RE: [PATCH] [SYSVIPC] Change shm_tot from int to size_t

[Makhlis, Lev]

Andries Brouwer wrote:
> On Fri, Jun 25, 2004 at 12:42:26PM -0500, Makhlis, Lev wrote:
> 
> > > Thirdly, shm_tot is transmitted to userspace (via the 
> SHM_INFO ioctl)
> > > as an unsigned long. If it is necessary to make it larger, then we
> > > must do something with this ioctl. For example, return -1 there
> > > in case the actual value does not fit in an unsigned long.
> > 
> > The SHM_INFO shmctl is actually how I found it in the first place.
> > But we have the same situation with many other values.  For example,
> > shm_ctlmax, shm_ctlall and shm_segsz can all potentially be 
> 64-bit wide
> > in the kernel and are exported into potentially 32-bit 
> userspace values.
> > We don't return -1 for any of those if they don't fit.  Is there a
> > special reason to do it in this case?
> 
> There is a good reason to do it always.
> If one truncates, then the result is always unreliable.
> If one replaces a too large value by -1, then any other value 
> is reliable.
> 

I've looked around and couldn't find anything that returns -1 as a value
(as opposed to returning an error) when the value won't fit in 32 bits.
Here's what I've found (assuming a 32-bit app in all cases):
* sys_statfs, which fails with EOVERFLOW if any of the values won't fit
* sys_sysinfo, which scales the values down up to a PAGE_SIZE factor,
and silently truncates them if they are still too large
* BLKGETSIZE ioctl, which fails with EFBIG if the kernel itself is 32-bit,
but silently truncates if the kernel is 64-bit
* BLKGETSIZE64 ioctl, which always silently truncates
* HDIO_GETGEO ioctl (.start is ulong), which always silently truncates
I'm sure there are some other examples...

What do you think about following the example of sys_statfs in sys_shmctl?
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