Re: [PATCH 6/6] vfs: introduce FMODE_NEG_OFFSET for allowingnegative f_pos

[KAMEZAWA Hiroyuki]

On Mon, 18 Jan 2010 11:13:04 +0900
OGAWA Hirofumi <hirofumi@xxxxxxxxxxxxxxxxxx> wrote:

> KAMEZAWA Hiroyuki <kamezawa.hiroyu@xxxxxxxxxxxxxx> writes:
> 
> > On Mon, 18 Jan 2010 10:38:27 +0900
> > OGAWA Hirofumi <hirofumi@xxxxxxxxxxxxxxxxxx> wrote:
> >
> >> KAMEZAWA Hiroyuki <kamezawa.hiroyu@xxxxxxxxxxxxxx> writes:
> >> 
> >> >> So, lseek() returns (uses) it?
> >> >
> >> > lseek can return negative value, as far as I know.
> >> 
> >> Umm..., how do you know the difference of -EOVERFLOW and fpos == -75? 
> >> 
> >
> > Ah, sorry. I read wrong.
> >
> > For /dev/mem, it uses its own lseek function which allows negative f_pos
> > value. Other usual file system doesn't allow negative f_pos.
> >
> > It's ok not to return -EOVEFLOW for /dev/mem because there is no file end.
> 
> No, no. I think it has the problem.
> 
> E.g. /dev/mem returns -75 as fpos, so, lseek(2) returns -75 to
> userland. Then the userland (e.g. glibc) convert it as
> error. I.e. finally, errno == -75, and lseek(3) returns -1, right?
> 
Maybe possible.

Hmm. Then, /dev/mem's llseek need some fix not to return pos < -PAGESIZE.
Wu-san, could you add additional bug fix to lseek()'s f_pos handling in
/dev/mem ?

Thanks,
-Kame


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