Re: [PATCH] mm/mseal: fix mseal documentation for 32-bit kernels

[Pedro Falcato <pfalcato@suse.de>]

On Fri, Jul 03, 2026 at 10:25:07AM +0800, Leon Hwang wrote:
> mseal.o is built only for 64-bit kernels, so 32-bit kernels fall back
> to sys_ni_syscall() and return -ENOSYS rather than -EPERM.
> 
> Document the -EINTR return from mmap_write_lock_killable(), fix the
> CONFIG_MSEAL_SYSTEM_MAPPINGS typo, and describe system mappings in
> terms of VM_SEALED_SYSMAP.
> 
> Signed-off-by: Leon Hwang <leon.hwang@linux.dev>
> ---
>  Documentation/userspace-api/mseal.rst | 18 ++++++++++--------
>  init/Kconfig                          |  2 +-
>  mm/mseal.c                            |  4 ++--
>  3 files changed, 13 insertions(+), 11 deletions(-)
> 
> diff --git a/Documentation/userspace-api/mseal.rst b/Documentation/userspace-api/mseal.rst
> index ea9b11a0bd89..1f1cf206670c 100644
> --- a/Documentation/userspace-api/mseal.rst
> +++ b/Documentation/userspace-api/mseal.rst
> @@ -50,8 +50,10 @@ mseal syscall signature
>           * The start address (``addr``) is not allocated.
>           * The end address (``addr`` + ``len``) is not allocated.
>           * A gap (unallocated memory) between start and end address.
> -      - **-EPERM**:
> -         * sealing is supported only on 64-bit CPUs, 32-bit is not supported.
> +      - **-EINTR**:
> +         * Interrupted while waiting for the mmap write lock.
> +      - **-ENOSYS**:
> +         * The kernel does not implement ``mseal()``.
>  
>     **Note about error return**:
>        - For above error cases, users can expect the given memory range is

Honestly, this whole thing needs to be deleted. We need a proper manpage.

> @@ -62,7 +64,8 @@ mseal syscall signature
>          memory range could happen. However, those cases should be rare.
>  
>     **Architecture support**:
> -      mseal only works on 64-bit CPUs, not 32-bit CPUs.
> +      mseal is built only for 64-bit kernels. 32-bit kernels return
> +      ``-ENOSYS``.

This LGTM.

>  
>     **Idempotent**:
>        users can call mseal multiple times. mseal on an already sealed memory
> @@ -131,20 +134,19 @@ Use cases
>  - Chrome browser: protect some security sensitive data structures.
>  
>  - System mappings:
> -  The system mappings are created by the kernel and includes vdso, vvar,
> +  The system mappings are created by the kernel and include vdso, vvar,
>    vvar_vclock, vectors (arm compat-mode), sigpage (arm compat-mode), uprobes.
>  
>    Those system mappings are readonly only or execute only, memory sealing can
> -  protect them from ever changing to writable or unmmap/remapped as different
> +  protect them from ever changing to writable or unmapped/remapped as different
>    attributes. This is useful to mitigate memory corruption issues where a
>    corrupted pointer is passed to a memory management system.

Also LGTM.

>  
>    If supported by an architecture (CONFIG_ARCH_SUPPORTS_MSEAL_SYSTEM_MAPPINGS),
> -  the CONFIG_MSEAL_SYSTEM_MAPPINGS seals all system mappings of this
> -  architecture.
> +  CONFIG_MSEAL_SYSTEM_MAPPINGS seals mappings marked with VM_SEALED_SYSMAP.

VM_SEALED_SYSMAP isn't meaningful to userspace.

>  
>    The following architectures currently support this feature: x86-64, arm64,
> -  loongarch and s390.
> +  loongarch, riscv, and s390.

This is also useless, every 64-bit architecture will support this.

>  
>    WARNING: This feature breaks programs which rely on relocating
>    or unmapping system mappings. Known broken software at the time
> diff --git a/init/Kconfig b/init/Kconfig
> index 5230d4879b1c..12bb39f637b1 100644
> --- a/init/Kconfig
> +++ b/init/Kconfig
> @@ -2112,7 +2112,7 @@ config ARCH_SUPPORTS_MSEAL_SYSTEM_MAPPINGS
>  	  from a kernel perspective.
>  
>  	  After the architecture enables this, a distribution can set
> -	  CONFIG_MSEAL_SYSTEM_MAPPING to manage access to the feature.
> +	  CONFIG_MSEAL_SYSTEM_MAPPINGS to manage access to the feature.
>  
>  	  For complete descriptions of memory sealing, please see
>  	  Documentation/userspace-api/mseal.rst
> diff --git a/mm/mseal.c b/mm/mseal.c
> index 9781647483d1..0464c7b94ab9 100644
> --- a/mm/mseal.c
> +++ b/mm/mseal.c
> @@ -132,8 +132,8 @@ static int mseal_apply(struct mm_struct *mm,
>   *   addr is not a valid address (not allocated).
>   *   end (start + len) is not a valid address.
>   *   a gap (unallocated memory) between start and end.
> - *  -EPERM:
> - *  - In 32 bit architecture, sealing is not supported.
> + *  -EINTR:
> + *   interrupted while waiting for the mmap write lock.
>   * Note:
>   *  user can call mseal(2) multiple times, adding a seal on an
>   *  already sealed memory is a no-action (no error).

And this whole header needs to be deleted as well. No one's looking at
kernel code for documentation (and if they are, we did a horrendous job
at actually documenting the thing).

-- 
Pedro