On Fri, Jun 6, 2025 at 12:45 AM Alexis Lothoré <alexis.loth...@bootlin.com> wrote: > > Hi Alexei, > > On Thu Jun 5, 2025 at 6:09 PM CEST, Alexei Starovoitov wrote: > > On Thu, Jun 5, 2025 at 12:35 AM Alexis Lothoré > > <alexis.loth...@bootlin.com> wrote: > >> > >> Hi Ihor, > >> > >> On Wed Jun 4, 2025 at 7:31 PM CEST, Ihor Solodrai wrote: > >> > On 6/4/25 2:02 AM, Alexis Lothoré wrote: > > [...] > > >> Thanks for the details ! I have missed this possibility, as I have been > >> assuming that DWARF info was exposing the needed info. I'll take a look at > >> it, but if those attributes can not be represented by DWARF, I'll have to > >> find another way of getting those packing/alignment modifications on data > >> type (eg: re-use/share btf__align_of from libbpf, as suggested by Andrii, > >> but it may not able to cover all cases). > > > > Not sure all the trouble is worth it. > > I feel it's a corner case. Something we don't need to fix. > > TBH I don't own any specific use case really needing this handling, so if > it does not feel worth the trouble, I'm fine with not trying to support > this. My effort is rather motivated by the goal of aligning the ARM64 > features with other platform, and so of getting rid of > tools/testing/selftests/bpf/DENYLIST.aarch64. > > For the record, this effort also showed that the same kind of issue affects > other platforms already supporting many args + structs passed by value ([1]) > - structs alignment with specific alignment constraints are not > specifically handled (eg: a struct with an __int128 as a top-level > member, leading to a 16 byte alignment requirement) > - packing and custom alignment is not handled > > From there, I could do two different things: > 1. do nothing, keep ARM64 as-is with the current version which has been > recently merged: ARM64 then denies attachment to any function trying to > pass a struct by value on stack. We keep the tracing_struct tests denied > for ARM64. Other platforms still allow to attach such functions, but may > be parsing wrongly arguments in those specific cases. > 2. add the constraint applied on ARM64 (refusing attachment when structs are > passed through stack) to other JIT compilers. Then update the > tracing_struct test to ensure this specific case is properly denied on > all platforms to avoid risking reading wrongly arguments passed through > stack when structs or large types are involved. > > I tend to think 2. is better, but let me know if you have a different > opinion here.
Agree. tracing_struct_many_args is working on x86, but assumptions about BTF being able to express everything about calling convention were not correct, so let's roll back.
