[PATCH v2] mm/mseal: fix mseal documentation for 32-bit kernels

[PATCH v2] mm/mseal: fix mseal documentation for 32-bit kernels

[Leon Hwang]

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.

Drop architecture description in mseal.rst, since the arch feature
doc has the latest state of mseal for each architecture.

Fix the CONFIG_MSEAL_SYSTEM_MAPPINGS typo in init/Kconfig.

Drop the whole comment of do_mseal() to avoid stale info in the
comment, as we have the doc in mseal.rst.

Acked-by: Lance Yang <lance.yang@linux.dev>
Signed-off-by: Leon Hwang <leon.hwang@linux.dev>
---
v1 -> v2:
* Drop -EINTR description in mseal.rst. (per Mathew)
* Drop VM_SEALED_SYSMAP change in mseal.rst. (per Pedro)
* Drop architecture description in mseal.rst. (per Pedro)
* Drop the whole comment of do_mseal(). (per Pedro)
* v1: https://lore.kernel.org/linux-mm/20260703022507.187457-1-leon.hwang@linux.dev/
---
 Documentation/userspace-api/mseal.rst | 14 ++++----
 init/Kconfig                          |  2 +-
 mm/mseal.c                            | 52 ---------------------------
 3 files changed, 7 insertions(+), 61 deletions(-)

diff --git a/Documentation/userspace-api/mseal.rst b/Documentation/userspace-api/mseal.rst
index ea9b11a0bd89..c039d782f9f5 100644
--- a/Documentation/userspace-api/mseal.rst
+++ b/Documentation/userspace-api/mseal.rst
@@ -50,8 +50,8 @@ 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.
+      - **-ENOSYS**:
+         * The kernel does not implement ``mseal()``.

    **Note about error return**:
       - For above error cases, users can expect the given memory range is
@@ -62,7 +62,8 @@ 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``.

    **Idempotent**:
       users can call mseal multiple times. mseal on an already sealed memory
@@ -131,11 +132,11 @@ 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.

@@ -143,9 +144,6 @@ Use cases
   the CONFIG_MSEAL_SYSTEM_MAPPINGS seals all system mappings of this
   architecture.

-  The following architectures currently support this feature: x86-64, arm64,
-  loongarch and s390.
-
   WARNING: This feature breaks programs which rely on relocating
   or unmapping system mappings. Known broken software at the time
   of writing includes CHECKPOINT_RESTORE, UML, gVisor, rr. Therefore
diff --git a/init/Kconfig b/init/Kconfig
index 9d91074c5d0a..a1402bd82026 100644
--- a/init/Kconfig
+++ b/init/Kconfig
@@ -2120,7 +2120,7 @@ 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..69a53207ab57 100644
--- a/mm/mseal.c
+++ b/mm/mseal.c
@@ -88,58 +88,6 @@ static int mseal_apply(struct mm_struct *mm,
 	return 0;
 }

-/*
- * mseal(2) seals the VM's meta data from
- * selected syscalls.
- *
- * addr/len: VM address range.
- *
- *  The address range by addr/len must meet:
- *   start (addr) must be in a valid VMA.
- *   end (addr + len) must be in a valid VMA.
- *   no gap (unallocated memory) between start and end.
- *   start (addr) must be page aligned.
- *
- *  len: len will be page aligned implicitly.
- *
- *   Below VMA operations are blocked after sealing.
- *   1> Unmapping, moving to another location, and shrinking
- *	the size, via munmap() and mremap(), can leave an empty
- *	space, therefore can be replaced with a VMA with a new
- *	set of attributes.
- *   2> Moving or expanding a different vma into the current location,
- *	via mremap().
- *   3> Modifying a VMA via mmap(MAP_FIXED).
- *   4> Size expansion, via mremap(), does not appear to pose any
- *	specific risks to sealed VMAs. It is included anyway because
- *	the use case is unclear. In any case, users can rely on
- *	merging to expand a sealed VMA.
- *   5> mprotect and pkey_mprotect.
- *   6> Some destructive madvice() behavior (e.g. MADV_DONTNEED)
- *      for anonymous memory, when users don't have write permission to the
- *	memory. Those behaviors can alter region contents by discarding pages,
- *	effectively a memset(0) for anonymous memory.
- *
- *  flags: reserved.
- *
- * return values:
- *  zero: success.
- *  -EINVAL:
- *   invalid input flags.
- *   start address is not page aligned.
- *   Address range (start + len) overflow.
- *  -ENOMEM:
- *   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.
- * Note:
- *  user can call mseal(2) multiple times, adding a seal on an
- *  already sealed memory is a no-action (no error).
- *
- *  unseal() is not supported.
- */
 int do_mseal(unsigned long start, size_t len_in, unsigned long flags)
 {
 	size_t len;
--
2.55.0