Re: [PATCH 1/2] x86/boot: move early_serial_base to .data section

[Pingfan Liu]

On Tue, May 7, 2019 at 4:28 PM Ingo Molnar <mingo@xxxxxxxxxx> wrote:
>
>
> * Pingfan Liu <kernelfans@xxxxxxxxx> wrote:
>
> > arch/x86/boot/compressed/head_64.S clears BSS after relocated. If early
> > serial is set up before clearing BSS, the early_serial_base will be reset
> > to 0.
> >
> > Initializing early_serial_base as -1 to push it to .data section.
>
> I'm wondering whether it's wise to clear the BSS after relocation to
> begin with. It already gets cleared once, and an implicit zeroing of all
> fields on kernel relocation sounds dubious to me.
>
After reading the code more closely, I think that the BSS is not fully
initialized to 0, exception the stack and heap.

Furthermore the BSS is not copied to the target address. We just copy [0, _bss).
> Is there a strong reason for that? I.e. is there some uninitialized or
> otherwise important-to-clear data there?
>
I guess the reason may be stack or heap can contain some position
dependent data. (While in practice, there is no such kind of data in
the code now days)

Thanks,
Pingfan