It can cause, or it causes ? Did you encounter the issue ?
Yes, in configs that result in the section layout I described, the crush is consistent.
>
> The init sections are ordered like this:
> .init.text
> .exit.text
> .init.data
>
> Currently, these sections aren't page aligned.
>
> Because the init code is mapped read-only at runtime and because the
> .init.text section can potentially reside on the same physical page as
> .init.data, the beginning of .init.data might be mapped read-only along
> with .init.text.
init code is mapped PAGE_KERNEL_TEXT.
Whether PAGE_KERNEL_TEXT is read-only or not depends on the selected options.
You are right, of course. Should I change the commit message to 'might be mapped' or something?
>
> Then when the kernel tries to modify a variable in .init.data (like
> kthreadd_done, used in kernel_init()) the kernel panics.
>
> To avoid this, I made these sections page aligned.
Should write this unpersonal, something like "To avoid this, make these sections page aligned"
Noted, thanks.
>
> Fixes: 060ef9d89d18 ("powerpc32: PAGE_EXEC required for inittext")
> Signed-off-by: Ariel Marcovitch <ariel.marcovitch@gmail.com>
> ---
> arch/powerpc/kernel/vmlinux.lds.S | 7 +++++++
> 1 file changed, 7 insertions(+)
>
> diff --git a/arch/powerpc/kernel/vmlinux.lds.S b/arch/powerpc/kernel/vmlinux.lds.S
> index 326e113d2e45..e3a7c90c03f4 100644
> --- a/arch/powerpc/kernel/vmlinux.lds.S
> +++ b/arch/powerpc/kernel/vmlinux.lds.S
> @@ -179,6 +179,11 @@ SECTIONS
> #endif
> } :text
>
> + /* .init.text is made RO and .exit.text is not, so we must
> + * ensure these sections reside in separate physical pages.
> + */
> + . = ALIGN(PAGE_SIZE);
> +
In principle, as it is text, it should be made RO as well. But what happens at the begining doesn't
really matter, anyway .exit.text should never be executed and is discarded together with init text.
So, I think it is OK the live with it as is for the time being.
Making it page aligned makes sense anyway.
Should we make _einittext page aligned instead, just like _etext ?
Yes, this will probably be better (because when _einittext is not aligned,
the part of the page after _einittext is mapped RO implicitly, and it's
hard to notice from the code). I suppose you mean something like this:
_sinittext = .;
INIT_TEXT
+
+ . = ALIGN(.);
_einittext = .;
> /* .exit.text is discarded at runtime, not link time,
> * to deal with references from __bug_table
> */
> @@ -186,6 +191,8 @@ SECTIONS
> EXIT_TEXT
> }
>
> + . = ALIGN(PAGE_SIZE);
> +
Here for sure, as you explain in the coming log, this needs to be separated from init text. So
making it aligned is a must.
> .init.data : AT(ADDR(.init.data) - LOAD_OFFSET) {
> INIT_DATA
> }
>
> base-commit: 1398820fee515873379809a6415930ad0764b2f6
>
Christophe
Thanks for your time,
Ariel Marcovitch