Re: FW: Linux kernel file offset pointer races

Marcelo Tosatti
2004-08-12T15:06:30Z

Hi Andrey,

On Thu, Aug 12, 2004 at 11:55:31AM +0400, Andrey Savochkin wrote:
> On Wed, Aug 11, 2004 at 06:14:30PM -0300, Marcelo Tosatti wrote:
> > On Wed, Aug 11, 2004 at 06:26:02PM +0400, Andrey Savochkin wrote:
> > > BTW, f_pos assignments are non-atomic on IA-32 since it's a 64-bit value.
> > > The file position is protected by the BKL in llseek(), but I do not see any
> > > serialization neither in sys_read() nor in generic_file_read() and other
> > > methods.
> > >
> > > Have we accepted that the file position may be corrupted after crossing 2^32
> > > boundary by 2 processes reading in parallel from the same file?
> > > Or am I missing something?
> >
> > Yes, as far as I know, parallel users of the same file descriptions (which
> > can race on 64-bit architectures) is expected, we dont care about handling it.
> >
> > Behaviour is undefined.
>
> I prefer explainable behaviours :)

Do the locking in the concurrent userspace pos users, then :)

> If 2 processes start reading at offset 0xfffffffe, and one of them reads 1
> byte and the second 2 bytes, I can expect the file position be 0xffffffff,
> 0x100000000, 0x100000001, or, in the worst case, 0xfffffffe again.
> But 0x1ffffffff will be a real surprise.
>
> For a bigger surprise, we can kill one of the processes with SIGFPE if we
> find that the processes perform such an "incorrect" parallel read and the
> file position has changed behind us ;)
> But we don't want that much undefined behaviour, do we? :)

quoting viro:

"FWIW, anybody who mixes read() and lseek() on the same descriptor without
logics in their code serializing those are also getting what they'd asked
for - GIGO. It should not break the kernel, obviously, but kernel has no
business being nice to inherently racy userland code. BTW, you do realize
that lseek() in the middle of read() on the same descriptor will be lost?
On regular files, on quite a few Unices, Linux included."

We dont guarantee serialization of a file's descriptor
(I mistyped "file description" in the last email) f_pos.
Thats up to the user application.

Makes sense to me.