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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