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

[Leon Hwang]

On 2026/7/3 17:44, Pedro Falcato wrote:
> 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 at 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.

$ man mseal
No manual entry for mseal

When searching "mseal manual" using Google, this doc is the first entry.

So, this change is worthy.

> 
>> @@ -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.


Got it. Will drop this change.

> 
>>  
>>    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.


Do you mean dropping this sentence, or this change?

> 
>>  
>>    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).
> 


Just to confirm, do you mean removing the entire function comment above
do_mseal()?

Thanks,
Leon