Re: [PATCH dwarves v3 5/5] btf_encoder: Align .BTF section to 8 bytes

[Giuliano Procida <gprocida@google.com>]

On Mon, 8 Feb 2021 at 22:30, Andrii Nakryiko <andrii.nakryiko@gmail.com> wrote:
>
> On Fri, Feb 5, 2021 at 5:42 AM Giuliano Procida <gprocida@google.com> wrote:
> >
> > This is to avoid misaligned access to BTF type structs when
> > memory-mapping ELF objects.
> >
> > Signed-off-by: Giuliano Procida <gprocida@google.com>
> > ---
>
> I trust you did verify that it actually works in cases where
> previously .BTF was mis-aligned?
>

Yes. :-)

> Acked-by: Andrii Nakryiko <andrii@kernel.org>
>
> >  libbtf.c | 9 +++++++++
> >  1 file changed, 9 insertions(+)
> >
> > diff --git a/libbtf.c b/libbtf.c
> > index 9f4abb3..6754a17 100644
> > --- a/libbtf.c
> > +++ b/libbtf.c
> > @@ -744,6 +744,14 @@ static int btf_elf__write(const char *filename, struct btf *btf)
> >                 goto out;
> >         }
> >
> > +       /*
> > +        * We'll align .BTF to 8 bytes to cater for all architectures. It'd be
> > +        * nice if we could fetch this value from somewhere. The BTF
> > +        * specification does not discuss alignment and its trailing string
> > +        * table is not currently padded to any particular alignment.
> > +        */
> > +       const size_t btf_alignment = 8;
> > +
> >         /*
> >          * First we check if there is already a .BTF section present.
> >          */
> > @@ -821,6 +829,7 @@ static int btf_elf__write(const char *filename, struct btf *btf)
> >                 elf_error("elf_getshdr(btf_scn) failed");
> >                 goto out;
> >         }
> > +       btf_shdr.sh_addralign = btf_alignment;
> >         btf_shdr.sh_entsize = 0;
> >         btf_shdr.sh_flags = 0;
> >         if (dot_btf_offset)
> > --
> > 2.30.0.478.g8a0d178c01-goog
> >
>
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