linux-kernel - [PATCH] Using MAP_SHARE_VALIDATE in mmap without fd

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Message-ID: <20230310171617.wqnqs42l2viwjsz5@archlinux>
Date:   Fri, 10 Mar 2023 18:16:17 +0100
From:   Nils Hartmann <nils1hartmann@...il.com>
To:     Andrew Morton <akpm@...ux-foundation.org>
Cc:     linux-mm@...ck.org, linux-kernel@...r.kernel.org
Subject: [PATCH] Using MAP_SHARE_VALIDATE in mmap without fd

Hey,
I have a rather simple question about the MAP_SHARED_VALIDATE flag in mmap.
When used without a file pointer, EINVAL is returned. Is there a reason for this?
I researched a bit but could not find anything. I attached a simple patch that adds MAP_SHARE_VALIDATE to the flags switch and checks for invalid flags.

Signed-off-by: Nils Hartmann <nils1hartmann@...il.com>
---
 mm/mmap.c | 9 +++++++--
 1 file changed, 7 insertions(+), 2 deletions(-)

diff --git a/mm/mmap.c b/mm/mmap.c
index 740b54be3..fd7db51af 100644
--- a/mm/mmap.c
+++ b/mm/mmap.c
@@ -1196,6 +1196,7 @@ unsigned long do_mmap(struct file *file, unsigned long addr,
 {
 	struct mm_struct *mm = current->mm;
 	vm_flags_t vm_flags;
+	unsigned long flags_mask;
 	int pkey = 0;
 
 	validate_mm(mm);
@@ -1266,14 +1267,14 @@ unsigned long do_mmap(struct file *file, unsigned long addr,
 	if (mlock_future_check(mm, vm_flags, len))
 		return -EAGAIN;
 
+	flags_mask = LEGACY_MAP_MASK;
 	if (file) {
 		struct inode *inode = file_inode(file);
-		unsigned long flags_mask;
 
 		if (!file_mmap_ok(file, inode, pgoff, len))
 			return -EOVERFLOW;
 
-		flags_mask = LEGACY_MAP_MASK | file->f_op->mmap_supported_flags;
+		flags_mask |= file->f_op->mmap_supported_flags;
 
 		switch (flags & MAP_TYPE) {
 		case MAP_SHARED:
@@ -1327,6 +1328,10 @@ unsigned long do_mmap(struct file *file, unsigned long addr,
 		}
 	} else {
 		switch (flags & MAP_TYPE) {
+		case MAP_SHARED_VALIDATE:
+			if (flags & ~flags_mask)
+				return -EOPNOTSUPP;
+			fallthrough;
 		case MAP_SHARED:
 			if (vm_flags & (VM_GROWSDOWN|VM_GROWSUP))
 				return -EINVAL;
-- 
2.39.2