linux-kernel - [PATCH v3 02/10] mm/mremap: refactor initial parameter sanity checks

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Message-ID: <87f17c67a4a477e92645189bb2f67049a6973ece.1752232673.git.lorenzo.stoakes@oracle.com>
Date: Fri, 11 Jul 2025 12:38:16 +0100
From: Lorenzo Stoakes <lorenzo.stoakes@...cle.com>
To: Andrew Morton <akpm@...ux-foundation.org>
Cc: Peter Xu <peterx@...hat.com>, Alexander Viro <viro@...iv.linux.org.uk>,
        Christian Brauner <brauner@...nel.org>, Jan Kara <jack@...e.cz>,
        "Liam R . Howlett" <Liam.Howlett@...cle.com>,
        Vlastimil Babka <vbabka@...e.cz>, Jann Horn <jannh@...gle.com>,
        Pedro Falcato <pfalcato@...e.de>, Rik van Riel <riel@...riel.com>,
        linux-mm@...ck.org, linux-fsdevel@...r.kernel.org,
        linux-kernel@...r.kernel.org, linux-kselftest@...r.kernel.org
Subject: [PATCH v3 02/10] mm/mremap: refactor initial parameter sanity checks

We are currently checking some things later, and some things
immediately. Aggregate the checks and avoid ones that need not be made.

Simplify things by aligning lengths immediately. Defer setting the delta
parameter until later, which removes some duplicate code in the hugetlb
case.

We can safely perform the checks moved from mremap_to() to
check_mremap_params() because:

* If we set a new address via vrm_set_new_addr(), then this is guaranteed
  to not overlap nor to position the new VMA past TASK_SIZE, so there's no
  need to check these later.

* We can simply page align lengths immediately. We do not need to check for
  overlap nor TASK_SIZE sanity after hugetlb alignment as this asserts
  addresses are huge-aligned, then huge-aligns lengths, rounding down. This
  means any existing overlap would have already been caught.

Moving things around like this lays the groundwork for subsequent changes
to permit operations on batches of VMAs.

No functional change intended.

Signed-off-by: Lorenzo Stoakes <lorenzo.stoakes@...cle.com>
Reviewed-by: Vlastimil Babka <vbabka@...e.cz>
---
 mm/mremap.c | 29 ++++++++++++++---------------
 1 file changed, 14 insertions(+), 15 deletions(-)

diff --git a/mm/mremap.c b/mm/mremap.c
index 1815095c4bca..a00da0288c37 100644
--- a/mm/mremap.c
+++ b/mm/mremap.c
@@ -1415,14 +1415,6 @@ static unsigned long mremap_to(struct vma_remap_struct *vrm)
 	struct mm_struct *mm = current->mm;
 	unsigned long err;
 
-	/* Is the new length or address silly? */
-	if (vrm->new_len > TASK_SIZE ||
-	    vrm->new_addr > TASK_SIZE - vrm->new_len)
-		return -EINVAL;
-
-	if (vrm_overlaps(vrm))
-		return -EINVAL;
-
 	if (vrm->flags & MREMAP_FIXED) {
 		/*
 		 * In mremap_to().
@@ -1527,7 +1519,12 @@ static unsigned long check_mremap_params(struct vma_remap_struct *vrm)
 	 * for DOS-emu "duplicate shm area" thing. But
 	 * a zero new-len is nonsensical.
 	 */
-	if (!PAGE_ALIGN(vrm->new_len))
+	if (!vrm->new_len)
+		return -EINVAL;
+
+	/* Is the new length or address silly? */
+	if (vrm->new_len > TASK_SIZE ||
+	    vrm->new_addr > TASK_SIZE - vrm->new_len)
 		return -EINVAL;
 
 	/* Remainder of checks are for cases with specific new_addr. */
@@ -1546,6 +1543,10 @@ static unsigned long check_mremap_params(struct vma_remap_struct *vrm)
 	if (flags & MREMAP_DONTUNMAP && vrm->old_len != vrm->new_len)
 		return -EINVAL;
 
+	/* Target VMA must not overlap source VMA. */
+	if (vrm_overlaps(vrm))
+		return -EINVAL;
+
 	/*
 	 * move_vma() need us to stay 4 maps below the threshold, otherwise
 	 * it will bail out at the very beginning.
@@ -1622,8 +1623,6 @@ static bool align_hugetlb(struct vma_remap_struct *vrm)
 	if (vrm->new_len > vrm->old_len)
 		return false;
 
-	vrm_set_delta(vrm);
-
 	return true;
 }
 
@@ -1723,14 +1722,13 @@ static unsigned long do_mremap(struct vma_remap_struct *vrm)
 	struct vm_area_struct *vma;
 	unsigned long res;
 
+	vrm->old_len = PAGE_ALIGN(vrm->old_len);
+	vrm->new_len = PAGE_ALIGN(vrm->new_len);
+
 	res = check_mremap_params(vrm);
 	if (res)
 		return res;
 
-	vrm->old_len = PAGE_ALIGN(vrm->old_len);
-	vrm->new_len = PAGE_ALIGN(vrm->new_len);
-	vrm_set_delta(vrm);
-
 	if (mmap_write_lock_killable(mm))
 		return -EINTR;
 	vrm->mmap_locked = true;
@@ -1753,6 +1751,7 @@ static unsigned long do_mremap(struct vma_remap_struct *vrm)
 		goto out;
 	}
 
+	vrm_set_delta(vrm);
 	vrm->remap_type = vrm_remap_type(vrm);
 
 	/* Actually execute mremap. */
-- 
2.50.0