[PATCH] arm64: bpf: Fix UBSAN misaligned access in BPF JIT

[Fuad Tabba]

Hi Will,

On Tue, 24 Feb 2026 at 22:11, Will Deacon <will at kernel.org> wrote:
>
> On Tue, Feb 24, 2026 at 09:29:15AM +0000, Fuad Tabba wrote:
> > struct bpf_plt contains a u64 'target' field. The BPF JIT allocator
> > was using an alignment of 4 bytes (sizeof(u32)), which could lead
> > to the 'target' field being misaligned in the JIT buffer.
> >
> > Increase the alignment requirement to 8 bytes (sizeof(u64)) in
> > bpf_jit_binary_pack_alloc() to guarantee proper alignment for
> > struct bpf_plt.
> >
> > Fixes: b2ad54e1533e9 ("bpf, arm64: Implement bpf_arch_text_poke() for arm64")
> > Signed-off-by: Fuad Tabba <tabba at google.com>
> > ---
> >  arch/arm64/net/bpf_jit_comp.c | 2 +-
> >  1 file changed, 1 insertion(+), 1 deletion(-)
> >
> > diff --git a/arch/arm64/net/bpf_jit_comp.c b/arch/arm64/net/bpf_jit_comp.c
> > index 356d33c7a4ae..adf84962d579 100644
> > --- a/arch/arm64/net/bpf_jit_comp.c
> > +++ b/arch/arm64/net/bpf_jit_comp.c
> > @@ -2119,7 +2119,7 @@ struct bpf_prog *bpf_int_jit_compile(struct bpf_prog *prog)
> >       extable_offset = round_up(prog_size + PLT_TARGET_SIZE, extable_align);
> >       image_size = extable_offset + extable_size;
> >       ro_header = bpf_jit_binary_pack_alloc(image_size, &ro_image_ptr,
> > -                                           sizeof(u32), &header, &image_ptr,
> > +                                           sizeof(u64), &header, &image_ptr,
> >                                             jit_fill_hole);
>
> Did you see a functional issue with this or are you just trying to squash
> a UBSAN splat? I can't see an issue with the code and it seems a bit
> over-the-top to over-align the whole JIT buffer just because of structure
> alignment rules that don't really make sense for the actual PLT.

I didn't see a functional issue. I was debugging something else, and
ran into the UBSAN splat.

> Does marking 'struct bpf_plt' as __packed help?

It does. It drops the compiler's alignment assumption silencing the
UBSAN warning without touching the allocator, and the generated
assembly is identical. I'll respin a v2 with that fix instead.

Thanks,
/fuad

>
> Will