Re: [RFC dwarves] btf_encoder: Remove duplicates from functions entries

[Nick Alcock <nick.alcock@oracle.com>]

On 21 Jul 2025, Jiri Olsa verbalised:
> On Mon, Jul 21, 2025 at 12:41:00PM +0100, Alan Maguire wrote:
>> On 17/07/2025 16:25, Jiri Olsa wrote:
>> > Menglong reported issue where we can have function in BTF which has
>> > multiple addresses in kallsysm [1].
>> > 
>> > Rather than filtering this in runtime, let's teach pahole to remove
>> > such functions.
>> > 
>> > Removing duplicate records from functions entries that have more
>> > at least one different address. This way btf_encoder__find_function
>> > won't find such functions and they won't be added in BTF.
>> > 
>> > In my setup it removed 428 functions out of 77141.
>> >
>> 
>> Is such removal necessary? If the presence of an mcount annotation is
>> the requirement, couldn't we just utilize
>> 
>> /sys/kernel/tracing/available_filter_functions_addrs
>> 
>> to map name to address safely? It identifies mcount-containing functions
>> and some of these appear to be duplicates, for example there I see
>> 
>> ffffffff8376e8b4 acpi_attr_is_visible
>> ffffffff8379b7d4 acpi_attr_is_visible
>
> for that we'd need new interface for loading fentry/fexit.. programs, right?
>
> the current interface to get fentry/fexit.. attach address is:
>   - user specifies function name, that translates to btf_id
>   - in kernel that btf id translates back to function name
>   - kernel uses kallsyms_lookup_name or find_kallsyms_symbol_value
>     to get the address
>
> so we don't really know which address user wanted in the first place
>
> I think we discussed this issue some time ago, but I'm not sure what
> the proposal was at the end (function address stored in BTF?)

Function address, translation unit name, *some* disambiguator. Really
both seem like they might be useful in different situations.

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