Re: [PATCH 6/7] ovl: intercept mmap

[Amir Goldstein]

On Thu, Nov 24, 2016 at 12:55 PM, Miklos Szeredi <mszeredi@xxxxxxxxxx> wrote:
> ... in order to handle the corner case when the file is copied up after
> being opened read-only and mapped shared.
>
> Can be verified with the following script:
>
>  - 8< - - - - - 8< - - - - - 8< - - - - - 8< - - - -
> cd /
> rm -rf /tmp/ovl-rorw-test
> mkdir /tmp/ovl-rorw-test
> cd /tmp/ovl-rorw-test
> cat << EOF > rorw-map.c
> #include <fcntl.h>
> #include <unistd.h>
> #include <string.h>
> #include <err.h>
> #include <sys/mman.h>
>
> int main(int argc, char *argv[])
> {
>         int rofd, rwfd;
>         int ret;
>         char buf[4];
>         char *addr;
>
>         rofd = open(argv[1], O_RDONLY);
>         if (rofd == -1)
>                 err(1, "ro open");
>
>         addr = mmap(NULL, 4, PROT_READ, MAP_SHARED, rofd, 0);
>         if (addr == MAP_FAILED)
>                 err(1, "ro mmap");
>
>         if (memcmp(addr, "bubu", 4) == 0)
>                 errx(1, "identical startup data");
>
>         rwfd = open(argv[1], O_WRONLY);
>         if (rwfd == -1)
>                 err(1, "rw open");
>
>         ret = write(rwfd, "bubu", 4);
>         if (ret == -1)
>                 err(1, "write");
>         if (ret < 4)
>                 errx(1, "short write");
>
>         if (memcmp(addr, "bubu", 4) != 0)
>                 errx(1, "bad mmap data");
>
>         ret = read(rofd, buf, 4);
>         if (ret == -1)
>                 err(1, "read");
>         if (ret < 4)
>                 errx(1, "short read");
>         if (memcmp(buf, "bubu", 4) != 0)
>                 errx(1, "bad read data");
>
>         return 0;
> }
> EOF


Good timing :-)
I just started working on an xfstest this morning to bring this corner
case into attention.
Once I get the xfs_io commands in order, you could use them for a
shorter commit message.
Unless you like it that way...

> gcc -o rorw-map rorw-map.c
> mkdir -p mnt lower upper work
> echo baba > lower/foo
> mount -t overlay overlay -olowerdir=lower,upperdir=upper,workdir=work mnt
> ./rorw-map mnt/foo
> umount mnt
>  - 8< - - - - - 8< - - - - - 8< - - - - - 8< - - - -
>
> No output means success, "rorw-map: bad mmap data" means failure.
>
> Signed-off-by: Miklos Szeredi <mszeredi@xxxxxxxxxx>
> ---
>  fs/overlayfs/inode.c | 23 ++++++++++++++++++++++-
>  1 file changed, 22 insertions(+), 1 deletion(-)
>
> diff --git a/fs/overlayfs/inode.c b/fs/overlayfs/inode.c
> index 09c6f99bd5db..25b31c6ebe9e 100644
> --- a/fs/overlayfs/inode.c
> +++ b/fs/overlayfs/inode.c
> @@ -11,6 +11,7 @@
>  #include <linux/slab.h>
>  #include <linux/xattr.h>
>  #include <linux/posix_acl.h>
> +#include <linux/mm.h>
>  #include <linux/module.h>
>  #include <linux/file.h>
>  #include <linux/hashtable.h>
> @@ -392,6 +393,25 @@ static ssize_t ovl_read_iter(struct kiocb *iocb, struct iov_iter *to)
>         return ret;
>  }
>
> +static int ovl_mmap(struct file *file, struct vm_area_struct *vma)
> +{
> +       if (likely(ovl_file_is_lower(file)))
> +               return OVL_CALL_REAL_FOP(file, mmap(file, vma));
> +
> +       file = filp_clone_open(file);
> +       if (IS_ERR(file))
> +               return PTR_ERR(file);
> +
> +       fput(vma->vm_file);
> +       /* transfer ref: */
> +       vma->vm_file = file;
> +
> +       if (!file->f_op->mmap)
> +               return -ENODEV;
> +
> +       return file->f_op->mmap(file, vma);
> +}
> +
>  static struct ovl_fops *ovl_fops_find(const struct file_operations *orig)
>  {
>         struct ovl_fops *ofop;
> @@ -431,9 +451,10 @@ static struct ovl_fops *ovl_fops_get(struct file *file)
>                 /* Intercept these: */
>                 if (orig->read_iter)
>                         ofop->fops.read_iter = ovl_read_iter;
> +               if (orig->mmap)
> +                       ofop->fops.mmap = ovl_mmap;
>
>                 /* These will need to be intercepted: */
> -               ofop->fops.mmap = orig->mmap;
>                 ofop->fops.fsync = orig->fsync;
>
>                 /*
> --
> 2.5.5
>
> --
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