On 08/09/2025 19:35, Will Deacon wrote:
On Thu, Jul 03, 2025 at 05:42:11PM +0800, Huang Shijie wrote:
As per admin guide documentation, "rodata=on" should be the default on
platforms. Documentation/admin-guide/kernel-parameters.txt describes
these options as
rodata= [KNL,EARLY]
on Mark read-only kernel memory as read-only (default).
off Leave read-only kernel memory writable for debugging.
full Mark read-only kernel memory and aliases as read-only
[arm64]
But on arm64 platform, "rodata=full" is the default instead.
Please mention RODATA_FULL_DEFAULT_ENABLED here.
okay.
This patch implements the following changes.
- Make "rodata=on" behaviour same as the original "rodata=full"
You should mention that this gives us parity with x86.
No problem.
- Make "rodata=noalias" (new) behaviour same as the original "rodata=on"
- Drop the original "rodata=full"
- Add comment for arch_parse_debug_rodata()
- Update kernel-parameters.txt as required
These last two are self-evident from the code and don't need to be listed
here.
After this patch, the "rodata=on" will be the default on arm64 platform
as well.
Reviewed-by: Christoph Lameter (Ampere) <cl@xxxxxxxxxx>
Signed-off-by: Huang Shijie <shijie@xxxxxxxxxxxxxxxxxxxxxx>
---
.../admin-guide/kernel-parameters.txt | 2 +-
arch/arm64/include/asm/setup.h | 28 +++++++++++++++++--
2 files changed, 27 insertions(+), 3 deletions(-)
diff --git a/Documentation/admin-guide/kernel-parameters.txt b/Documentation/admin-guide/kernel-parameters.txt
index ee0735c6b8e2..3590bdc8d9a5 100644
--- a/Documentation/admin-guide/kernel-parameters.txt
+++ b/Documentation/admin-guide/kernel-parameters.txt
@@ -6354,7 +6354,7 @@
rodata= [KNL,EARLY]
on Mark read-only kernel memory as read-only (default).
off Leave read-only kernel memory writable for debugging.
- full Mark read-only kernel memory and aliases as read-only
+ noalias Use more block mappings, may have better performance.
[arm64]
This isn't particularly helpful documentation and I think we need to mention
the linear alias rather than talk about the page-table structure.
How about:
noalias Mark read-only kernel memory as read-only but retain
writable aliases in the direct map for regions outside
of the kernel image. [arm64]
?
Okay, thanks.
diff --git a/arch/arm64/include/asm/setup.h b/arch/arm64/include/asm/setup.h
index ba269a7a3201..6b994d0881d1 100644
--- a/arch/arm64/include/asm/setup.h
+++ b/arch/arm64/include/asm/setup.h
@@ -13,6 +13,30 @@
extern phys_addr_t __fdt_pointer __initdata;
extern u64 __cacheline_aligned boot_args[4];
+/*
+ * rodata=on (default)
+ *
+ * This applies read-only attributes to VM areas and to the linear
+ * alias of the backing pages as well. This prevents code or read-
+ * only data from being modified (inadvertently or intentionally),
+ * via another mapping for the same memory page.
+ *
+ * But this might cause linear map region to be mapped down to base
+ * pages, which may adversely affect performance in some cases.
+ *
+ * rodata=off
+ *
+ * This provides more block mappings and contiguous hints for linear
+ * map region which would minimize TLB footprint. This also leaves
+ * read-only kernel memory writable for debugging.
+ *
+ * rodata=noalias
+ *
+ * This provides more block mappings and contiguous hints for linear
+ * map region which would minimize TLB footprint. This leaves the linear
+ * alias of read-only mappings in the vmalloc space writeable, making
+ * them susceptible to inadvertent modification by software.
+ */
Please remove this comment. If you want to keep it, this information
belongs either in the commit message (to justify the performance impact)
Okay, I can move it to commit message.
Thanks
Huang Shijie
