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

[Amir Goldstein]

On Thu, Nov 24, 2016 at 8:03 PM, Amir Goldstein <amir73il@xxxxxxxxx> wrote:
> On Thu, Nov 24, 2016 at 3:25 PM, Amir Goldstein <amir73il@xxxxxxxxx> wrote:
>> 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
>
> Well, there is a bug/feature is xfs_io that puts it in a spin when an
> open command is
> used from command line, so I will need to fix that before sumbitting
> the xfstest, but for
> documentation purpose, you can use the following command in place of rorw-map:
>
> $ xfs_io -r -c "mmap -r 0 4" -c "open foo" -c "pwrite -S 0x61 0 4" -c
> "mread -v 0 4" foo |tail -n 1
> 00000000:  62 61 62 61  baba
>
> Naturally, 'baba' is bad. 'aaaa' is good.
>

Or maybe it was meant to be used this way, which does not blow up:

$ xfs_io << EOF
open -r foo
mmap -r 0 4
open foo
pwrite -S 0x61 0 4
mread -v 0 4
EOF

>>> 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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