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[dm-devel] Re: [PATCH] User-configurable HDIO_GETGEO for dm volumes



"Darrick J. Wong" <djwong us ibm com> wrote:
>
> ...
> > That's brave - we take the hd_geometry straight from userspace without
> > checking anything?
> 
> My original approach didn't work anyway; libdevmapper thinks that a 
> target message is a string and would stop copying at the first null it 
> saw.  Since you're also concerned about being locked into a particular 
> hd_geometry structure layout, I respun the patch so that dmsetup passes 
> a string to the dm configuration code; now dm performs some basic 
> range/sanity checks.  However, the patch doesn't check that the CHS 
> values make sense or are even close to the real disk size; if somebody 
> in userspace wants to configure a 150G dm device to have the same 
> geometry as a 360K floppy disk, so be it.  Geometries seem to be rather 
> inaccurate anyway.
> 
> Or, were you worried that I'm dereferencing a userspace pointer in the 
> kernel?  The code that calls _setgeo handles that properly.

Well, I was just asking whether you'd thought about it...

> > Will this code dtrt if userspace is 32-bit and the kernel is 64-bit?
> 
> There shouldn't be any 32/64 mis-match issues with passing a string.  If 
> one tries to pass too-large values, -EINVAL is returned.

Yup, strings work.

> > > struct hd_geometry looks like something which different compilers could lay
> > out differently, perhaps even different gcc versions.  We're relying upon
> > the userspace representation being identical to the kernel's
> > representation.
> > 
> > It means that struct hd_geometry becomes part of the kernel ABI.  We can
> > never again change it and neither we (nor the compiler) can ever change its
> > layout.  That's dangerous.  I'd suggest that you not use hd_geometry in
> > this way (unless we're already using it that way, which might be the case).
> 
> hd_geometry is already part of the kernel ABI because the HDIO_GETGEO 
> ioctl takes a pointer to a struct hd_geometry in userspace and fills it 
> out.

argh.

> 
> Signed-off-by: Darrick J. Wong <djwong us ibm com>
> 

We don't seem to have a changelog any more.


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