Discussion:
[PATCH 6/9] arm64/build: Use common DISCARDS in linker script
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Will Deacon
2020-03-17 21:52:57 UTC
Permalink
Use the common DISCARDS rule for the linker script in an effort to
regularize the linker script to prepare for warning on orphaned
sections.
---
arch/arm64/kernel/vmlinux.lds.S | 8 ++------
1 file changed, 2 insertions(+), 6 deletions(-)
diff --git a/arch/arm64/kernel/vmlinux.lds.S b/arch/arm64/kernel/vmlinux.lds.S
index 497f9675071d..c61d9ab3211c 100644
--- a/arch/arm64/kernel/vmlinux.lds.S
+++ b/arch/arm64/kernel/vmlinux.lds.S
@@ -6,6 +6,7 @@
*/
#define RO_EXCEPTION_TABLE_ALIGN 8
+#define RUNTIME_DISCARD_EXIT
#include <asm-generic/vmlinux.lds.h>
#include <asm/cache.h>
@@ -19,7 +20,6 @@
/* .exit.text needed in case of alternative patching */
#define ARM_EXIT_KEEP(x) x
-#define ARM_EXIT_DISCARD(x)
OUTPUT_ARCH(aarch64)
ENTRY(_text)
@@ -94,12 +94,8 @@ SECTIONS
* matching the same input section name. There is no documented
* order of matching.
*/
+ DISCARDS
/DISCARD/ : {
- ARM_EXIT_DISCARD(EXIT_TEXT)
- ARM_EXIT_DISCARD(EXIT_DATA)
- EXIT_CALL
- *(.discard)
- *(.discard.*)
*(.interp .dynamic)
*(.dynsym .dynstr .hash .gnu.hash)
*(.eh_frame)
Acked-by: Will Deacon <***@kernel.org>

Will
Will Deacon
2020-03-17 21:56:14 UTC
Permalink
We don't want to depend on the linker's orphan section placement
heuristics as these can vary between linkers, and may change between
versions. All sections need to be explicitly named in the linker
script.
Explicitly include debug sections when they're present. Add .eh_frame*
to discard as it seems that these are still generated even though
-fno-asynchronous-unwind-tables is being specified. Add .plt and
.data.rel.ro to discards as they are not actually used. Add .got.plt
to the image as it does appear to be mapped near .data. Finally enable
orphan section warnings.
Hmm, I don't understand what .got.plt is doing here. Please can you
elaborate?

Will
Kees Cook
2020-03-17 23:01:16 UTC
Permalink
Post by Will Deacon
We don't want to depend on the linker's orphan section placement
heuristics as these can vary between linkers, and may change between
versions. All sections need to be explicitly named in the linker
script.
Explicitly include debug sections when they're present. Add .eh_frame*
to discard as it seems that these are still generated even though
-fno-asynchronous-unwind-tables is being specified. Add .plt and
.data.rel.ro to discards as they are not actually used. Add .got.plt
to the image as it does appear to be mapped near .data. Finally enable
orphan section warnings.
Hmm, I don't understand what .got.plt is doing here. Please can you
elaborate?
I didn't track it down, but it seems to have been present (and merged
into the kernel .data) for a while now. I can try to track this down if
you want?
--
Kees Cook
Nick Desaulniers
2020-03-17 23:10:53 UTC
Permalink
Post by Kees Cook
Post by Will Deacon
We don't want to depend on the linker's orphan section placement
heuristics as these can vary between linkers, and may change between
versions. All sections need to be explicitly named in the linker
script.
Explicitly include debug sections when they're present. Add .eh_frame*
to discard as it seems that these are still generated even though
-fno-asynchronous-unwind-tables is being specified. Add .plt and
.data.rel.ro to discards as they are not actually used. Add .got.plt
to the image as it does appear to be mapped near .data. Finally enable
orphan section warnings.
Hmm, I don't understand what .got.plt is doing here. Please can you
elaborate?
I didn't track it down, but it seems to have been present (and merged
into the kernel .data) for a while now. I can try to track this down if
you want?
Yes, the presence of a procedure linkage table makes sense for symbol
interposition and lazy binding in userspace executables with runtime
shared object loading support, but not so much the kernel, I would
think. (Though someone did just recently ask me if loadable kernel
modules could interpose weakly defined symbols in the kernel, and if
so what happens on unload. I have no idea and suspect kernel modules
cannot do that, but I have looked into the kernel's runtime relocation
support.)
--
Thanks,
~Nick Desaulniers
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