[PATCH V5 0/2] arm64: refactor the rodata=xxx

[PATCH V5 0/2] arm64: refactor the rodata=xxx

[Huang Shijie]

From Documentation/admin-guide/kernel-parameters.txt, we know that:
 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]

So the "rodata=on" is the default.

But the current code does not follow the document, it makes "rodata=full"
as the default.

This patch set follows Anshuman Khandual's suggetions.
It makes the "rodata=on" as the default, and removes the CONFIG_RODATA_FULL_DEFAULT_ENABLED.

v5:
  Rebase this patch set with linux-next20250627

v4:
  Follows Anshuman Khandual/Ard Biesheuvel's suggetions:
  - Change commit message format.
  - Change the titile name.
  - others
  https://lists.infradead.org/pipermail/linux-arm-kernel/2024-December/985629.html

v3:
  Follows Anshuman Khandual's suggetions:
  - Merge patch 1 and patch 3 into one patch.
  - Remove patch 4
  - update comments and document.
   https://lists.infradead.org/pipermail/linux-arm-kernel/2024-December/984344.html

v2:
  Follows Will's suggetions.
  Add a new file fine-tuning-tips.rst for the expert users.
   https://lists.infradead.org/pipermail/linux-arm-kernel/2024-November/981190.html

v1:
   https://lists.infradead.org/pipermail/linux-arm-kernel/2024-October/971415.html


Huang Shijie (2):
  arm64: refactor the rodata=xxx
  arm64/Kconfig: Remove CONFIG_RODATA_FULL_DEFAULT_ENABLED

 .../admin-guide/kernel-parameters.txt         |  2 +-
 arch/arm64/Kconfig                            | 14 ----------
 arch/arm64/include/asm/setup.h                | 28 +++++++++++++++++--
 arch/arm64/mm/pageattr.c                      |  2 +-
 4 files changed, 28 insertions(+), 18 deletions(-)

-- 
2.40.1

[PATCH V5 1/2] arm64: refactor the rodata=xxx

[Huang Shijie]

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. This patch
implements the following changes.

 - Make "rodata=on" behaviour same as the original "rodata=full"
 - 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

After this patch, the "rodata=on" will be the default on arm64 platform
as well.

Signed-off-by: Huang Shijie <shijie@os.amperecomputing.com>
---
 .../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..e0cd6dac26d3 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]
 
 	rockchip.usb_uart
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.
+ */
 static inline bool arch_parse_debug_rodata(char *arg)
 {
 	extern bool rodata_enabled;
@@ -21,7 +45,7 @@ static inline bool arch_parse_debug_rodata(char *arg)
 	if (!arg)
 		return false;
 
-	if (!strcmp(arg, "full")) {
+	if (!strcmp(arg, "on")) {
 		rodata_enabled = rodata_full = true;
 		return true;
 	}
@@ -31,7 +55,7 @@ static inline bool arch_parse_debug_rodata(char *arg)
 		return true;
 	}
 
-	if (!strcmp(arg, "on")) {
+	if (!strcmp(arg, "noalias")) {
 		rodata_enabled = true;
 		rodata_full = false;
 		return true;
-- 
2.40.1

Re: [PATCH V5 1/2] arm64: refactor the rodata=xxx

[Randy Dunlap]

On 6/29/25 8:02 PM, 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. This patch
> implements the following changes.
> 
>  - Make "rodata=on" behaviour same as the original "rodata=full"
>  - 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
> 
> After this patch, the "rodata=on" will be the default on arm64 platform
> as well.
> 
> Signed-off-by: Huang Shijie <shijie@os.amperecomputing.com>
> ---
>  .../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..e0cd6dac26d3 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.

Add space after comma, please.                 ^

>  		        [arm64]
>  
>  	rockchip.usb_uart
> 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.
> + */
>  static inline bool arch_parse_debug_rodata(char *arg)
>  {
>  	extern bool rodata_enabled;
> @@ -21,7 +45,7 @@ static inline bool arch_parse_debug_rodata(char *arg)
>  	if (!arg)
>  		return false;
>  
> -	if (!strcmp(arg, "full")) {
> +	if (!strcmp(arg, "on")) {
>  		rodata_enabled = rodata_full = true;
>  		return true;
>  	}
> @@ -31,7 +55,7 @@ static inline bool arch_parse_debug_rodata(char *arg)
>  		return true;
>  	}
>  
> -	if (!strcmp(arg, "on")) {
> +	if (!strcmp(arg, "noalias")) {
>  		rodata_enabled = true;
>  		rodata_full = false;
>  		return true;

-- 
~Randy

[PATCH V5 2/2] arm64/Kconfig: Remove CONFIG_RODATA_FULL_DEFAULT_ENABLED

[Huang Shijie]

After patch "arm64: refacotr the rodata=xxx",
the "rodata=on" becomes the default.

     ......................................
	if (!strcmp(arg, "on")) {
		rodata_enabled = rodata_full = true;
		return true;
	}
     ......................................

The rodata_full is always "true" via "rodata=on" and does not
depend on the config RODATA_FULL_DEFAULT_ENABLED anymore,
so it can be dropped.

Reviewed-by: Anshuman Khandual <anshuman.khandual@arm.com>
Signed-off-by: Huang Shijie <shijie@os.amperecomputing.com>
---
 arch/arm64/Kconfig       | 14 --------------
 arch/arm64/mm/pageattr.c |  2 +-
 2 files changed, 1 insertion(+), 15 deletions(-)

diff --git a/arch/arm64/Kconfig b/arch/arm64/Kconfig
index f9f988c2cab7..12a70f10f7bb 100644
--- a/arch/arm64/Kconfig
+++ b/arch/arm64/Kconfig
@@ -1694,20 +1694,6 @@ config MITIGATE_SPECTRE_BRANCH_HISTORY
 	  When taking an exception from user-space, a sequence of branches
 	  or a firmware call overwrites the branch history.
 
-config RODATA_FULL_DEFAULT_ENABLED
-	bool "Apply r/o permissions of VM areas also to their linear aliases"
-	default y
-	help
-	  Apply read-only attributes of VM areas 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 of the same memory page. This additional enhancement can
-	  be turned off at runtime by passing rodata=[off|on] (and turned on
-	  with rodata=full if this option is set to 'n')
-
-	  This requires the linear region to be mapped down to pages,
-	  which may adversely affect performance in some cases.
-
 config ARM64_SW_TTBR0_PAN
 	bool "Emulate Privileged Access Never using TTBR0_EL1 switching"
 	depends on !KCSAN
diff --git a/arch/arm64/mm/pageattr.c b/arch/arm64/mm/pageattr.c
index 04d4a8f676db..667aff1efe49 100644
--- a/arch/arm64/mm/pageattr.c
+++ b/arch/arm64/mm/pageattr.c
@@ -20,7 +20,7 @@ struct page_change_data {
 	pgprot_t clear_mask;
 };
 
-bool rodata_full __ro_after_init = IS_ENABLED(CONFIG_RODATA_FULL_DEFAULT_ENABLED);
+bool rodata_full __ro_after_init = true;
 
 bool can_set_direct_map(void)
 {
-- 
2.40.1