[RFC PATCH 0/5] Resend - Use procfs to change a syscall behavior

Dave Hansen dave at linux.vnet.ibm.com
Thu Jul 10 12:04:03 PDT 2008

On Thu, 2008-07-10 at 20:45 +0200, Pavel Machek wrote:
> On Thu 2008-07-10 10:53:35, Dave Hansen wrote:
> > On Thu, 2008-07-10 at 10:54 +0200, Pavel Machek wrote:
> > > 
> > > If you don't see a backward compatibility problem here, perhaps you
> > > should not be hacking kernel...? The way ids are assigned is certainly
> > > part of syscall semantics (applications rely on), at least for open.
> > 
> > We also used to have a pretty defined ordering for handing out address
> > space with mmap().  That all changed with address space randomization.
> > Are file descriptors different somehow?
> > 
> > Anyway, it's not like we're actually changing existing behavior.  An
> > application has to do something special and new to trigger this new
> > behavior.  Nobody is going to stumble over it, and it will *not* break
> > backward compatibility.
> It will break compatibility, but not in a way you expect. There's
> application called "subterfugue" that monitors other applications
> using ptrace and enforces security policy (or does other stuff). Such
> hacks depend on existing syscalls behaving in a way they are
> specified...
> Then you'll have to update open.2 man page:
>        Given a pathname for a file, open() returns a file descriptor,
> a small, non-
>        negative integer for use in  subsequent  system  calls
> (read(2),  write(2),
>        lseek(2),  fcntl(2),  etc.).   The  file descriptor returned by
> a successful
>        call will be the lowest-numbered file descriptor not currently
> open for  the
>        process.
> ...you'll need to add "unless someone write some number in file in
> /proc somewhere"... hmm... is new behaviour even POSIX compliant?
> open() is specified in POSIX...

Yup, that's true.  Good point.

> Ok, so it will not break too many apps... but echo "123 >
> /proc/something" breaking bash (etc) is not nice.
> (Plus proposed interface is so ugly that this discussion is moot.)

Yes, I agree that the current proposed interface is too ugly to live. :)

-- Dave

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