Re: [PATCHv4 2/3] kbuild: Introduce build-salt linker section and config option

Masahiro Yamada
2018-06-13T06:07:48Z

Hi.

2018-06-12 9:32 GMT+09:00 Laura Abbott <labbott@redhat.com>:
>
> The build id generated from --build-id can be generated in several different
> ways, with the default being the sha1 on the output of the linked file. For
> distributions, it can be useful to make sure this ID is unique, even if the
> actual file contents don't change. The easiest way to do this is to insert
> a section with some data.
>
> Introduce a macro to insert a linker section which will be filled
> with a hex value. This will ensure the build id can be changed just via
> a config option. Users who don't care about this can leave the
> default value and strip the section.
>
> Suggested-by: Linus Torvalds <torvalds@linux-foundation.org>
> Signed-off-by: Laura Abbott <labbott@redhat.com>
> ---
> include/asm-generic/vmlinux.lds.h | 6 ++++++
> init/Kconfig | 9 +++++++++
> scripts/module-common.lds.S | 4 ++++
> 3 files changed, 19 insertions(+)
>
> diff --git a/include/asm-generic/vmlinux.lds.h b/include/asm-generic/vmlinux.lds.h
> index e373e2e10f6a..4af7e683aad2 100644
> --- a/include/asm-generic/vmlinux.lds.h
> +++ b/include/asm-generic/vmlinux.lds.h
> @@ -830,6 +830,12 @@
> #define PERCPU_DECRYPTED_SECTION
> #endif
>
> +#define BUILD_SALT \
> + . = ALIGN(32); \
> + .salt : AT(ADDR(.salt) - LOAD_OFFSET) { \
> + LONG(0xffaa5500); \
> + . = ALIGN(32); \
> + } = CONFIG_BUILD_ID_SALT \

What is 0xffaa5500 ?

Is it another salt in addition to CONFIG_BUILD_ID_SALT ?

Or, does it have a special meaning?

> /*
> * Default discarded sections.
> diff --git a/init/Kconfig b/init/Kconfig
> index d2b8b2ea097e..eb92ccfe4ecb 100644
> --- a/init/Kconfig
> +++ b/init/Kconfig
> @@ -1967,3 +1967,12 @@ config ARCH_HAS_SYNC_CORE_BEFORE_USERMODE
> # <asm/syscall_wrapper.h>.
> config ARCH_HAS_SYSCALL_WRAPPER
> def_bool n
> +
> +config BUILD_ID_SALT
> + hex "Build ID Salt"
> + default 0x12345678
> + help
> + The build ID is used to link binaries and their debug info. Setting
> + this option will use the value in the calculation of the build id.
> + This is mostly useful for distributions which want to ensure the
> + build is unique between builds. It's safe to leave the default.

If you run "make menuconfig",
you will notice this looks so strange;
"Build ID Salt" is displayed in the top level menu.

> diff --git a/scripts/module-common.lds.S b/scripts/module-common.lds.S
> index d61b9e8678e8..3c8410270ac1 100644
> --- a/scripts/module-common.lds.S
> +++ b/scripts/module-common.lds.S
> @@ -3,6 +3,9 @@
> * Archs are free to supply their own linker scripts. ld will
> * combine them automatically.
> */
> +
> +#include <asm-generic/vmlinux.lds.h>
> +

You are pulling many macros in <asm-generic/vmlinux.lds.h>
into modules, VDSO, etc.

You also need to touch
arch/*/kernel/vmlinux.lds.S
of all architectures.

This is not so nice.

> SECTIONS {
> /DISCARD/ : {
> *(.discard)
> @@ -23,4 +26,5 @@ SECTIONS {
> .init_array 0 : ALIGN(8) { *(SORT(.init_array.*)) *(.init_array) }
>
> __jump_table 0 : ALIGN(8) { KEEP(*(__jump_table)) }
> + BUILD_SALT
> }
> --
> 2.18.0.rc1

I was playing with a different approach.

Instead of touching linker scripts around,
how about putting the salt into an elfnote?

The compiler puts the build ID
into the '.note.gnu.build-id' section.

So, I guess it is sensible to put
the salt into '.note.*' section as well.

I attached my trial below:

- I moved 'config BUILD_SALT' to a better location
so that it will be displayed in the "General setup" menu.

- I added 'range 0 0xffffffff' to avoid warnings.

- I renamed the config symbol to CONFIG_BUILD_SALT
and change the default to 0x0.
Of course, this is just bike-shed things, though..

- It looks like 'owner=Linux, type_id=0'
is already used in VDSO.
https://github.com/torvalds/linux/blob/v4.17/arch/arm64/kernel/vdso/note.S#L26

I chose 0x100 for the type id, but it could be a different value

diff --git a/arch/x86/entry/vdso/vdso-note.S b/arch/x86/entry/vdso/vdso-note.S
index 79a071e..7942317 100644
--- a/arch/x86/entry/vdso/vdso-note.S
+++ b/arch/x86/entry/vdso/vdso-note.S
@@ -3,6 +3,7 @@
* Here we can supply some information useful to userland.
*/

+#include <linux/build-salt.h>
#include <linux/uts.h>
#include <linux/version.h>
#include <linux/elfnote.h>
@@ -10,3 +11,5 @@
ELFNOTE_START(Linux, 0, "a")
.long LINUX_VERSION_CODE
ELFNOTE_END
+
+BUILD_SALT
diff --git a/arch/x86/entry/vdso/vdso32/note.S
b/arch/x86/entry/vdso/vdso32/note.S
index 9fd51f2..e78047d 100644
--- a/arch/x86/entry/vdso/vdso32/note.S
+++ b/arch/x86/entry/vdso/vdso32/note.S
@@ -4,6 +4,7 @@
* Here we can supply some information useful to userland.
*/

+#include <linux/build-salt.h>
#include <linux/version.h>
#include <linux/elfnote.h>

@@ -14,6 +15,8 @@ ELFNOTE_START(Linux, 0, "a")
.long LINUX_VERSION_CODE
ELFNOTE_END

+BUILD_SALT
+
#ifdef CONFIG_XEN
/*
* Add a special note telling glibc's dynamic linker a fake hardware
diff --git a/include/linux/build-salt.h b/include/linux/build-salt.h
new file mode 100644
index 0000000..66e87c9
--- /dev/null
+++ b/include/linux/build-salt.h
@@ -0,0 +1,20 @@
+#ifndef __BUILD_SALT_H
+#define __BUILD_SALT_H
+
+#include <linux/elfnote.h>
+
+#define LINUX_ELFNOTE_BUILD_SALT 0x100
+
+#ifdef __ASSEMBLER__
+
+#define BUILD_SALT \
+ ELFNOTE(Linux, LINUX_ELFNOTE_BUILD_SALT, .long CONFIG_BUILD_SALT)
+
+#else
+
+#define BUILD_SALT \
+ ELFNOTE32("Linux", LINUX_ELFNOTE_BUILD_SALT, CONFIG_BUILD_SALT)
+
+#endif
+
+#endif /* __BUILD_SALT_H */
diff --git a/init/Kconfig b/init/Kconfig
index d2b8b2e..54b5828 100644
--- a/init/Kconfig
+++ b/init/Kconfig
@@ -92,6 +92,16 @@ config LOCALVERSION_AUTO

which is done within the script "scripts/setlocalversion".)

+config BUILD_SALT
+ hex "Build ID Salt"
+ default 0x0
+ range 0 0xffffffff
+ help
+ The build ID is used to link binaries and their debug info. Setting
+ this option will use the value in the calculation of the build id.
+ This is mostly useful for distributions which want to ensure the
+ build is unique between builds. It's safe to leave the default.
+
config HAVE_KERNEL_GZIP
bool

diff --git a/init/version.c b/init/version.c
index bfb4e3f..ef4012e 100644
--- a/init/version.c
+++ b/init/version.c
@@ -7,6 +7,7 @@
*/

#include <generated/compile.h>
+#include <linux/build-salt.h>
#include <linux/export.h>
#include <linux/uts.h>
#include <linux/utsname.h>
@@ -49,3 +50,5 @@ const char linux_proc_banner[] =
"%s version %s"
" (" LINUX_COMPILE_BY "@" LINUX_COMPILE_HOST ")"
" (" LINUX_COMPILER ") %s\n";
+
+BUILD_SALT;
diff --git a/scripts/mod/modpost.c b/scripts/mod/modpost.c
index 1663fb1..dc6d714 100644
--- a/scripts/mod/modpost.c
+++ b/scripts/mod/modpost.c
@@ -2125,10 +2125,13 @@ static int check_modname_len(struct module *mod)
**/
static void add_header(struct buffer *b, struct module *mod)
{
+ buf_printf(b, "#include <linux/build-salt.h>\n");
buf_printf(b, "#include <linux/module.h>\n");
buf_printf(b, "#include <linux/vermagic.h>\n");
buf_printf(b, "#include <linux/compiler.h>\n");
buf_printf(b, "\n");
+ buf_printf(b, "BUILD_SALT;\n");
+ buf_printf(b, "\n");
buf_printf(b, "MODULE_INFO(vermagic, VERMAGIC_STRING);\n");
buf_printf(b, "MODULE_INFO(name, KBUILD_MODNAME);\n");
buf_printf(b, "\n");

--
Best Regards
Masahiro Yamada