On Thu, May 18, 2023 at 11:26 AM Yonghong Song <yhs@xxxxxxxx> wrote:
> > I wonder now when the address will be stored as number (not string) we
> > could somehow generate relocation records and have the module loader
> > do the relocation automatically
> >
> > not sure how that works for vmlinux when it's loaded/relocated on boot
>
> Right, actual module address will mostly not match the one in dwarf.
> Some during module btf load, we should modify btf address as well
> for later use? Yes, may need to reuse some routines used in initial
> module relocation.
Few thoughts:
Initially I felt that single FUNC with multiple DECL_TAG(addr)
is better, since BTF for all funcs is the same and it's likely
one static inline function that the compiler decided not to inline
(like cpumask_weight), so when libbpf wants to attach prog to it
the kernel should automatically attach in all places.
But then noticed that actually different functions with
the same name and proto will be deduplicated into one.
Their bodies at different locations will be different.
Example: seq_show.
In this case it's better to let libbpf pick the exact one to attach.
Then realized that even the same function like cpumask_weight()
might have different body at different locations due to optimizations.
I don't think dwarf contains enough info to distinguish all the combinations.
Considering all that it's better to keep one BTF kind_func -> one addr.
If it's extended the way Alan is proposing with kind_flag
the dedup logic will not combine them due to different addresses.
Also turned out that the kernel doesn't validate decl_tag string.
The following code loads without error:
__attribute__((btf_decl_tag("\x10\xf0")));
I'm not sure whether we want to tighten decl_tag validation and how.
If we keep it as-is we can use func+decl_tag approach
to add 4 bytes of addr in the binary format (if 1st byte is not zero).
But it feels like a hack, since the kernel needs to be changed
anyway to adjust the addresses after module loading and kernel relocation.
So func with kind_flag seems like the best approach.
Regarding relocation of address in the kernel and modules...
We just need to add base_addr to all addrs-es recorded in BTF.
Both for kernel and for module BTFs.
Shouldn't be too complicated.
