[PATCH v2] powerpc: fix alignment bug whithin the init sections

From: Ariel Marcovitch
Date: Sat Jan 02 2021 - 15:16:10 EST


This is a bug that causes early crashes in builds with a
.exit.text section smaller than a page and a .init.text section that
ends in the beginning of a physical page (this is kinda random, which
might explain why this wasn't really encountered before).

The init sections are ordered like this:
.init.text
.exit.text
.init.data

Currently, these sections aren't page aligned.

Because the init code might become 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.

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, make _einittext page aligned and also align .exit.text
to make sure .init.data is always seperated from the text segments.

Fixes: 060ef9d89d18 ("powerpc32: PAGE_EXEC required for inittext")
Signed-off-by: Ariel Marcovitch <ariel.marcovitch@xxxxxxxxx>
---
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 6db90cdf11da..b6c765d8e7ee 100644
--- a/arch/powerpc/kernel/vmlinux.lds.S
+++ b/arch/powerpc/kernel/vmlinux.lds.S
@@ -187,6 +187,11 @@ SECTIONS
.init.text : AT(ADDR(.init.text) - LOAD_OFFSET) {
_sinittext = .;
INIT_TEXT
+
+ /* .init.text might be RO so we must
+ * ensure this section ends in a page boundary.
+ */
+ . = ALIGN(PAGE_SIZE);
_einittext = .;
#ifdef CONFIG_PPC64
*(.tramp.ftrace.init);
@@ -200,6 +205,8 @@ SECTIONS
EXIT_TEXT
}

+ . = ALIGN(PAGE_SIZE);
+
.init.data : AT(ADDR(.init.data) - LOAD_OFFSET) {
INIT_DATA
}

base-commit: 2c85ebc57b3e1817b6ce1a6b703928e113a90442
--
2.17.1