[PATCH 1/2] powerpc: Discard .rela* sections if CONFIG_RELOCATABLE is undefined

[PATCH 1/2] powerpc: Discard .rela* sections if CONFIG_RELOCATABLE is undefined

[H.J. Lu]

arch/powerpc/kernel/vmlinux.lds.S has

        DISCARDS
        /DISCARD/ : {
                *(*.EMB.apuinfo)
                *(.glink .iplt .plt .rela* .comment)
                *(.gnu.version*)
                *(.gnu.attributes)
                *(.eh_frame)
        }

Since .rela* sections are needed when CONFIG_RELOCATABLE is defined,
change to discard .rela* sections if CONFIG_RELOCATABLE is undefined.

Signed-off-by: H.J. Lu <hjl.tools@gmail.com>
Acked-by: Michael Ellerman <mpe@ellerman.id.au> (powerpc)
---
 arch/powerpc/kernel/vmlinux.lds.S | 5 ++++-
 1 file changed, 4 insertions(+), 1 deletion(-)

diff --git a/arch/powerpc/kernel/vmlinux.lds.S b/arch/powerpc/kernel/vmlinux.lds.S
index 31a0f201fb6f..4ba07734a210 100644
--- a/arch/powerpc/kernel/vmlinux.lds.S
+++ b/arch/powerpc/kernel/vmlinux.lds.S
@@ -366,9 +366,12 @@ SECTIONS
 	DISCARDS
 	/DISCARD/ : {
 		*(*.EMB.apuinfo)
-		*(.glink .iplt .plt .rela* .comment)
+		*(.glink .iplt .plt .comment)
 		*(.gnu.version*)
 		*(.gnu.attributes)
 		*(.eh_frame)
+#ifndef CONFIG_RELOCATABLE
+		*(.rela*)
+#endif
 	}
 }
-- 
2.25.4

[PATCH 2/2] Discard .note.gnu.property sections in generic NOTES

[H.J. Lu]

With the command-line option, -mx86-used-note=yes, the x86 assembler
in binutils 2.32 and above generates a program property note in a note
section, .note.gnu.property, to encode used x86 ISAs and features.  But
kernel linker script only contains a single NOTE segment:

PHDRS {
 text PT_LOAD FLAGS(5);
 data PT_LOAD FLAGS(6);
 percpu PT_LOAD FLAGS(6);
 init PT_LOAD FLAGS(7);
 note PT_NOTE FLAGS(0);
}
SECTIONS
{
...
 .notes : AT(ADDR(.notes) - 0xffffffff80000000) { __start_notes = .; KEEP(*(.not
e.*)) __stop_notes = .; } :text :note
...
}

The NOTE segment generated by kernel linker script is aligned to 4 bytes.
But .note.gnu.property section must be aligned to 8 bytes on x86-64 and
we get

[hjl@gnu-skx-1 linux]$ readelf -n vmlinux

Displaying notes found in: .notes
  Owner                Data size Description
  Xen                  0x00000006 Unknown note type: (0x00000006)
   description data: 6c 69 6e 75 78 00
  Xen                  0x00000004 Unknown note type: (0x00000007)
   description data: 32 2e 36 00
  xen-3.0              0x00000005 Unknown note type: (0x006e6558)
   description data: 08 00 00 00 03
readelf: Warning: note with invalid namesz and/or descsz found at offset 0x50
readelf: Warning:  type: 0xffffffff, namesize: 0x006e6558, descsize:
0x80000000, alignment: 8
[hjl@gnu-skx-1 linux]$

Since note.gnu.property section in kernel image is never used, this patch
discards .note.gnu.property sections in kernel linker script by adding

/DISCARD/ : {
  *(.note.gnu.property)
}

before kernel NOTE segment in generic NOTES.

Signed-off-by: H.J. Lu <hjl.tools@gmail.com>
Reviewed-by: Kees Cook <keescook@chromium.org>
---
 include/asm-generic/vmlinux.lds.h | 7 +++++++
 1 file changed, 7 insertions(+)

diff --git a/include/asm-generic/vmlinux.lds.h b/include/asm-generic/vmlinux.lds.h
index 71e387a5fe90..95cd678428f4 100644
--- a/include/asm-generic/vmlinux.lds.h
+++ b/include/asm-generic/vmlinux.lds.h
@@ -833,7 +833,14 @@
 #define TRACEDATA
 #endif
 
+/*
+ * Discard .note.gnu.property sections which are unused and have
+ * different alignment requirement from kernel note sections.
+ */
 #define NOTES								\
+	/DISCARD/ : {							\
+		*(.note.gnu.property)					\
+	}								\
 	.notes : AT(ADDR(.notes) - LOAD_OFFSET) {			\
 		__start_notes = .;					\
 		KEEP(*(.note.*))					\
-- 
2.25.4

Re: [PATCH 1/2] powerpc: Discard .rela* sections if CONFIG_RELOCATABLE is undefined

[Christophe Leroy]

Hi,

Le 28/04/2020 à 03:48, H.J. Lu a écrit :
> arch/powerpc/kernel/vmlinux.lds.S has
> 
>          DISCARDS
>          /DISCARD/ : {
>                  *(*.EMB.apuinfo)
>                  *(.glink .iplt .plt .rela* .comment)
>                  *(.gnu.version*)
>                  *(.gnu.attributes)
>                  *(.eh_frame)
>          }
> 
> Since .rela* sections are needed when CONFIG_RELOCATABLE is defined,
> change to discard .rela* sections if CONFIG_RELOCATABLE is undefined.

Your explanation and especially the subject are unclear.

 From the subject you understand that you are adding a discard of the 
.rela* sections if CONFIG_RELOCATABLE is undefined.

But when reading the patch, you see that it is the contrary: you are 
removing a discard of the .rela* sections if CONFIG_RELOCATABLE is defined.


So I think the subject could instead be:

	Only discard .rela* sections when CONFIG_RELOCATABLE is undefined

Or maybe better:

	Keep .rela* sections when CONFIG_RELOCATABLE is defined

And the explanation could be:

	Since .rela* sections are needed when CONFIG_RELOCATABLE
	is defined, don't discard .rela* sections if
	CONFIG_RELOCATABLE is defined.

> 
> Signed-off-by: H.J. Lu <hjl.tools@gmail.com>
> Acked-by: Michael Ellerman <mpe@ellerman.id.au> (powerpc)
> ---
>   arch/powerpc/kernel/vmlinux.lds.S | 5 ++++-
>   1 file changed, 4 insertions(+), 1 deletion(-)
> 
> diff --git a/arch/powerpc/kernel/vmlinux.lds.S b/arch/powerpc/kernel/vmlinux.lds.S
> index 31a0f201fb6f..4ba07734a210 100644
> --- a/arch/powerpc/kernel/vmlinux.lds.S
> +++ b/arch/powerpc/kernel/vmlinux.lds.S
> @@ -366,9 +366,12 @@ SECTIONS
>   	DISCARDS
>   	/DISCARD/ : {
>   		*(*.EMB.apuinfo)
> -		*(.glink .iplt .plt .rela* .comment)
> +		*(.glink .iplt .plt .comment)
>   		*(.gnu.version*)
>   		*(.gnu.attributes)
>   		*(.eh_frame)
> +#ifndef CONFIG_RELOCATABLE
> +		*(.rela*)
> +#endif
>   	}
>   }
> 

Christophe