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Date:   Thu, 29 Apr 2021 10:50:28 +0100
From:   Szabolcs Nagy <szabolcs.nagy@arm.com>
To:     Bill Wendling <morbo@google.com>
Cc:     Mark Brown <broonie@kernel.org>,
        Catalin Marinas <catalin.marinas@arm.com>,
        Kees Cook <keescook@google.com>,
        Ard Biesheuvel <ardb@kernel.org>,
        Will Deacon <will@kernel.org>,
        LKML <linux-kernel@vger.kernel.org>,
        linux-arm-kernel@lists.infradead.org,
        Daniel Kiss <Daniel.Kiss@arm.com>
Subject: Re: [PATCH] arm64/vdso: Discard .note.gnu.property sections in vDSO
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The 04/29/2021 02:23, Bill Wendling wrote:
> On Thu, Apr 29, 2021 at 12:55 AM Szabolcs Nagy <szabolcs.nagy@arm.com> wrote:
> > The 04/28/2021 12:31, Bill Wendling wrote:
> > > On Wed, Apr 28, 2021 at 12:21 PM Bill Wendling <morbo@google.com> wrote:
> > > > On Wed, Apr 28, 2021 at 10:40 AM Mark Brown <broonie@kernel.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.
> 
> This is a potential patch. I haven't had time to test it though.
> However, it does appear to format the section in the "expected" way,
> so one's able to grab the build IDs. Thoughts?


please show the program headers.


> 
> $ objdump -s -j .note ./arch/arm64/kernel/vdso/vdso.so
> 
> ./arch/arm64/kernel/vdso/vdso.so:     file format elf64-little
> 
> Contents of section .note:
>  0288 04000000 10000000 05000000 474e5500  ............GNU.
>  0298 000000c0 04000000 03000000 00000000  ................
>  02a8 06000000 04000000 00000000 4c696e75  ............Linu
>  02b8 78000000 000a0500 06000000 01000000  x...............
>  02c8 00010000 4c696e75 78000000 00000000  ....Linux.......
>  02d8 04000000 14000000 03000000 474e5500  ............GNU.
>  02e8 958db149 af5156cb 45309896 7a53ae8a  ...I.QV.E0..zS..
>  02f8 ef34e95c                             .4.\
> 
> diff --git a/arch/arm64/kernel/vdso/vdso.lds.S
> b/arch/arm64/kernel/vdso/vdso.lds.S
> index d808ad31e01f..d51e886c6223 100644
> --- a/arch/arm64/kernel/vdso/vdso.lds.S
> +++ b/arch/arm64/kernel/vdso/vdso.lds.S
> @@ -31,7 +31,13 @@ SECTIONS
>         .gnu.version_d  : { *(.gnu.version_d) }
>         .gnu.version_r  : { *(.gnu.version_r) }
> 
> -       .note           : { *(.note.*) }                :text   :note
> +       /*
> +        * Add the .note.gnu.property section first, as it's aligned to
> +        * 8-bytes, while other notes are aligned to 4-bytes.
> +        */
> +       . = ALIGN(8);
> +
> +       .note           : { *(.note.gnu.property) *(.note.*) }  :text   :note
> 
>         . = ALIGN(16);

--