Re: [PATCH bpf-next 03/12] libbpf: Fix an error in 64bit relocation value computation

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On 5/9/22 3:37 PM, Andrii Nakryiko wrote:
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?

The '|' also works. I used '|' in other places, so will change to use '|' as well.


                 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




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