From 73a5accbfd210bda7e88e969310f15f8e515dc86 Mon Sep 17 00:00:00 2001 From: Daniel Borkmann Date: Sun, 18 Oct 2026 13:33:52 +0200 Subject: [PATCH] bpf, x86: Mask the shift counts of the magic number division The kernel test robot reports for a clang 22 randconfig with UBSAN and ThinLTO: vmlinux.o: error: objtool: do_jit(): unexpected end of section .text.do_jit Fix by masking the counts to their range like rol64() does, which keeps the shifts defined for the sanitizer and removes the __ubsan_handle_shift_out_of_bounds() checks. The counts are below 32 and 64 anyway, so the emitted code does not change. Reproduced with clang 20 and the robot's config without LTO. Fixes: 02cfd66f3d9a ("bpf, x86: Divide by constants via multiplication with a magic number") Reported-by: kernel test robot Closes: https://lore.kernel.org/oe-kbuild-all/202610171029.DfoBvNXv-lkp@intel.com/ Signed-off-by: Daniel Borkmann Link: https://lore.kernel.org/bpf/20261018113352.730799-1-daniel@iogearbox.net --- arch/x86/net/bpf_jit_comp.c | 4 ++-- 1 file changed, 2 insertions(+), 2 deletions(-) diff --git a/arch/x86/net/bpf_jit_comp.c b/arch/x86/net/bpf_jit_comp.c index 11a4e4f87aac..d3843045dc71 100644 --- a/arch/x86/net/bpf_jit_comp.c +++ b/arch/x86/net/bpf_jit_comp.c @@ -2080,7 +2080,7 @@ struct bpf_div_magic { */ static void bpf_magic_udiv(u64 d, int lz, struct bpf_div_magic *mag) { - u64 mask = ~0ULL >> lz; + u64 mask = ~0ULL >> (lz & 63); u64 nc, delta, q1, r1, q2, r2; int p; @@ -2200,7 +2200,7 @@ static bool emit_div_const(u8 **pprog, const struct bpf_insn *insn, u32 dst_reg) * the add sequence. */ if (mag.add && !(imm32 & 1)) { - pre = __ffs(imm32); + pre = __ffs(imm32) & 31; bpf_magic_udiv(imm32 >> pre, pre, &mag); if (mag.add) return false;