Re: [PATCH v2 6/9] mm/mshare: Add mmap operation

Darrick J. Wong
2022-06-30T21:44:23Z

On Wed, Jun 29, 2022 at 04:53:57PM -0600, Khalid Aziz wrote:
> mmap is used to establish address range for mshare region and map the
> region into process's address space. Add basic mmap operation that
> supports setting address range. Also fix code to not allocate new
> mm_struct for files in msharefs that exist for information and not
> for defining a new mshare region.
>
> Signed-off-by: Khalid Aziz <khalid.aziz@oracle.com>
> Signed-off-by: Matthew Wilcox (Oracle) <willy@infradead.org>
> ---
> mm/mshare.c | 48 +++++++++++++++++++++++++++++++++++++++++-------
> 1 file changed, 41 insertions(+), 7 deletions(-)
>
> diff --git a/mm/mshare.c b/mm/mshare.c
> index d238b68b0576..088a6cab1e93 100644
> --- a/mm/mshare.c
> +++ b/mm/mshare.c
> @@ -9,7 +9,8 @@
> *
> *
> * Copyright (C) 2022 Oracle Corp. All rights reserved.
> - * Author: Khalid Aziz <khalid.aziz@oracle.com>
> + * Authors: Khalid Aziz <khalid.aziz@oracle.com>
> + * Matthew Wilcox <willy@infradead.org>
> *
> */
>
> @@ -60,9 +61,36 @@ msharefs_read(struct kiocb *iocb, struct iov_iter *iov)
> return ret;
> }
>
> +static int
> +msharefs_mmap(struct file *file, struct vm_area_struct *vma)
> +{
> + struct mshare_data *info = file->private_data;
> + struct mm_struct *mm = info->mm;
> +
> + /*
> + * If this mshare region has been set up once already, bail out
> + */
> + if (mm->mmap_base != 0)
> + return -EINVAL;
> +
> + if ((vma->vm_start | vma->vm_end) & (PGDIR_SIZE - 1))
> + return -EINVAL;
> +
> + mm->mmap_base = vma->vm_start;
> + mm->task_size = vma->vm_end - vma->vm_start;
> + if (!mm->task_size)
> + mm->task_size--;
> + info->minfo->start = mm->mmap_base;
> + info->minfo->size = mm->task_size;

So, uh, if the second mmap() caller decides to ignore the mshare_info,
should they get an -EINVAL here since the memory mappings won't be at
the same process virtual address?

> + vma->vm_flags |= VM_SHARED_PT;
> + vma->vm_private_data = info;
> + return 0;
> +}
> +
> static const struct file_operations msharefs_file_operations = {
> .open = msharefs_open,
> .read_iter = msharefs_read,
> + .mmap = msharefs_mmap,
> .llseek = no_llseek,
> };
>
> @@ -119,7 +147,12 @@ msharefs_fill_mm(struct inode *inode)
> goto err_free;
> }
> info->mm = mm;
> - info->minfo = NULL;
> + info->minfo = kzalloc(sizeof(struct mshare_info), GFP_KERNEL);
> + if (info->minfo == NULL) {
> + retval = -ENOMEM;
> + goto err_free;
> + }
> +
> refcount_set(&info->refcnt, 1);
> inode->i_private = info;
>
> @@ -128,13 +161,14 @@ msharefs_fill_mm(struct inode *inode)
> err_free:
> if (mm)
> mmput(mm);
> + kfree(info->minfo);
> kfree(info);
> return retval;
> }
>
> static struct inode
> *msharefs_get_inode(struct super_block *sb, const struct inode *dir,
> - umode_t mode)
> + umode_t mode, bool newmm)
> {
> struct inode *inode = new_inode(sb);
> if (inode) {
> @@ -147,7 +181,7 @@ static struct inode
> case S_IFREG:
> inode->i_op = &msharefs_file_inode_ops;
> inode->i_fop = &msharefs_file_operations;
> - if (msharefs_fill_mm(inode) != 0) {
> + if (newmm && msharefs_fill_mm(inode) != 0) {
> discard_new_inode(inode);
> inode = ERR_PTR(-ENOMEM);
> }
> @@ -177,7 +211,7 @@ msharefs_mknod(struct user_namespace *mnt_userns, struct inode *dir,
> struct inode *inode;
> int err = 0;
>
> - inode = msharefs_get_inode(dir->i_sb, dir, mode);
> + inode = msharefs_get_inode(dir->i_sb, dir, mode, true);
> if (IS_ERR(inode))
> return PTR_ERR(inode);
>
> @@ -267,7 +301,7 @@ prepopulate_files(struct super_block *s, struct inode *dir,
> if (!dentry)
> return -ENOMEM;
>
> - inode = msharefs_get_inode(s, dir, S_IFREG | files->mode);
> + inode = msharefs_get_inode(s, dir, S_IFREG | files->mode, false);

I was wondering why the information files were getting their own
mshare_data.

TBH I'm not really sure what the difference is between mshare_data and
mshare_info, since those names are not especially distinct.

> if (!inode) {
> dput(dentry);
> return -ENOMEM;
> @@ -301,7 +335,7 @@ msharefs_fill_super(struct super_block *sb, struct fs_context *fc)
> sb->s_d_op = &msharefs_d_ops;
> sb->s_time_gran = 1;
>
> - inode = msharefs_get_inode(sb, NULL, S_IFDIR | 0777);
> + inode = msharefs_get_inode(sb, NULL, S_IFDIR | 0777, false);

Is it wise to default to world-writable? Surely whatever userspace
software wraps an msharefs can relax permissions as needed.

--D

> if (!inode) {
> err = -ENOMEM;
> goto out;
> --
> 2.32.0
>