Re: LLVM bug when storing unpacked struct?

["Julian P Samaroo" <jpsamaroo@jpsamaroo.me>]

On Wed Jun 2, 2021 at 12:19 PM CDT, Yonghong Song wrote:
>
>
> On 6/2/21 9:57 AM, Julian P Samaroo wrote:
> > This is my first LKML email, so let me know if I'm doing something wrong! :)
> > 
> > I believe I've found a bug in LLVM's generation of BPF bytecode, and would like
> > to get advice on whether this is truly a bug before considering writing a
> > patch.
> > 
> > When storing an unpacked struct such as { i64, i32 } to the stack (as part of
> > writing a struct-typed map key), LLVM 11.0.1 generates BPF bytecode like the
> > following:
> > 
> > ...
> > 2: (b7) r1 = 2
> > 3: (63) *(u32 *)(r10 -24) = r1
> > 4: (b7) r1 = 4
> > 5: (7b) *(u64 *)(r10 -32) = r1
> > ...
> > 8: (bf) r3 = r10
> > 9: (07) r3 += -32
> > ...
> > 13: (85) call bpf_map_update_elem#2
> > invalid indirect read from stack off -32+12 size 16
> > 
> > The verifier understandably complains about this when verifying a call that
> > uses these stack slots, such as bpf_map_update_elem, because the associated map
> > definition has a key size of 16 bytes, not 12 bytes as this bytecode would
> > suggest. In my particular case that generated this code, my frontend doesn't
> > have the notion of packed structs, so I can't workaround this by making the
> > struct packed.
> > 
> > My belief is that for unpacked structs, LLVM should emit these stores as 64-bit
> > stores, which should be OK since the padding bytes are going to be zero (from
> > my limited understanding of LLVM structs). Does this seem like a reasonable
>
> Your assumption about padding bytes to be zero is not correct. Except
> explicitly requesting to fill padding bytes with zero e.g., using
> __builtin_memset(), the compiler doesn't need to write to padding bytes.
> So this is not a compiler bug.
>
> The best approach is to do manual padding or using __builtin_memset()
> before assigning values to each individual field.
>

Ok, that makes sense to me! Thanks for pointing that out :)

> > change to make? I'm also unable to test this on LLVM 12 (my language hasn't yet
> > updated to support that version), so this could have possibly already been
> > fixed; please let me know if so!
> > 
> > Julian
> >