Re: [PATCH v2 1/3] vfs: ftruncate check IS_APPEND() on real inode

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On Fri, Apr 7, 2017 at 4:10 PM, Miklos Szeredi <miklos@xxxxxxxxxx> wrote:
> On Fri, Apr 7, 2017 at 11:01 AM, Amir Goldstein <amir73il@xxxxxxxxx> wrote:
>> ftruncate an overlayfs inode was checking IS_APPEND() on
>> overlay inode, but overlay inode does not have the S_APPEND flag.
>>
>> Set 'inode' var to file_inode() so all checks are performed on
>> the real inode and use the overlay inode/sb explicitly for
>> freeze protection and file lock checks.
>
> If you'd just add the file_inode() to the IS_APPEND check then the
> patch would look a lot less complicated and the effect should be
> exactly the same.  Am I missing something?
>

Technically you are right.
Social engineering wise, its quite hard to look at a random
vfs function and say what 'inode' stands.

I surveyed some functions and found that in most cases
where file->f_inode is set a local var named inode stands
for file_inode() and locks_inode() is the exception, so this
patch brings do_sys_ftruncate() into compliance.


> Thanks,
> Miklos
>
>>
>> Signed-off-by: Amir Goldstein <amir73il@xxxxxxxxx>
>> ---
>>  fs/open.c | 8 ++++----
>>  1 file changed, 4 insertions(+), 4 deletions(-)
>>
>> diff --git a/fs/open.c b/fs/open.c
>> index 949cef2..b7d5ab1 100644
>> --- a/fs/open.c
>> +++ b/fs/open.c
>> @@ -182,7 +182,7 @@ static long do_sys_ftruncate(unsigned int fd, loff_t length, int small)
>>                 small = 0;
>>
>>         dentry = f.file->f_path.dentry;
>> -       inode = dentry->d_inode;
>> +       inode = file_inode(f.file);
>>         error = -EINVAL;
>>         if (!S_ISREG(inode->i_mode) || !(f.file->f_mode & FMODE_WRITE))
>>                 goto out_putf;
>> @@ -196,13 +196,13 @@ static long do_sys_ftruncate(unsigned int fd, loff_t length, int small)
>>         if (IS_APPEND(inode))
>>                 goto out_putf;
>>
>> -       sb_start_write(inode->i_sb);
>> -       error = locks_verify_truncate(inode, f.file, length);
>> +       sb_start_write(dentry->d_sb);
>> +       error = locks_verify_truncate(d_inode(dentry), f.file, length);
>>         if (!error)
>>                 error = security_path_truncate(&f.file->f_path);
>>         if (!error)
>>                 error = do_truncate(dentry, length, ATTR_MTIME|ATTR_CTIME, f.file);
>> -       sb_end_write(inode->i_sb);
>> +       sb_end_write(dentry->d_sb);
>>  out_putf:
>>         fdput(f);
>>  out:
>> --
>> 2.7.4
>>
--
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