linux-kernel - RE: [PATCH v4 02/24] powerpc/pseries: Fix alignment of PLPKS structures and buffers

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Date:   Thu, 26 Jan 2023 17:31:09 +0000
From:   David Laight <David.Laight@...LAB.COM>
To:     'Segher Boessenkool' <segher@...nel.crashing.org>,
        Michael Ellerman <mpe@...erman.id.au>
CC:     "gjoyce@...ux.ibm.com" <gjoyce@...ux.ibm.com>,
        Andrew Donnellan <ajd@...ux.ibm.com>,
        "erichte@...ux.ibm.com" <erichte@...ux.ibm.com>,
        "gregkh@...uxfoundation.org" <gregkh@...uxfoundation.org>,
        "nayna@...ux.ibm.com" <nayna@...ux.ibm.com>,
        "linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>,
        "zohar@...ux.ibm.com" <zohar@...ux.ibm.com>,
        "sudhakar@...ux.ibm.com" <sudhakar@...ux.ibm.com>,
        "ruscur@...sell.cc" <ruscur@...sell.cc>,
        "joel@....id.au" <joel@....id.au>,
        "bgray@...ux.ibm.com" <bgray@...ux.ibm.com>,
        "linux-integrity@...r.kernel.org" <linux-integrity@...r.kernel.org>,
        "gcwilson@...ux.ibm.com" <gcwilson@...ux.ibm.com>,
        "linuxppc-dev@...ts.ozlabs.org" <linuxppc-dev@...ts.ozlabs.org>
Subject: RE: [PATCH v4 02/24] powerpc/pseries: Fix alignment of PLPKS
 structures and buffers

From: Segher Boessenkool
> Sent: 26 January 2023 17:19
> 
> On Thu, Jan 26, 2023 at 12:09:53AM +1100, Michael Ellerman wrote:
> > Andrew Donnellan <ajd@...ux.ibm.com> writes:
> > > A number of structures and buffers passed to PKS hcalls have alignment
> > > requirements, which could on occasion cause problems:
> > >
> > > - Authorisation structures must be 16-byte aligned and must not cross a
> > >   page boundary
> > >
> > > - Label structures must not cross page boundaries
> > >
> > > - Password output buffers must not cross page boundaries
> > >
> > > Round up the allocations of these structures/buffers to the next power of
> > > 2 to make sure this happens.
> >
> > It's not the *next* power of 2, it's the *nearest* power of 2, including
> > the initial value if it's already a power of 2.
> 
> It's not the nearest either, the nearest power of two to 65 is 64.  You
> could say "but, round up" to which I would say "round?"  :-P
> 
> "Adjust the allocation size to be the smallest power of two greater than
> or equal to the given size."
> 
> "Pad to a power of two" in shorthand.  "Padded to a power of two if
> necessary" if you want to emphasise it can be a no-op.

Changing the size to kzalloc() doesn't help.
The alignment depends on the allocator and is only required to have
a relatively small alignment (ARCH_MINALIGN?) regardless of the size.

IIRC one of the allocators adds a small header to every item.
It won't return 16 byte aligned items at all.

	David

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