Re: [Xen-devel] [PATCH v4 12/16] bug/x86/arm: Align bug_frames sections.

[Konrad Rzeszutek Wilk <konrad.wilk@xxxxxxxxxx>]

On Mon, Sep 19, 2016 at 04:19:55PM -0400, Konrad Rzeszutek Wilk wrote:
> > > > Signed-off-by: Konrad Rzeszutek Wilk <konrad.wilk@xxxxxxxxxx>
> > 
> > I forgot to mention that with those NITs fixed:
> > 
> > Reviewed-by: Julien Grall <julien.grall@xxxxxxx>
> 
> Thanks.
> 
> However I noticed (and I recall having it replaced in earlier
> versions so I must have in a hurry missed it), but this:
> 
> > > > --- a/xen/include/asm-arm/bug.h
> > > > +++ b/xen/include/asm-arm/bug.h
> > > > @@ -52,6 +52,7 @@ struct bug_frame {
> > > > 
> > > > ".popsection\n"                                                    \
> > > >           ".pushsection .bug_frames." __stringify(type) ", \"a\",
> > > > %progbits\n"\
> > > > 
> > > > "4:\n"                                                             \
> > > > +         ".align 4\n"                                                  
> > > >      \
> 
> [fixed up, your mailer mangled it]
> This should have been:
> 
> diff --git a/xen/include/asm-arm/bug.h b/xen/include/asm-arm/bug.h
> index 773d63e..affe64f 100644
> --- a/xen/include/asm-arm/bug.h
> +++ b/xen/include/asm-arm/bug.h
> @@ -52,7 +52,7 @@ struct bug_frame {
>           ".popsection\n"                                                    \
>           ".pushsection .bug_frames." __stringify(type) ", \"a\", 
> %progbits\n"\
>           "4:\n"                                                             \
> -         ".align 4\n"                                                       \
> +         ".p2align 4\n"                                                     \

Argh. '.p2align 2' !

For reference, here is the full patch:

From ecf25f594118042364367d0bfacb3ee252222046 Mon Sep 17 00:00:00 2001
From: Konrad Rzeszutek Wilk <konrad.wilk@xxxxxxxxxx>
Date: Wed, 7 Sep 2016 11:57:05 -0400
Subject: [PATCH] bug/x86/arm: Align bug_frames sections.

Most of the WARN_ON or BUG_ON sections are properly aligned on
x86. However on ARM and on x86 assembler the 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.

Enforcing bug_frames to have the proper alignment across all
architectures and in both C and x86 makes them all the same.

Furthermore on x86 the bloat-o-meter detects that with this
change:

add/remove: 0/0 grow/shrink: 0/0 up/down: 0/0 (0)
function                                     old     new   delta

On ARM32:
add/remove: 1/0 grow/shrink: 0/1 up/down: 384/-288 (96)
function                                     old     new   delta
gnttab_unpopulate_status_frames                -     384    +384
do_grant_table_op                          10808   10520    -288

And ARM64:
add/remove: 1/2 grow/shrink: 0/1 up/down: 4164/-4236 (-72)
function                                     old     new   delta
gnttab_map_grant_ref                           -    4164   +4164
do_grant_table_op                           9892    9836     -56
grant_map_exists                             300       -    -300
__gnttab_map_grant_ref                      3880       -   -3880

Reviewed-by: Julien Grall <julien.grall@xxxxxxx>
Acked-by: Jan Beulich <jbeulich@xxxxxxxx> [x86 parts]
Signed-off-by: Konrad Rzeszutek Wilk <konrad.wilk@xxxxxxxxxx>
---
Cc: Julien Grall <julien.grall@xxxxxxx>
Cc: Stefano Stabellini <sstabellini@xxxxxxxxxx>
Cc: Jan Beulich <jbeulich@xxxxxxxx>
Cc: Andrew Cooper <andrew.cooper3@xxxxxxxxxx>

v3: First submission. Replaces the "livepatch/elf: Adjust section aligment to 
word"
    patch.
v4: Remove the .ALIGN(4) in xen.lds.S for x86 (the only place needing
    that change).
    Update the commit description with correct x86 results
    Remove the . = ALIGN(4); in linker filer on x86 [the only file needing the 
change]
v5: Add Jan's Ack on x86 parts.
v6: Added Julien's Reviewed-by
    s/aligment/alignment/
    s/align 4/p2align 2/ as the align semnatics varies by platforms, while
    p2align is the same across all of them.
---
 xen/arch/x86/xen.lds.S    | 1 -
 xen/include/asm-arm/bug.h | 1 +
 xen/include/asm-x86/bug.h | 1 +
 3 files changed, 2 insertions(+), 1 deletion(-)

diff --git a/xen/arch/x86/xen.lds.S b/xen/arch/x86/xen.lds.S
index d903c31..7676de9 100644
--- a/xen/arch/x86/xen.lds.S
+++ b/xen/arch/x86/xen.lds.S
@@ -79,7 +79,6 @@ SECTIONS
   .rodata : {
        _srodata = .;
        /* Bug frames table */
-       . = ALIGN(4);
        __start_bug_frames = .;
        *(.bug_frames.0)
        __stop_bug_frames_0 = .;
diff --git a/xen/include/asm-arm/bug.h b/xen/include/asm-arm/bug.h
index 68353e1..4704e2d 100644
--- a/xen/include/asm-arm/bug.h
+++ b/xen/include/asm-arm/bug.h
@@ -52,6 +52,7 @@ struct bug_frame {
          ".popsection\n"                                                    \
          ".pushsection .bug_frames." __stringify(type) ", \"a\", %progbits\n"\
          "4:\n"                                                             \
+         ".p2align 2\n"                                                     \
          ".long (1b - 4b)\n"                                                \
          ".long (2b - 4b)\n"                                                \
          ".long (3b - 4b)\n"                                                \
diff --git a/xen/include/asm-x86/bug.h b/xen/include/asm-x86/bug.h
index c5d2d4c..9bb4a19 100644
--- a/xen/include/asm-x86/bug.h
+++ b/xen/include/asm-x86/bug.h
@@ -98,6 +98,7 @@ extern const struct bug_frame __start_bug_frames[],
     .popsection
 
     .pushsection .bug_frames.\type, "a", @progbits
+        .p2align 2
         .L\@bf:
         .long (.L\@ud - .L\@bf) + \
                ((\line >> BUG_LINE_LO_WIDTH) << BUG_DISP_WIDTH)
-- 
2.4.11


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