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

[Andrii Nakryiko <andrii.nakryiko@gmail.com>]

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:
> >>
> >> In order to properly JIT the trampolines needed to attach BPF programs
> >> to functions, some architectures like ARM64 need to know about the
> >> alignment needed for the function arguments. Such alignment can
> >> generally be deduced from the argument size, but that's not completely
> >> true for composite types. In the specific case of ARM64, the AAPCS64 ABI
> >> defines that a composite type which needs to be passed through stack
> >> must be aligned on the maximum between 8 and the largest alignment
> >> constraint of its first-level members. So the JIT compiler needs more
> >> information about the arguments to make sure to generate code that
> >> respects those alignment constraints.
> >>
> >> For struct arguments, add information about the size of the largest
> >> first-level member in the struct btf_func_model to allow the JIT
> >> compiler to guess the needed alignment. The information is quite
> >
> > I might be missing something, but how can the *size* of the field be
> > used to calculate that argument's *alignment*? i.e., I don't
> > understand why arg_largest_member_size needs to be calculated instead
> > of arg_largest_member_alignment...
>
> 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.

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))).

>
> >> specific, but it allows to keep arch-specific concerns (ie: guessing the
> >> final needed alignment for an argument) isolated in each JIT compiler.
> >
> > couldn't all this information be calculated in the JIT compiler (if
> > JIT needs that) from BTF?
>
> From what I understand, the JIT compiler does not have access to BTF info,
> only a substract from it, arranged in a struct btf_func_model ? This
> struct btf_func_model already has size info for standard types, but for
> structs we need some additional info about the members, hence this
> arg_largest_member_alignment addition in btf_func_model.
>
> Thanks,
>
> Alexis
>
> --
> Alexis Lothoré, Bootlin
> Embedded Linux and Kernel engineering
> https://bootlin.com
>