linux-kernel - Re: [PATCH 6/7] ovl: intercept mmap

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Date:   Thu, 24 Nov 2016 20:03:25 +0200
From:   Amir Goldstein <amir73il@...il.com>
To:     Miklos Szeredi <mszeredi@...hat.com>
Cc:     linux-unionfs@...r.kernel.org,
        linux-fsdevel <linux-fsdevel@...r.kernel.org>,
        linux-kernel <linux-kernel@...r.kernel.org>
Subject: Re: [PATCH 6/7] ovl: intercept mmap

On Thu, Nov 24, 2016 at 3:25 PM, Amir Goldstein <amir73il@...il.com> wrote:
> On Thu, Nov 24, 2016 at 12:55 PM, Miklos Szeredi <mszeredi@...hat.com> 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.

>> umount mnt
>>  - 8< - - - - - 8< - - - - - 8< - - - - - 8< - - - -
>>
>> No output means success, "rorw-map: bad mmap data" means failure.
>>
>> Signed-off-by: Miklos Szeredi <mszeredi@...hat.com>
>> ---
>>  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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