[PATCH v3 05/17] alternative/x86/arm32: Align altinstructions (and altinstr_replacement) sections.

[Konrad Rzeszutek Wilk <konrad@kernel.org>]

This is very similar to 137c59b9ff3f7a214f03b52d9c00a0a02374af1f
"bug/x86/arm: Align bug_frames sections."

On ARM and on x86 the C and assembler macros don't include
any alignment information - hence they end up being the default
byte granularity.

On ARM32 it is paramount that the alignment is word-size (4)
otherwise if one tries to use (uint32_t*) access (such
as livepatch ELF relocations) we get a Data Abort.

Specifically this issue was observed on ARM32 with a cross compiler for
the livepatches. Mainly the livepatches .data section size was not
padded to the section alignment:

ARM32 native:
Contents of section .rodata:
 0000 68695f66 756e6300 63686563 6b5f666e  hi_func.check_fn
 0010 63000000 78656e5f 65787472 615f7665  c...xen_extra_ve
 0020 7273696f 6e000000                    rsion...

ARM32 cross compiler:
Contents of section .rodata:
 0000 68695f66 756e6300 63686563 6b5f666e  hi_func.check_fn
 0010 63000000 78656e5f 65787472 615f7665  c...xen_extra_ve
 0020 7273696f 6e00                        rsion.

And when we loaded it the next section would be put right behind it:

native:

(XEN) livepatch.c:413: livepatch: xen_hello_world: Loaded .text at 00a02000
(XEN) livepatch.c:413: livepatch: xen_hello_world: Loaded .rodata at 00a04024
(XEN) livepatch.c:413: livepatch: xen_hello_world: Loaded .altinstructions at 00a0404c

cross compiler:
(XEN) livepatch.c:413: livepatch: xen_hello_world: Loaded .text at 00a02000
(XEN) livepatch.c:413: livepatch: xen_hello_world: Loaded .rodata at 00a04024
(XEN) livepatch.c:413: livepatch: xen_hello_world: Loaded .altinstructions at 00a0404a

(See 4a vs 4c)

native readelf:
  [ 4] .rodata           PROGBITS        00000000 000164 000028 00   A  0   0  4
  [ 5] .altinstructions  PROGBITS        00000000 00018c 00000c 00   A  0   0  1

cross compiler readelf --sections:
  [ 4] .rodata           PROGBITS        00000000 000164 000026 00   A  0   0  4
  [ 5] .altinstructions  PROGBITS        00000000 00018a 00000c 00   A  0   0  1

And as can be seen the .altinstructions have alignment of 1 which from
'man elf' is: "Values of zero and one mean no alignment is required."
which means we can ignore it.

Enforcing .altinstructions (and also .altinstr_replacement for
completness on ARM) to have the proper alignment across all
architectures and in both C and x86 makes them all the same.

On x86 the bloat-o-meter detects that with this change the file shrinks:
add/remove: 1/0 grow/shrink: 0/2 up/down: 156/-367 (-211)
function                                     old     new   delta
get_page_from_gfn                              -     156    +156
do_mmu_update                               4578    4569      -9
do_mmuext_op                                5604    5246    -358
Total: Before=3170439, After=3170228, chg -0.01%

But as found adding even "#Hi!\n" will casue this optimization, so the
bloat-o-meter value here is useless.

While on ARM 32/64:
add/remove: 0/0 grow/shrink: 0/0 up/down: 0/0 (0)
function                                     old     new   delta
Total: Before=822563, After=822563, chg +0.00%

Also since the macros have the alignment coded in them there is no need
to do that for the xen.lds.S anymore.

Signed-off-by: Konrad Rzeszutek Wilk <konrad.wilk@oracle.com>
---
v2: - First version
v3: - Figured out the x86 bloat-o-meter results.
    - Removed the .ALIGN from xen.lds.S
    - Removed the .p2align on .altinstr_replacement per Jan's request.
    - Put most of the commit description for the original issue
---
 xen/arch/arm/xen.lds.S            | 2 --
 xen/arch/x86/xen.lds.S            | 1 -
 xen/include/asm-arm/alternative.h | 4 ++++
 xen/include/asm-x86/alternative.h | 1 +
 4 files changed, 5 insertions(+), 3 deletions(-)

diff --git a/xen/arch/arm/xen.lds.S b/xen/arch/arm/xen.lds.S
index c9b9546435..447d33888f 100644
--- a/xen/arch/arm/xen.lds.S
+++ b/xen/arch/arm/xen.lds.S
@@ -155,11 +155,9 @@ SECTIONS
        __initcall_end = .;
 
 #ifdef CONFIG_HAS_ALTERNATIVE
-       . = ALIGN(4);
        __alt_instructions = .;
        *(.altinstructions)
        __alt_instructions_end = .;
-       . = ALIGN(4);
        *(.altinstr_replacement)
 #endif
 
diff --git a/xen/arch/x86/xen.lds.S b/xen/arch/x86/xen.lds.S
index d5e8821d41..9eb42048d5 100644
--- a/xen/arch/x86/xen.lds.S
+++ b/xen/arch/x86/xen.lds.S
@@ -202,7 +202,6 @@ SECTIONS
         * "Alternative instructions for different CPU types or capabilities"
         * Think locking instructions on spinlocks.
         */
-       . = ALIGN(8);
         __alt_instructions = .;
         *(.altinstructions)
         __alt_instructions_end = .;
diff --git a/xen/include/asm-arm/alternative.h b/xen/include/asm-arm/alternative.h
index 6cc9d0dc5f..cd1373fdd5 100644
--- a/xen/include/asm-arm/alternative.h
+++ b/xen/include/asm-arm/alternative.h
@@ -54,9 +54,11 @@ int apply_alternatives(const struct alt_instr *start, const struct alt_instr *en
 	oldinstr "\n"							\
 	"662:\n"							\
 	".pushsection .altinstructions,\"a\"\n"				\
+	".p2align 2\n"							\
 	ALTINSTR_ENTRY(feature)						\
 	".popsection\n"							\
 	".pushsection .altinstr_replacement, \"a\"\n"			\
+	".p2align 2\n"							\
 	"663:\n\t"							\
 	newinstr "\n"							\
 	"664:\n\t"							\
@@ -84,6 +86,7 @@ int apply_alternatives(const struct alt_instr *start, const struct alt_instr *en
 	.if \enable
 661:	\insn1
 662:	.pushsection .altinstructions, "a"
+	.p2align 2
 	altinstruction_entry 661b, 663f, \cap, 662b-661b, 664f-663f
 	.popsection
 	.pushsection .altinstr_replacement, "ax"
@@ -103,6 +106,7 @@ int apply_alternatives(const struct alt_instr *start, const struct alt_instr *en
 .macro alternative_if_not cap, enable = 1
 	.if \enable
 	.pushsection .altinstructions, "a"
+	.p2align 2
 	altinstruction_entry 661f, 663f, \cap, 662f-661f, 664f-663f
 	.popsection
 661:
diff --git a/xen/include/asm-x86/alternative.h b/xen/include/asm-x86/alternative.h
index db4f08e0e7..79430fdf05 100644
--- a/xen/include/asm-x86/alternative.h
+++ b/xen/include/asm-x86/alternative.h
@@ -56,6 +56,7 @@ extern void alternative_instructions(void);
 
 #define ALTERNATIVE_N(newinstr, feature, number)	\
 	".pushsection .altinstructions,\"a\"\n"		\
+	".p2align 2\n"					\
 	ALTINSTR_ENTRY(feature, number)			\
 	".section .discard,\"a\",@progbits\n"		\
 	DISCARD_ENTRY(number)				\
-- 
2.13.3


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