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bpf, x86: Mask the shift counts of the magic number division

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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: 02cfd66f3d ("bpf, x86: Divide by constants via multiplication with a magic number")
Reported-by: kernel test robot <lkp@intel.com>
Closes: https://lore.kernel.org/oe-kbuild-all/202610171029.DfoBvNXv-lkp@intel.com/
Signed-off-by: Daniel Borkmann <daniel@iogearbox.net>
Link: https://lore.kernel.org/bpf/20261018113352.730799-1-daniel@iogearbox.net
This commit is contained in:
Daniel Borkmann 2026-10-18 13:33:52 +02:00
parent a6303535c2
commit 73a5accbfd
1 changed files with 2 additions and 2 deletions
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4
arch/x86/net/bpf_jit_comp.c
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@ -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;