On Sun, May 1, 2022 at 12:00 PM Yonghong Song <yhs@xxxxxx> wrote:
>
> Currently, the 64bit relocation value in the instruction
> is computed as follows:
> __u64 imm = insn[0].imm + ((__u64)insn[1].imm << 32)
>
> Suppose insn[0].imm = -1 (0xffffffff) and insn[1].imm = 1.
> With the above computation, insn[0].imm will first sign-extend
> to 64bit -1 (0xffffffffFFFFFFFF) and then add 0x1FFFFFFFF,
> producing incorrect value 0xFFFFFFFF. The correct value
> should be 0x1FFFFFFFF.
>
> Changing insn[0].imm to __u32 first will prevent 64bit sign
> extension and fix the issue.
>
> Signed-off-by: Yonghong Song <yhs@xxxxxx>
> ---
> tools/lib/bpf/relo_core.c | 2 +-
> 1 file changed, 1 insertion(+), 1 deletion(-)
>
> diff --git a/tools/lib/bpf/relo_core.c b/tools/lib/bpf/relo_core.c
> index 2ed94daabbe5..f25ffd03c3b1 100644
> --- a/tools/lib/bpf/relo_core.c
> +++ b/tools/lib/bpf/relo_core.c
> @@ -1024,7 +1024,7 @@ int bpf_core_patch_insn(const char *prog_name, struct bpf_insn *insn,
> return -EINVAL;
> }
>
> - imm = insn[0].imm + ((__u64)insn[1].imm << 32);
> + imm = (__u32)insn[0].imm + ((__u64)insn[1].imm << 32);
great catch, it should also probably be written as | instead of + operation?
> if (res->validate && imm != orig_val) {
> pr_warn("prog '%s': relo #%d: unexpected insn #%d (LDIMM64) value: got %llu, exp %llu -> %llu\n",
> prog_name, relo_idx,
> --
> 2.30.2
>
