On Tue, 8 Aug 2017 10:53:56 +0900
Masahiro Yamada [off-list ref] wrote:

Hi Nicholas,

2017-07-26 21:46 GMT+09:00 Nicholas Piggin [off-list ref]:

quoted

The .data and .bss sections were modified in the generic linker script to
pull in sections named .data.<C identifier>, which are generated by gcc with
-ffunction-sections and -fdata-sections options.

The problem with this pattern is it can also match section names that Linux
defines explicitly, e.g., .data.unlikely. This can cause Linux sections to
get moved into the wrong place.

The way to avoid this is to use ".." separators for explicit section names
(the dot character is valid in a section name but not a C identifier).
However currently there are sections which don't follow this rule, so for
now just disable the wild card by default.

Example: http://marc.info/?l=linux-arm-kernel&m=150106824024221&w=2

Cc: <redacted> # 4.9
Fixes: b67067f1176df ("kbuild: allow archs to select link dead code/data elimination")
Signed-off-by: Nicholas Piggin <npiggin@gmail.com>
---
 include/asm-generic/vmlinux.lds.h | 38 ++++++++++++++++++++++++++------------
 1 file changed, 26 insertions(+), 12 deletions(-)

diff --git a/include/asm-generic/vmlinux.lds.h b/include/asm-generic/vmlinux.lds.h
index da0be9a8d1de..9623d78f8494 100644
--- a/include/asm-generic/vmlinux.lds.h
+++ b/include/asm-generic/vmlinux.lds.h

@@ -60,6 +60,22 @@
 #define ALIGN_FUNCTION()  . = ALIGN(8)

 /*
+ * LD_DEAD_CODE_DATA_ELIMINATION option enables -fdata-sections, which
+ * generates .data.identifier sections, which need to be pulled in with
+ * .data. We don't want to pull in .data..other sections, which Linux
+ * has defined. Same for text and bss.
+ */
+#ifdef CONFIG_LD_DEAD_CODE_DATA_ELIMINATION
+#define TEXT_MAIN .text .text.[0-9a-zA-Z_]*
+#define DATA_MAIN .data .data.[0-9a-zA-Z_]*
+#define BSS_MAIN .bss .bss.[0-9a-zA-Z_]*
+#else
+#define TEXT_MAIN .text
+#define DATA_MAIN .data
+#define BSS_MAIN .bss
+#endif  

From the git-log, I think you are planning to rename
  .data.unlikely   ->   .data..unlikey
then the problem will be fixed.

("git grep data.unlikely" gave me only 7 instances.)


Why is this patch necessary?


You are trying to migrate some architectures to DCDE,
so this #ifdef does not solve the problem, I think.

Well basically I want to preserve previous behaviour and avoid
regressions as far as possible. Patches to rename existing
sections I think would not be so suitable for @stable. And
there could be other things we've missed.

Yes I would like to ensure all our sections use .. separators
which (hopefully). Until then I think this is a minimal fix.

Thanks,
Nick