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

[Alexei Starovoitov <alexei.starovoitov@gmail.com>]

On Thu, Apr 24, 2025 at 6:38 AM Alexis Lothoré
<alexis.lothore@bootlin.com> wrote:
>
> Hi Xu,
>
> On Thu Apr 24, 2025 at 2:00 PM CEST, Xu Kuohai wrote:
> > On 4/24/2025 3:24 AM, Alexis Lothoré wrote:
> >> 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:
>
> [...]
>
> >> 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.
> >>
> >
> > With DWARF info, we might not need to detect the structure alignment anymore,
> > since the DW_AT_location attribute tells us where the structure parameter is
> > located on the stack, and DW_AT_byte_size gives us the size of the structure.
>
> I am not sure to follow you here, because DWARF info is not accessible
> from kernel at runtime, right ? Or are you meaning that we could, at build
> time, enrich the BTF info embedded in the kernel thanks to DWARF info ?

Sounds like arm64 has complicated rules for stack alignment and
stack offset computation for passing 9th+ argument.

Since your analysis shows:
"there are about 200 functions accept 9 to 12 arguments, so adding support
for up to 12 function arguments."
I say, let's keep the existing limitation:
        if (nregs > 8)
                return -ENOTSUPP;

If there is a simple and dumb way to detect that arg9+ are scalars
with simple stack passing rules, then, sure, let's support those too,
but fancy packed/align(x)/etc let's ignore.