[PATCH 3/3] mm/mseal: remove further superfluous comments, do_mseal()

[Lorenzo Stoakes (ARM)]

There's no need to abstract do_mseal() any longer so put the system call
implementation in the system call declaration.

The comment around do_mseal() is strangely formatted, overly long and adds
a lot of superfluous information that the code already provides, so boil it
down to the essentials.

Signed-off-by: Lorenzo Stoakes (ARM) <ljs@xxxxxxxxxx>
---
 mm/mseal.c | 74 ++++++++++++++------------------------------------------------
 1 file changed, 16 insertions(+), 58 deletions(-)

diff --git a/mm/mseal.c b/mm/mseal.c
index 5930551d84f2..d01ab35d3f0f 100644
--- a/mm/mseal.c
+++ b/mm/mseal.c
@@ -99,60 +99,24 @@ void mseal_mmap_page_zero(void)
 }
 
 /*
- * mseal(2) seals the VM's meta data from
- * selected syscalls.
+ * Seal VMAs in the specified input range to prevent an attacker replacing what
+ * is mapped in the range with something else.
  *
- * addr/len: VM address range.
+ * Disallows:
+ * - VMA unmapping, remapping or shrinking.
+ * - Overwriting the VMA with another one via mmap(), mremap() or similar.
+ * - Alteration of properties via mprotect()/pkey_mprotect().
+ * - Destructive madvise() behaviours (like MADV_DONTNEED) on anonymous read-only
+ *   ranges.
  *
- *  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.
+ * Since unmapped ranges can be mapped at any time, the input range must span
+ * mapped ranges only.
  *
- *  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.
+ * The flags parameter is currently reserved.
  */
-static int do_mseal(unsigned long start, size_t len_in, unsigned long flags)
+SYSCALL_DEFINE3(mseal, unsigned long, start, size_t, len, unsigned long, flags)
 {
-	size_t len;
+	size_t len_aligned;
 	unsigned long end;
 
 	/* Verify flags not set. */
@@ -163,12 +127,12 @@ static int do_mseal(unsigned long start, size_t len_in, unsigned long flags)
 	if (!PAGE_ALIGNED(start))
 		return -EINVAL;
 
-	len = PAGE_ALIGN(len_in);
+	len_aligned = PAGE_ALIGN(len);
 	/* Check to see whether len was rounded up from small -ve to zero. */
-	if (len_in && !len)
+	if (len && !len_aligned)
 		return -EINVAL;
 
-	end = start + len;
+	end = start + len_aligned;
 	if (end < start)
 		return -EINVAL;
 
@@ -177,9 +141,3 @@ static int do_mseal(unsigned long start, size_t len_in, unsigned long flags)
 
 	return mseal(start, end);
 }
-
-SYSCALL_DEFINE3(mseal, unsigned long, start, size_t, len, unsigned long,
-		flags)
-{
-	return do_mseal(start, len, flags);
-}

-- 
2.55.0