RE: [PATCH v2 08/10] hash: fix unaligned access in predictable RSS

[Konstantin Ananyev <konstantin.ananyev@huawei.com>]

> On Tue, Jul 1, 2025 at 10:36 AM Konstantin Ananyev
> <konstantin.ananyev@huawei.com> wrote:
> > > Caught by UBSan:
> > >
> > > ../lib/hash/rte_thash.c:421:8: runtime error: load of misaligned address
> > >       0x0001816c2da3 for type 'uint32_t' (aka 'unsigned int'),
> > >       which requires 4 byte alignment
> > >
> > > Fixes: 28ebff11c2dc ("hash: add predictable RSS")
> > > Cc: stable@dpdk.org
> > >
> > > Signed-off-by: David Marchand <david.marchand@redhat.com>
> > > ---
> > >  lib/hash/rte_thash.c | 6 +++---
> > >  1 file changed, 3 insertions(+), 3 deletions(-)
> > >
> > > diff --git a/lib/hash/rte_thash.c b/lib/hash/rte_thash.c
> > > index 6c662bf14f..6d4dbea6d7 100644
> > > --- a/lib/hash/rte_thash.c
> > > +++ b/lib/hash/rte_thash.c
> > > @@ -415,10 +415,10 @@ generate_subkey(struct rte_thash_ctx *ctx, struct thash_lfsr *lfsr,
> > >  static inline uint32_t
> > >  get_subvalue(struct rte_thash_ctx *ctx, uint32_t offset)
> > >  {
> > > -     uint32_t *tmp, val;
> > > +     uint32_t tmp, val;
> > >
> > > -     tmp = (uint32_t *)(&ctx->hash_key[offset >> 3]);
> > > -     val = rte_be_to_cpu_32(*tmp);
> > > +     memcpy(&tmp, &ctx->hash_key[offset >> 3], sizeof(tmp));
> > > +     val = rte_be_to_cpu_32(tmp);
> >
> > Just wonder do you guys consider it as a real one?
> > AFAIK, all architectures that we care about do support unaligned load for 32-bit integers.
> 
> Well this is undefined behavior, regardless of what the architecture support.
> And the compiler may end up generating wrong code.

Probably, though AFAIK, we do have a lot of code that load 32-bit values from possibly
non-aligned addresses (nearly all packet parsing does that).
I wonder why it only complained only about that one? 
BTW, would our 'unaligned_uint32_t' type help here?

> I could revisit this change with the aliasing trick (used for rte_memcpy).
> 
> 
> --
> David Marchand
>