[PATCH akpm/next v2] lib: crc32: constify crc32 lookup table

From: Daniel Borkmann
Date: Sat Jan 03 2015 - 06:14:41 EST


Commit 8f243af42ade ("sections: fix const sections for crc32 table")
removed the compile-time generated crc32 tables from the RO sections,
because it conflicts with the definition of __cacheline_aligned
which puts all such aligned data into .data..cacheline_aligned section
optimized for wasting less space, and can cause alignment issues when
used in combination with const with some gcc versions like 4.7.0 due
to a gcc bug [1].

Given that most gcc versions should have the fix by now, we can just
use ____cacheline_aligned, which only aligns the data but doesn't move
it into specific sections as opposed to __cacheline_aligned. In case
of gcc versions having the mentioned bug, the alignment attribute will
have no effect, but the data will still be made RO.

After patch tables are in RO:

$ nm -v lib/crc32.o | grep -1 -E "crc32c?table"
0000000000000000 t arch_local_irq_enable
0000000000000000 r crc32ctable_le
0000000000000000 t crc32_exit
--
0000000000000960 t test_buf
0000000000002000 r crc32table_be
0000000000004000 r crc32table_le
000000001d1056e5 A __crc_crc32_be

[1] https://gcc.gnu.org/bugzilla/show_bug.cgi?id=52181

Signed-off-by: Daniel Borkmann <dborkman@xxxxxxxxxx>
Cc: Joe Mario <jmario@xxxxxxxxxx>
---
v1 -> v2:
- Removed gcc test case.

lib/gen_crc32table.c | 6 +++---
1 file changed, 3 insertions(+), 3 deletions(-)

diff --git a/lib/gen_crc32table.c b/lib/gen_crc32table.c
index 71fcfcd..d83a372 100644
--- a/lib/gen_crc32table.c
+++ b/lib/gen_crc32table.c
@@ -109,7 +109,7 @@ int main(int argc, char** argv)

if (CRC_LE_BITS > 1) {
crc32init_le();
- printf("static u32 __cacheline_aligned "
+ printf("static const u32 ____cacheline_aligned "
"crc32table_le[%d][%d] = {",
LE_TABLE_ROWS, LE_TABLE_SIZE);
output_table(crc32table_le, LE_TABLE_ROWS,
@@ -119,7 +119,7 @@ int main(int argc, char** argv)

if (CRC_BE_BITS > 1) {
crc32init_be();
- printf("static u32 __cacheline_aligned "
+ printf("static const u32 ____cacheline_aligned "
"crc32table_be[%d][%d] = {",
BE_TABLE_ROWS, BE_TABLE_SIZE);
output_table(crc32table_be, LE_TABLE_ROWS,
@@ -128,7 +128,7 @@ int main(int argc, char** argv)
}
if (CRC_LE_BITS > 1) {
crc32cinit_le();
- printf("static u32 __cacheline_aligned "
+ printf("static const u32 ____cacheline_aligned "
"crc32ctable_le[%d][%d] = {",
LE_TABLE_ROWS, LE_TABLE_SIZE);
output_table(crc32ctable_le, LE_TABLE_ROWS,
--
1.7.11.7

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