modname("vmlinux", &count)) {
>>                   if (!init_kernel_kallsyms())
> 
> No, even if you create a extension which doesn't require symbols from
> "vmlinux", once you load it, the check_ksyms_require_modname() will
> still be true and init_kernel_kallsyms() is called.
> 
> The reason is, with the introduction of makedumpfile.ld, and the line
> "INIT_MOD_SYM(vmlinux, _stext);" in kallsyms.c, makedumpfile and
> extensions have a similar section structure, you can regard
> makedumpfile itself as an extension:
> 
> $ readelf -S makedumpfile | grep -E 'ksyms|ktypes'
>    [25] .init_ksyms       PROGBITS         000000000046ee90  0006de90
>    [26] .init_ktypes      PROGBITS         000000000046eeb8  0006deb8
> $ readelf -S extensions/sample.so | grep -E 'ksyms|ktypes'
>    [24] .init_ksyms       PROGBITS         0000000000003108  00002108
>    [25] .init_ktypes      PROGBITS         0000000000003110  00002110
> 
> If makedumpfile doesn't load any extensions, then makedumpfile doesn't
> need to initialize its .init_ksyms/ktypes section, the behaviour stays
> the same as before;
> If makedumpfile load at least one extensions, then the extension
> subsystem will register all .init_ksyms/ktypes sections, both
> extenison's and makedumpfile's.
> 
> This is like, extension say "I need sym1 & sym2 to be ready before I
> can run", then makedumpfile say "OK, I got it, but before I can
> resolve your sym1 & sym2, I need to resolve some symbol(e.g. vmlinux's
> _stext) formyself first". So check_ksyms_require_modname("vmlinux",
> &count) will be true.

hmm, but as far as I've tested with "extensions/sample.c" on RHEL10.2:

1) add INIT_MOD_SYM(xfs, xfsstats) and GET_MOD_SYM(xfs, xfsstats)

ok

Loaded extension: /share/tmp/makedumpfile/extensions/sample.so
sample.so: The address of xfsstats is: ffffffffc0c0b540   <<--- ok
sample.so: The address of init_task is: ffffffffb4412940
sample.so: The size of task_struct is: 10240 bytes
sample.so: The offset of member mm within task_struct is: 2704 bytes
sample.so: The size of member mm within task_struct is: 8 bytes
sample.so: Your kernel is using maple tree in mm_struct

2) remove all "vmlinux" entries (only INIT_MOD_SYM(xfs, xfsstats))

skipped

Loaded extension: /share/tmp/makedumpfile/extensions/sample.so
check_required_ksyms_all_resolved: Symbol xfsstats in xfs not found
init_extensions: Skip 1th extension

3) restore INIT_MOD_SYM(vmlinux, init_task) and GET_MOD_SYM(vmlinux, init_task)

still skipped

Loaded extension: /share/tmp/makedumpfile/extensions/sample.so
check_required_ksyms_all_resolved: Symbol xfsstats in xfs not found
init_extensions: Skip 1th extension

4) also restore INIT_MOD_STRUCT_MEMBER(vmlinux, task_struct, mm) and
    GET_MOD_STRUCT_MEMBER_SSIZE(vmlinux, task_struct, mm)

ok

Loaded extension: /share/tmp/makedumpfile/extensions/sample.so
sample.so: The address of xfsstats is: ffffffffc0c0b540
sample.so: The address of init_task is: ffffffffb4412940
sample.so: The size of task_struct is: 10240 bytes


So, it looks like, with the current condition check in init_kallsyms_btf()
(check_ksyms_require_modname and check_ktypes_require_modname),
a module needs to use INIT_* for vmlinux's kallsyms and BTF.

I'm still missing something..?

Thanks,
Kazu