Re: [PATCH bpf-next 3/3] libbpf: Fix dumping __int128

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

On Tue, 2021-10-12 at 05:52 +0200, Andrii Nakryiko wrote:
> On Tue, Oct 12, 2021 at 4:32 AM Ilya Leoshkevich <iii@linux.ibm.com>
> wrote:
> > 
> > On s390 __int128 can be 8-byte aligned, therefore in libbpf will
> > occasionally consider variables of this type non-aligned and try to
> > dump them as a bitfield, which is supported for at most 64-bit
> > integers.
> > 
> > Fix by using the same trick as btf_dump_float_data(): copy non-
> > aligned
> > values to the local buffer.
> > 
> > Signed-off-by: Ilya Leoshkevich <iii@linux.ibm.com>
> > ---
> >  tools/lib/bpf/btf_dump.c | 9 ++++++---
> >  1 file changed, 6 insertions(+), 3 deletions(-)
> > 
> > diff --git a/tools/lib/bpf/btf_dump.c b/tools/lib/bpf/btf_dump.c
> > index ab45771d0cb4..d8264c1762e8 100644
> > --- a/tools/lib/bpf/btf_dump.c
> > +++ b/tools/lib/bpf/btf_dump.c
> > @@ -1672,9 +1672,10 @@ static int btf_dump_int_data(struct btf_dump
> > *d,
> >  {
> >         __u8 encoding = btf_int_encoding(t);
> >         bool sign = encoding & BTF_INT_SIGNED;
> > +       char buf[16] __aligned(16);
> >         int sz = t->size;
> > 
> > -       if (sz == 0) {
> > +       if (sz == 0 || sz > sizeof(buf)) {
> >                 pr_warn("unexpected size %d for id [%u]\n", sz,
> > type_id);
> >                 return -EINVAL;
> >         }
> > @@ -1682,8 +1683,10 @@ static int btf_dump_int_data(struct btf_dump
> > *d,
> >         /* handle packed int data - accesses of integers not
> > aligned on
> >          * int boundaries can cause problems on some platforms.
> >          */
> > -       if (!ptr_is_aligned(data, sz))
> > -               return btf_dump_bitfield_data(d, t, data, 0, 0);
> > +       if (!ptr_is_aligned(data, sz)) {
> 
> I think ptr_is_aligned() logic is wrong. We should probably not
> assume
> that __int128 has 16-byte alignment. Can you try fixing it by using
> btf__align_of() API to get the natural alignment?

Ok, but this patch makes sense anyway, doesn't it? I can fix
ptr_is_aligned() separately.

> 
> > +               memcpy(buf, data, sz);
> > +               data = buf;
> > +       }
> > 
> >         switch (sz) {
> >         case 16: {
> > --
> > 2.31.1
> >