Re: [PATCH] bpf: x86: avoid link error with CONFIG_SMP=n

[Daniel Borkmann]

On 4/4/24 2:33 PM, Arnd Bergmann wrote:
> From: Arnd Bergmann <arnd@xxxxxxxx>
>
> On UP systems, this_cpu_off is not defined, so the new percpu code in bpf
> fails to link:
>
> x86_64-linux-ld: vmlinux.o: in function `do_jit':
> bpf_jit_comp.c:(.text+0xbab14): undefined reference to `this_cpu_off'
>
> Use offset zero on UP builds instead.
>
> Fixes: 7bdbf7446305 ("bpf: add special internal-only MOV instruction to resolve per-CPU addrs")
> Signed-off-by: Arnd Bergmann <arnd@xxxxxxxx>
> ---
> I assume this is not the correct fix, or at least not the best one, so
> please treat this as a bug report. It does address the link failure for
> me, so I applied this to my randconfig build tree.
> ---
>   arch/x86/net/bpf_jit_comp.c | 5 ++++-
>   1 file changed, 4 insertions(+), 1 deletion(-)
>
> diff --git a/arch/x86/net/bpf_jit_comp.c b/arch/x86/net/bpf_jit_comp.c
> index d6ebb9136f3c..8b8eebb89a9b 100644
> --- a/arch/x86/net/bpf_jit_comp.c
> +++ b/arch/x86/net/bpf_jit_comp.c
> @@ -1383,7 +1383,10 @@ static int do_jit(struct bpf_prog *bpf_prog, int *addrs, u8 *image, u8 *rw_image
>   				EMIT3(0x0F, 0x44, add_2reg(0xC0, AUX_REG, dst_reg));
>   				break;
>   			} else if (insn_is_mov_percpu_addr(insn)) {
> -				u32 off = (u32)(unsigned long)&this_cpu_off;
> +				u32 off = 0;
> +
> +				if (IS_ENABLED(CONFIG_SMP))
> +					off = (u32)(unsigned long)&this_cpu_off;
>   

Thanks, there was already a fix in bpf-next tree in here:
https://git.kernel.org/pub/scm/linux/kernel/git/bpf/bpf-next.git/commit/?id=1e9e0b85255e6eca6036b59d8a5fbca6501905ac

>   				/* mov <dst>, <src> (if necessary) */
>   				EMIT_mov(dst_reg, src_reg);