Re: [PATCHv4 0/3] Salted build ids via linker sections

Michael Ellerman
2018-06-12T06:53:44Z

Laura Abbott <labbott@redhat.com> writes:
> Hi,
>
> This is v4 of the series to allow unique build ids in the kernel. As a
> reminder of the context:
>
> ""
> In Fedora, the debug information is packaged separately (foo-debuginfo) and
> can be installed separately. There's been a long standing issue where only one
> version of a debuginfo info package can be installed at a time. Mark Wielaard
> made an effort for Fedora 27 to allow parallel installation of debuginfo (see
> https://fedoraproject.org/wiki/Changes/ParallelInstallableDebuginfo for
> more details)
>
> Part of the requirement to allow this to work is that build ids are
> unique between builds. The existing upstream rpm implementation ensures
> this by re-calculating the build-id using the version and release as a
> seed. This doesn't work 100% for the kernel because of the vDSO which is
> its own binary and doesn't get updated. After poking holes in a few of my
> ideas, there was a discussion with some people from the binutils team about
> adding --build-id-salt to let ld do the calculation debugedit is doing. There
> was a counter proposal made to add in the salt while building. The
> easiest proposal was to add an item in the linker script vs. linking in
> an object since we need the salt to go in every module as well as the
> kernel and vmlinux.
> ""
>
> v4 takes Linus' suggestion of using linker fill to insert the build id.
> This removes the need to use a generated header which makes things much
> easier. One change is that because this section isn't .comment it won't
> get stripped automatically. This is pretty small but I also know people
> can be picky so I'm open to opinions or suggestions here.
>
> Laura Abbott (3):
> scripts: Preprocess module-common.lds
> kbuild: Introduce build-salt linker section and config option
> x86: Add build salt to the vDSO and kernel linker scripts

Hi Laura,

Here's a patch to get it working on powerpc. Seems to work as expected.

cheers

From fc5e22e4873956f9328e401ee5dd2835f4884db9 Mon Sep 17 00:00:00 2001
From: Michael Ellerman <mpe@ellerman.id.au>
Date: Tue, 12 Jun 2018 14:52:34 +1000
Subject: [PATCH] powerpc: Add support for BUILD_SALT in kernel, modules & VDSO

This patch adds support for BUILD_SALT in the kernel, modules and
VDSO. See the commit that adds BUILD_SALT for more info.

Kernel:
0:mon> d c000000001340840
c000000001340840 0055aaff12345678 1234567812345678 |.U...4Vx.4Vx.4Vx|
c000000001340850 1234567812345678 1234567812345678 |.4Vx.4Vx.4Vx.4Vx|

Module:

$ cat/sys/module/kvm/sections/.salt
0xd0000000064385e8
...
0:mon> d d0000000064385e8
d0000000064385e8 0055aaff12345678 1234567812345678 |.U...4Vx.4Vx.4Vx|
d0000000064385f8 1234567812345678 1234567812345678 |.4Vx.4Vx.4Vx.4Vx|

vdso:
(gdb) x/4xw (0x7ffff7f80000 + 0x4a0)
0x7ffff7f804a0: 0xffaa5500 0x78563412 0x78563412 0x78563412

Signed-off-by: Michael Ellerman <mpe@ellerman.id.au>
---
arch/powerpc/kernel/vdso32/vdso32.lds.S | 2 ++
arch/powerpc/kernel/vdso64/vdso64.lds.S | 2 ++
arch/powerpc/kernel/vmlinux.lds.S | 1 +
3 files changed, 5 insertions(+)

diff --git a/arch/powerpc/kernel/vdso32/vdso32.lds.S b/arch/powerpc/kernel/vdso32/vdso32.lds.S
index 099a6db14e67..c06a12607777 100644
--- a/arch/powerpc/kernel/vdso32/vdso32.lds.S
+++ b/arch/powerpc/kernel/vdso32/vdso32.lds.S
@@ -4,6 +4,7 @@
* library
*/
#include <asm/vdso.h>
+#include <asm-generic/vmlinux.lds.h>

#ifdef __LITTLE_ENDIAN__
OUTPUT_FORMAT("elf32-powerpcle", "elf32-powerpcle", "elf32-powerpcle")
@@ -25,6 +26,7 @@ SECTIONS
.gnu.version_d : { *(.gnu.version_d) }
.gnu.version_r : { *(.gnu.version_r) }

+ BUILD_SALT
.note : { *(.note.*) } :text :note

. = ALIGN(16);
diff --git a/arch/powerpc/kernel/vdso64/vdso64.lds.S b/arch/powerpc/kernel/vdso64/vdso64.lds.S
index 256fb9720298..ace69258446a 100644
--- a/arch/powerpc/kernel/vdso64/vdso64.lds.S
+++ b/arch/powerpc/kernel/vdso64/vdso64.lds.S
@@ -4,6 +4,7 @@
* library
*/
#include <asm/vdso.h>
+#include <asm-generic/vmlinux.lds.h>

#ifdef __LITTLE_ENDIAN__
OUTPUT_FORMAT("elf64-powerpcle", "elf64-powerpcle", "elf64-powerpcle")
@@ -25,6 +26,7 @@ SECTIONS
.gnu.version_d : { *(.gnu.version_d) }
.gnu.version_r : { *(.gnu.version_r) }

+ BUILD_SALT
.note : { *(.note.*) } :text :note

. = ALIGN(16);
diff --git a/arch/powerpc/kernel/vmlinux.lds.S b/arch/powerpc/kernel/vmlinux.lds.S
index 5baac79df97e..59635369ceea 100644
--- a/arch/powerpc/kernel/vmlinux.lds.S
+++ b/arch/powerpc/kernel/vmlinux.lds.S
@@ -348,6 +348,7 @@ SECTIONS
}

BUG_TABLE
+ BUILD_SALT

. = ALIGN(PAGE_SIZE);
_edata = .;
--
2.14.1