723939 - Linker needs to align to 16k boundaries on armv6

Status: RESOLVED FIXED

(Core :: mozglue, defect)

Description

[(not currently active) Ted Mielczarek]

After some detective work by glandium, sewardj, and jbramley, we found out that the reason the linker is failing on my armv6 builds is because there's a 16k alignment requirement.

Comment 1

[Mike Hommey [:glandium]]

Attachment: When reserving memory for the loaded library, ensure correct alignment for future MAP_SHARED mappings

Comment 2

[Mike Hommey [:glandium]]

As mentioned on IRC, another part of the problem is that the static linker doesn't help either, and we want it to align segments at 16k boundaries (that is, we need (p_vaddr - p_offset) % 16384 == 0).
This can be done with -Wl,-z,max-page-size=0x4000 in LDFLAGS. I'm not completely sure which of configure.in or mozconfig this should live in.

Comment 3

[(not currently active) Ted Mielczarek]

I think we should automatically add that to LDFLAGS in configure if we're targeting Android-armv6.

Comment 4

[Mike Hommey [:glandium]]

armv6 or less, since armv5 code can run on armv6.

Comment 5

[(dormant account)]

Comment on attachment 595355 [details] [diff] [review]
When reserving memory for the loaded library, ensure correct alignment for future MAP_SHARED mappings

seems harmless

Comment 6

[Mike Hommey [:glandium]]

Mmmmm interestingly, this breaks the linker on desktop :-/

Comment 7

[Mike Hommey [:glandium]]

Attachment: When reserving memory for the loaded library, ensure correct alignment for future MAP_SHARED mappings

I'm not entirely sure why it was breaking desktop and not android, but there seems to be pretty bad side effects keeping the mapping MAP_SHARED. So once we get a base address that can hold the library and that will work with MAP_SHARED later, just remap it MAP_PRIVATE instead.

Comment 8

[Mike Hommey [:glandium]]

Attachment: part 2 - Ensure 16k alignment of ELF segments when building for ARM < v7

Comment 9

[Mike Hommey [:glandium]]

Attachment: part 2 - Ensure 16k alignment of ELF segments when building for ARM < v7

There were single quotes instead of double quotes in the previous patch

Comment 10

[Mike Hommey [:glandium]]

Attachment: part 2 - Ensure 16k alignment of ELF segments when building for ARM < v7

This applies the flag to nspr, too.

Comment 11

[(not currently active) Ted Mielczarek]

Comment on attachment 597872 [details] [diff] [review]
part 2 - Ensure 16k alignment of ELF segments when building for ARM < v7

Review of attachment 597872 [details] [diff] [review]:
-----------------------------------------------------------------

::: configure.in
@@ +4091,5 @@
> +dnl Using the custom linker on ARMv6 requires 16k alignment of ELF segments.
> +if test -n "$MOZ_LINKER"; then
> +  if test "$CPU_ARCH" = arm; then
> +    dnl Determine the target ARM architecture (5 for ARMv5, v5T, v5E, etc.; 6 for ARMv6, v6K, etc.)
> +    ARM_ARCH=`${CC-cc} ${CFLAGS} -dM -E - < /dev/null | sed -n 's/.*__ARM_ARCH_\([[0-9]]*\).*/\1/p'`

Kind of awkward, but I don't have a better suggestion.

@@ +4095,5 @@
> +    ARM_ARCH=`${CC-cc} ${CFLAGS} -dM -E - < /dev/null | sed -n 's/.*__ARM_ARCH_\([[0-9]]*\).*/\1/p'`
> +    dnl When building for < ARMv7, we need to ensure 16k alignment of ELF segments
> +    if test -n "$ARM_ARCH" && test "$ARM_ARCH" -lt 7; then
> +      LDFLAGS="$LDFLAGS -Wl,-z,max-page-size=0x4000"
> +      _SUBDIR_LDFLAGS="$LDFLAGS"

Do we really want to stick all of our LDFLAGS in SUBDIR_LDFLAGS, or just this one thing?

Comment 12

[Mike Hommey [:glandium]]

https://hg.mozilla.org/integration/mozilla-inbound/rev/8a9d1df1c762
https://hg.mozilla.org/integration/mozilla-inbound/rev/a69b347c4d35

Comment 13

[Ed Morley [:emorley]]

https://hg.mozilla.org/mozilla-central/rev/8a9d1df1c762
https://hg.mozilla.org/mozilla-central/rev/a69b347c4d35