Re: [PATCH RFC bpf-next 1/4] bpf: add struct largest member size in func model

["Alexis Lothoré" <alexis.lothore@bootlin.com>]

Hi Andrii,

On Wed Apr 23, 2025 at 7:15 PM CEST, Andrii Nakryiko wrote:
> On Thu, Apr 17, 2025 at 12:14 AM Alexis Lothoré
> <alexis.lothore@bootlin.com> wrote:
>>
>> Hi Andrii,
>>
>> On Wed Apr 16, 2025 at 11:24 PM CEST, Andrii Nakryiko wrote:
>> > On Fri, Apr 11, 2025 at 1:32 PM Alexis Lothoré (eBPF Foundation)
>> > <alexis.lothore@bootlin.com> wrote:

[...]

>> Indeed I initially checked whether I could return directly some alignment
>> info from btf, but it then involves the alignment computation in the btf
>> module. Since there could be minor differences between architectures about
>> alignment requirements, I though it would be better to in fact keep alignment
>> computation out of the btf module. For example, I see that 128 bits values
>> are aligned on 16 bytes on ARM64, while being aligned on 8 bytes on S390.
>>
>> And since for ARM64, all needed alignments are somehow derived from size
>> (it is either directly size for fundamental types, or alignment of the
>> largest member for structs, which is then size of largest member),
>> returning the size seems to be enough to allow the JIT side to compute
>> alignments.
>
> If you mean the size of "primitive" field and/or array element
> (applied recursively for all embedded structs/unions) then yes, that's
> close enough. But saying just "largest struct member" is wrong,
> because for
>
> struct blah {
>     struct {
>         int whatever[128];
>     } heya;
> };
>
>
> blah.heya has a large size, but alignment is still just 4 bytes.

Indeed, that's another case making my proposal fail :)

> I'd suggest looking at btf__align_of() in libbpf (tools/lib/bpf/btf.c)
> to see how we calculate alignment there. It seems to work decently
> enough. It won't cover any arch-specific extra rules like double
> needing 16-byte alignment (I vaguely remember something like that for
> some architectures, but I might be misremembering), or anything
> similar. It also won't detect (I don't think it's possible without
> DWARF) artificially increased alignment with attribute((aligned(N))).

Thanks for the pointer, I'll take a look at it. The more we discuss this
series, the less member size sounds relevant for what I'm trying to achieve
here.

Following Xu's comments, I have been thinking about how I could detect the
custom alignments and packing on structures, and I was wondering if I could
somehow benefit from __attribute__ encoding in BTF info ([1]). But
following your hint, I also see some btf_is_struct_packed() in
tools/lib/bpf/btf_dump.c that could help. I'll dig this further and see if
I can manage to make something work with all of this.

Thanks,

Alexis

[1] https://lore.kernel.org/bpf/20250130201239.1429648-1-ihor.solodrai@linux.dev/

-- 
Alexis Lothoré, Bootlin
Embedded Linux and Kernel engineering
https://bootlin.com