Problem using mandatory locks (other apps can read/delete etc)

Problem using mandatory locks (other apps can read/delete etc)

[E. Abbink]

Hi,

I'm trying to solve a problem using mandatory locks but am having some
difficulty in doing so. (if there's a more appropriate place for
discussing this please ignore the rest of this post. pointers to that
place would be appreciated ;) )

my problem:

when I lock a file with a mandatory write lock (ie. fcntl, +s-x bits and
mand mount option. for code see below) it is still possible:

- for me to rm the file in question
- for the file to be read by an other process

It's not possible to cat to the file or cp another file over it (as
expected). For my application I want a locked file to be completely
locked/protected from other processes.

If I retry the same without setting the +s-x bits the cp & cat succeed,
so something special is being done in the first case.


According to all docs (i can find) mandatory locks should block both the 
read and write system calls (i cant find anything in it regarding unlink 
though...). In my case however it seems as if only write calls are
blocked but not read calls?

If anyone could shed any light on this it would be much appreciated.

Esger



system details:

linux 2.4.17 partially suse & xfs patched kernel
filesystem is reiserfs
i386 dual pIII450

test app code:

int main ()
{
    int fd ;


    fd = open ("image.jpg", O_RDWR) ;

    if (fd == -1)
    {
        printf ("error %d while opening file\n", errno) ;
        exit (1) ;
    }

    struct flock lock ;

    lock.l_type = F_WRLCK ;
    lock.l_whence = SEEK_SET ;
    lock.l_start = 0 ;
    lock.l_len = 0 ;
    lock.l_pid = 0 ; // ignored

    int err = fcntl (fd, F_SETLK, &lock) ;

    if (err == -1)
    {
        printf ("error %d while locking file\n", errno) ;
        exit (1) ;
    }
    else
        printf ("file locked\n") ;


    while (1)
        sleep (1) ;
}






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Re: Problem using mandatory locks (other apps can read/delete etc)

[Denis Vlasenko]

On 10 April 2002 15:08, E. Abbink wrote:
> I'm trying to solve a problem using mandatory locks but am having some
> difficulty in doing so. (if there's a more appropriate place for
> discussing this please ignore the rest of this post. pointers to that
> place would be appreciated ;) )
>
> my problem:
>
> when I lock a file with a mandatory write lock (ie. fcntl, +s-x bits and
> mand mount option. for code see below) it is still possible:
>
> - for me to rm the file in question
> - for the file to be read by an other process

[snip]

>     lock.l_type = F_WRLCK ;   <================
>     lock.l_whence = SEEK_SET ;
>     lock.l_start = 0 ;
>     lock.l_len = 0 ;
>     lock.l_pid = 0 ; // ignored
>
>     int err = fcntl (fd, F_SETLK, &lock) ;

I know nothing about file locking in Unix, but it looks like you
requested write lock, i.e. forbid writing to a file. Why are you
surprised that reads are allowed?

Probably someone else would comment on why rm is working, though...
--
vda

Re: Problem using mandatory locks (other apps can read/delete etc)

[Alex Riesen]

On Thu, Apr 11, 2002 at 09:37:37AM -0200, Denis Vlasenko wrote:
> On 10 April 2002 15:08, E. Abbink wrote:
> > I'm trying to solve a problem using mandatory locks but am having some
> > difficulty in doing so. (if there's a more appropriate place for
> > discussing this please ignore the rest of this post. pointers to that
> > place would be appreciated ;) )
> >
> > my problem:
> >
> > when I lock a file with a mandatory write lock (ie. fcntl, +s-x bits and
> > mand mount option. for code see below) it is still possible:
> >
> > - for me to rm the file in question
> > - for the file to be read by an other process
> 
> [snip]
> 
> >     lock.l_type = F_WRLCK ;   <================
> >     lock.l_whence = SEEK_SET ;
> >     lock.l_start = 0 ;
> >     lock.l_len = 0 ;
> >     lock.l_pid = 0 ; // ignored
> >
> >     int err = fcntl (fd, F_SETLK, &lock) ;
> 
> I know nothing about file locking in Unix, but it looks like you
> requested write lock, i.e. forbid writing to a file. Why are you
> surprised that reads are allowed?

That's advisory write lock. Question is: why open call to read the file
succeeded?

> 
> Probably someone else would comment on why rm is working, though...

Why not? Apparently he has write permission on the directory which contain
the file, and is the owner of that file.

By the way, where are you changing file permissions to +s-x?

Re: Problem using mandatory locks (other apps can read/delete etc)

[E. Abbink]

> On 10 April 2002 15:08, E. Abbink wrote:
> > I'm trying to solve a problem using mandatory locks but am having some
> > difficulty in doing so. (if there's a more appropriate place for
> > discussing this please ignore the rest of this post. pointers to that
> > place would be appreciated ;) )
> >
> > my problem:
> >
> > when I lock a file with a mandatory write lock (ie. fcntl, +s-x bits and
> > mand mount option. for code see below) it is still possible:
> >
> > - for me to rm the file in question
> > - for the file to be read by an other process
> 
> [snip]
> 
> >     lock.l_type = F_WRLCK ;   <================
> >     lock.l_whence = SEEK_SET ;
> >     lock.l_start = 0 ;
> >     lock.l_len = 0 ;
> >     lock.l_pid = 0 ; // ignored
> >
> >     int err = fcntl (fd, F_SETLK, &lock) ;
> 
> I know nothing about file locking in Unix, but it looks like you
> requested write lock, i.e. forbid writing to a file. Why are you
> surprised that reads are allowed?
> 
> Probably someone else would comment on why rm is working, though...
> --
> vda

as i understand it what is called a "write" lock is actually an
exclusive lock (ie both read/write). Also, afaik, you cant set both a
read & write lock through fcntl which supports that assumption.
 
Esger

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Re: Problem using mandatory locks (other apps can read/delete etc)

[E. Abbink]

> On Thu, Apr 11, 2002 at 09:37:37AM -0200, Denis Vlasenko wrote:
> > On 10 April 2002 15:08, E. Abbink wrote:
> > > I'm trying to solve a problem using mandatory locks but am having some
> > > difficulty in doing so. (if there's a more appropriate place for
> > > discussing this please ignore the rest of this post. pointers to that
> > > place would be appreciated ;) )
> > >
> > > my problem:
> > >
> > > when I lock a file with a mandatory write lock (ie. fcntl, +s-x
bits and
> > > mand mount option. for code see below) it is still possible:
> > >
> > > - for me to rm the file in question
> > > - for the file to be read by an other process
> > 
> > [snip]
> > 
> > >     lock.l_type = F_WRLCK ;   <================
> > >     lock.l_whence = SEEK_SET ;
> > >     lock.l_start = 0 ;
> > >     lock.l_len = 0 ;
> > >     lock.l_pid = 0 ; // ignored
> > >
> > >     int err = fcntl (fd, F_SETLK, &lock) ;
> > 
> > I know nothing about file locking in Unix, but it looks like you
> > requested write lock, i.e. forbid writing to a file. Why are you
> > surprised that reads are allowed?
> 
> That's advisory write lock. Question is: why open call to read the file
> succeeded?

Normally it's an advisory lock, but if you do the appropriate actions
(mounting the fs with mand and setting the +s-x modebits, see
Documentation/mandatory.txt) the file becomes a mandatory lock candidate 
and fcntl will/should mandatory lock it.

> 
> > 
> > Probably someone else would comment on why rm is working, though...
> 
> Why not? Apparently he has write permission on the directory which contain
> the file, and is the owner of that file.
> 
> By the way, where are you changing file permissions to +s-x?
> 

by hand before the program is run.

> 
> 
> 

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