Re: [PATCH bpf-next v2 4/4] libbpf: Fix ptr_is_aligned() usages

[Ilya Leoshkevich <iii@linux.ibm.com>]

On Wed, 2021-10-20 at 11:44 -0700, Andrii Nakryiko wrote:
> On Wed, Oct 13, 2021 at 9:09 AM Ilya Leoshkevich <iii@linux.ibm.com>
> wrote:
> > 
> > Currently ptr_is_aligned() takes size, and not alignment, as a
> > parameter, which may be overly pessimistic e.g. for __i128 on s390,
> > which must be only 8-byte aligned. Fix by using btf__align_of()
> > where
> > possible - one notable exception is ptr_sz, for which there is no
> > corresponding type.
> > 
> > Signed-off-by: Ilya Leoshkevich <iii@linux.ibm.com>
> > ---
> >  tools/lib/bpf/btf_dump.c | 12 +++++-------
> >  1 file changed, 5 insertions(+), 7 deletions(-)
> > 
> > diff --git a/tools/lib/bpf/btf_dump.c b/tools/lib/bpf/btf_dump.c
> > index 25ce60828e8d..da345520892f 100644
> > --- a/tools/lib/bpf/btf_dump.c
> > +++ b/tools/lib/bpf/btf_dump.c
> > @@ -1657,9 +1657,9 @@ static int
> > btf_dump_base_type_check_zero(struct btf_dump *d,
> >         return 0;
> >  }
> > 
> > -static bool ptr_is_aligned(const void *data, int data_sz)
> > +static bool ptr_is_aligned(const void *data, int alignment)
> >  {
> > -       return ((uintptr_t)data) % data_sz == 0;
> > +       return ((uintptr_t)data) % alignment == 0;
> 
> btf__align_of() can return 0 on error and this will be div by 0. I
> think the better approach would be for ptr_is_aligned to accept
> struct
> btf *btf and __u32 type_id, call btf__align_of() based on btf and
> type
> id, handle 0 case pessimistically (assume not aligned).

I thought about this, but it won't cover the ptr_sz case. Maybe we
just need two functions - I'll give it a try tomorrow.