linux-kernel - Re: [PATCH] arm64/vdso: Discard .note.gnu.property sections in vDSO

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Message-ID: <20210429075454.GQ9028@arm.com>
Date:   Thu, 29 Apr 2021 08:54:55 +0100
From:   Szabolcs Nagy <szabolcs.nagy@....com>
To:     Bill Wendling <morbo@...gle.com>
Cc:     Mark Brown <broonie@...nel.org>,
        Catalin Marinas <catalin.marinas@....com>,
        Kees Cook <keescook@...gle.com>,
        Ard Biesheuvel <ardb@...nel.org>,
        Will Deacon <will@...nel.org>,
        LKML <linux-kernel@...r.kernel.org>,
        linux-arm-kernel@...ts.infradead.org,
        Daniel Kiss <Daniel.Kiss@....com>
Subject: Re: [PATCH] arm64/vdso: Discard .note.gnu.property sections in vDSO

The 04/28/2021 12:31, Bill Wendling wrote:
> On Wed, Apr 28, 2021 at 12:21 PM Bill Wendling <morbo@...gle.com> wrote:
> > On Wed, Apr 28, 2021 at 10:40 AM Mark Brown <broonie@...nel.org> wrote:
> > > On Wed, Apr 28, 2021 at 06:28:47PM +0100, Catalin Marinas wrote:
> > > > On Fri, Apr 23, 2021 at 01:51:59PM -0700, Bill Wendling wrote:
> > >
> > > > > Since the note.gnu.property section in the vDSO is not checked by the
> > > > > dynamic linker, discard the .note.gnu.property sections in the vDSO.
> > >
> > > > Can we not instead fix the linker script to preserve the
> > > > .note.gnu.property, correctly aligned? It doesn't take much space and
> > > > while we don't use it now, it has the BTI information about the binary.
> > >
> > > > Cc'ing a few others who were involved in the BTI support.
> > >
> > > Not just BTI, we also flag PAC usage in there too and could add other
> > > extensions going forwards.  While the note isn't actively used by
> > > anything right now due to the kernel mapping the vDSO prior to userspace
> > > starting it is part of the ABI and something could end up wanting to use
> > > it and getting confused if it's not there.  It would be much better to
> > > fix the alignment issue.
> >
> > If there's only one of the 8-byte aligned sections guaranteed, we
> > could place it first in the note. Otherwise, we will have to change
> > the alignment of the note (or somehow merge multiple notes).
> >
> I should have clarified that there's only one *entry* in the
> .note.gnu.properties section, and if not then is it possible to merge
> multiple entries into one. (Excuse my ignorance if this is already the
> case.)

.note.gnu.property should go to PT_GNU_PROPERTY and it
should be merged following rules specified in
https://github.com/hjl-tools/linux-abi/wiki/Linux-Extensions-to-gABI
and
https://github.com/ARM-software/abi-aa/blob/master/aaelf64/aaelf64.rst#program-property

it may also be covered by a (8byte aligned) PT_NOTE, but
that's not a requirement on aarch64 (x86 requires it for
compatibility with old dynamic linker, but since the vdso
is handled specially that may not be relevant either).

i don't know how this works in linker scripts.