Re: [PATCH v7] livepatch: Clear relocation targets on a module removal

[Song Liu <song@kernel.org>]

On Thu, Jan 5, 2023 at 3:20 AM Petr Mladek <pmladek@suse.com> wrote:
[...]
>
> The ideal solution would be to add checks into apply_relocated_add().
> It would make it more robust. In that case, clear_relocated_add()
> would need to clear everything.
>
> But this is not the case on powerpc and s390 at the moment.
> In this case, I suggest to clear only relocations that
> are checked in apply_relocated_add().
>
> But it should be done without duplicating the code.
>
> It would actually make sense to compute the value that was
> used in apply_relocated_add() and check that we are clearing
> the value. If we try to clear some other value than we
> probably do something wrong.
>
> This might actually be a solution. We could compute
> the value in both situations. Then we could have
> a common function for writing.
>
> This write function would check that it replaces zero
> with the value in apply_relocate_add() and that it replaces
> the value with zero in clear_relocate_add().

I like this idea. But I am not quite sure whether we can do it
for all those complicated cases.

Btw: I am confused with this one:

                case R_PPC64_REL16_HA:
                        /* Subtract location pointer */
                        value -= (unsigned long)location;
                        value = ((value + 0x8000) >> 16);
                        *((uint16_t *) location)
                                = (*((uint16_t *) location) & ~0xffff)
                                | (value & 0xffff);
                        break;

(*((uint16_t *) location) & ~0xffff) should always be zero, no?

Thanks,
Song