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

[Arnd Bergmann <arnd@kernel.org>]

From: Arnd Bergmann <arnd@arndb.de>

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@arndb.de>
---
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;
 
 				/* mov <dst>, <src> (if necessary) */
 				EMIT_mov(dst_reg, src_reg);
-- 
2.39.2