linux-kernel - [RFC][PATCH 37/42] do_shmat(): grab shp->shm_file earlier, switch to alloc_file

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Message-Id: <20180711022206.12571-37-viro@ZenIV.linux.org.uk>
Date:   Wed, 11 Jul 2018 03:22:01 +0100
From:   Al Viro <viro@...IV.linux.org.uk>
To:     Linus Torvalds <torvalds@...ux-foundation.org>
Cc:     linux-fsdevel@...r.kernel.org, linux-kernel@...r.kernel.org,
        Miklos Szeredi <mszeredi@...hat.com>
Subject: [RFC][PATCH 37/42] do_shmat(): grab shp->shm_file earlier, switch to alloc_file_clone()

From: Al Viro <viro@...iv.linux.org.uk>

Signed-off-by: Al Viro <viro@...iv.linux.org.uk>
---
 ipc/shm.c | 39 ++++++++++++++++++---------------------
 1 file changed, 18 insertions(+), 21 deletions(-)

diff --git a/ipc/shm.c b/ipc/shm.c
index 051a3e1fb8df..0cebcf74b669 100644
--- a/ipc/shm.c
+++ b/ipc/shm.c
@@ -1354,14 +1354,13 @@ long do_shmat(int shmid, char __user *shmaddr, int shmflg,
 	struct shmid_kernel *shp;
 	unsigned long addr = (unsigned long)shmaddr;
 	unsigned long size;
-	struct file *file;
+	struct file *file, *base;
 	int    err;
 	unsigned long flags = MAP_SHARED;
 	unsigned long prot;
 	int acc_mode;
 	struct ipc_namespace *ns;
 	struct shm_file_data *sfd;
-	struct path path;
 	fmode_t f_mode;
 	unsigned long populate = 0;
 
@@ -1435,46 +1434,44 @@ long do_shmat(int shmid, char __user *shmaddr, int shmflg,
 		goto out_unlock;
 	}
 
-	path = shp->shm_file->f_path;
-	path_get(&path);
+	/*
+	 * We need to take a reference to the real shm file to prevent the
+	 * pointer from becoming stale in cases where the lifetime of the outer
+	 * file extends beyond that of the shm segment.  It's not usually
+	 * possible, but it can happen during remap_file_pages() emulation as
+	 * that unmaps the memory, then does ->mmap() via file reference only.
+	 * We'll deny the ->mmap() if the shm segment was since removed, but to
+	 * detect shm ID reuse we need to compare the file pointers.
+	 */
+	base = get_file(shp->shm_file);
 	shp->shm_nattch++;
-	size = i_size_read(d_inode(path.dentry));
+	size = i_size_read(file_inode(base));
 	ipc_unlock_object(&shp->shm_perm);
 	rcu_read_unlock();
 
 	err = -ENOMEM;
 	sfd = kzalloc(sizeof(*sfd), GFP_KERNEL);
 	if (!sfd) {
-		path_put(&path);
+		fput(base);
 		goto out_nattch;
 	}
 
-	file = alloc_file(&path, f_mode,
-			  is_file_hugepages(shp->shm_file) ?
+	file = alloc_file_clone(base, f_mode,
+			  is_file_hugepages(base) ?
 				&shm_file_operations_huge :
 				&shm_file_operations);
 	err = PTR_ERR(file);
 	if (IS_ERR(file)) {
 		kfree(sfd);
-		path_put(&path);
+		fput(base);
 		goto out_nattch;
 	}
 
-	file->private_data = sfd;
-	file->f_mapping = shp->shm_file->f_mapping;
 	sfd->id = shp->shm_perm.id;
 	sfd->ns = get_ipc_ns(ns);
-	/*
-	 * We need to take a reference to the real shm file to prevent the
-	 * pointer from becoming stale in cases where the lifetime of the outer
-	 * file extends beyond that of the shm segment.  It's not usually
-	 * possible, but it can happen during remap_file_pages() emulation as
-	 * that unmaps the memory, then does ->mmap() via file reference only.
-	 * We'll deny the ->mmap() if the shm segment was since removed, but to
-	 * detect shm ID reuse we need to compare the file pointers.
-	 */
-	sfd->file = get_file(shp->shm_file);
+	sfd->file = base;
 	sfd->vm_ops = NULL;
+	file->private_data = sfd;
 
 	err = security_mmap_file(file, prot, flags);
 	if (err)
-- 
2.11.0