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

[Will Deacon]

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@xxxxxxxxxx>
> ---
>  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.

Does marking 'struct bpf_plt' as __packed help?

Will