Re: [tip: perf/core] perf/x86: Annotate struct bts_buffer with __counted_by()

[Thorsten Blum <thorsten.blum@linux.dev>]

On 5. Mar 2025, at 12:02, Ingo Molnar wrote:
> * Thorsten Blum <thorsten.blum@linux.dev> wrote:
>> On 5. Mar 2025, at 10:18, Ingo Molnar wrote:
>>> Actually, on a second thought:
>>> 
>>>> - buf = kzalloc_node(offsetof(struct bts_buffer, buf[nbuf]), GFP_KERNEL, node);
>>>> + buf = kzalloc_node(struct_size(buf, buf, nbuf), GFP_KERNEL, node);
>>> 
>>> Firstly, in what world is 'buf, buf' more readable? One is a member of 
>>> a structure, the other is the name of the structure - and they match, 
>>> which shows that this function's naming conventions are a mess.
>>> 
>>> Which should be fixed first ...
>> 
>> Yes, I noticed this too, but since buf->buf[] is used all over the place
>> (also in other functions), I didn't rename it in this patch.
>> 
>> We could just keep offsetof(struct bts_buffer, buf[nbuf]), or use
>> struct_size_t(struct bts_buffer, buf, nbuf) and still benefit from
>> additional compile-time checks, or rename the local variable to struct
>> bts_buffer *bts and use struct_size(bts, buf, nbuf), for example. Any
>> preferences or other ideas?
> 
> To clean up this code before changing it, so that the changes become 
> obvious to review.
> 
> Please also split out the annotation for instrumentation, it's separate 
> from any struct_size() changes, right?

Yes, I'll send a v2 with the __counted_by() annotation and submit a
separate patch for struct_size() and other changes.

>>> I'm also not sure the code is correct ...
>> 
>> Which part of it?
> 
> The size calculation. On a second reading I *think* it's correct, but 
> it's unnecessarily confusing due to the buf<->buf aliasing.
> 
> So in a cleaned up version of the code:
> 
> - If we name 'struct bts_buffer' objects 'bb'
> - and bb:buf[] is the var-array
> - and we rename 'nbuf' to 'nr_buf' (the number of bb:buf[] elements)
> 
> then the code right now does:
> 
>       bb = kzalloc_node(offsetof(struct bts_buffer, bb[nr_buf]), GFP_KERNEL, node);
> 
> ... which looks correct.

If bb:buf[] is the flexible array, it should be buf[nr_buf] like this:

	bb = kzalloc_node(offsetof(struct bts_buffer, buf[nr_buf]), GFP_KERNEL, node);

Thanks,
Thorsten