On Fri, Dec 18, 2020 at 5:39 PM Christophe Leroy <
christophe.leroy@csgroup.eu> wrote:

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.

quoted

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?

quoted

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.

quoted

Fixes: 060ef9d89d18 ("powerpc32: PAGE_EXEC required for inittext")
Signed-off-by: Ariel Marcovitch <redacted>
---
  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

quoted

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,

quoted

       * 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.

quoted

      .init.data : AT(ADDR(.init.data) - LOAD_OFFSET) {
              INIT_DATA
      }

base-commit: 1398820fee515873379809a6415930ad0764b2f6

Christophe

Thanks for your time,
        Ariel Marcovitch