[PATCH] ARM: force linker to use PIC veneers

On 26 March 2015 at 14:19, Dave P Martin [off-list ref] wrote:

On Thu, Mar 26, 2015 at 01:05:09PM +0000, Ard Biesheuvel wrote:

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On 26 March 2015 at 13:53, Dave P Martin [off-list ref] wrote:

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On Thu, Mar 26, 2015 at 12:24:01PM +0000, Ard Biesheuvel wrote:

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On 26 March 2015 at 13:22, Russell King - ARM Linux
[off-list ref] wrote:

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On Thu, Mar 26, 2015 at 01:20:16PM +0100, Ard Biesheuvel wrote:

[...]

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Any specific concerns? The --pic-veneer switch dates back to 2009, so
it's been around for a while

BX.  Not every CPU supports BX.

The bx is used for the Thumb -> ARM switch inside the veneer only, so
if you build for ARM these won't appear in the output

Agreed that's what's _supposed_ to happen.

Note that --use-blx is an override and might introduce BXs into a kernel
that otherwise doesn't have them.  But in any case, this option doesn't
show a benefit for the scenario under discussion, hence my dropping the
request (conditional on THUMB2_KERNEL in any case).

Actually, i am not quite sure. I hadn't realised that there is no
branch-without-link counterpart of 'blx <label>' so even if I still
potted a number of bx pc/nop sequences, I can't really say if there
were quite as many as before.

Where not out of range, the compiler should just branch to the
destination, with no veneer.  This would apply to optimised tail-
calls, for example -- but with an ordinary branch, not BX.  The
linker would only ever fix it up as B{L}X for one of two reasons:
either the destination is in a different instruction set, or
Thumb code is branching to a veneer (veneers are always ARM).

THUMB2_KERNEL generates a kernel which is very nearly 100% Thumb
code, so BX should be very rare when veneers are not involved.

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Ard, can you should build a couple of !THUMB2_KERNEL/pre-v5
configurations with your change and check, just to be sure?

Will do.

ARM kernel with --pic-veneer:
=============================
$ readelf -A vmlinux
Attribute Section: aeabi
File Attributes
  Tag_CPU_name: "4T"
  Tag_CPU_arch: v4T
  Tag_ARM_ISA_use: Yes
  Tag_THUMB_ISA_use: Thumb-1
  [...]

$ size vmlinux
   text   data    bss    dec    hex filename
38287123 13496224 9589540 61372887 3a879d7 vmlinux

$ grep -c veneer System.map
493

Veneers look like this:

c247d53c <__sys_write_veneer>:
c247d53c:       e59fc000        ldr     ip, [pc]
c247d540:       e08ff00c        add     pc, pc, ip
c247d544:       fdd3aa8c        .word   0xfdd3aa8c

c247d548 <__init_rt_rq_veneer>:
c247d548:       e59fc000        ldr     ip, [pc]
c247d54c:       e08ff00c        add     pc, pc, ip
c247d550:       fdbf9600        .word   0xfdbf9600

c247d554 <__read_boot_clock_veneer>:
c247d554:       e59fc000        ldr     ip, [pc]
c247d558:       e08ff00c        add     pc, pc, ip
c247d55c:       fdb91310        .word   0xfdb91310

Thumb2 kernel with --pic-veneer
===============================
Attribute Section: aeabi
File Attributes
  Tag_CPU_name: "7-A"
  Tag_CPU_arch: v7
  Tag_CPU_arch_profile: Application
  Tag_ARM_ISA_use: Yes
  Tag_THUMB_ISA_use: Thumb-2
  [...]

$ size vmlinux
   text   data    bss    dec    hex filename
29353740 13644128 9614052 52611920 322cb50 vmlinux

$ grep -c veneer System.map
1928

Veneers look like this:

c014f9d8 <____down_interruptible_veneer>:
c014f9d8:       e59fc004        ldr     ip, [pc, #4]
c014f9dc:       e08fc00c        add     ip, pc, ip
c014f9e0:       e12fff1c        bx      ip
c014f9e4:       0128de45        .word   0x0128de45

c014f9e8 <___raw_write_unlock_veneer>:
c014f9e8:       4778            bx      pc
c014f9ea:       46c0            nop
c014f9ec:       e59fc004        ldr     ip, [pc, #4]
c014f9f0:       e08fc00c        add     ip, pc, ip
c014f9f4:       e12fff1c        bx      ip
c014f9f8:       012901e1        .word   0x012901e1

c014f9fc <____mutex_unlock_slowpath_veneer>:
c014f9fc:       e59fc004        ldr     ip, [pc, #4]
c014fa00:       e08fc00c        add     ip, pc, ip
c014fa04:       e12fff1c        bx      ip
c014fa08:       0128d8e1        .word   0x0128d8e1

$ arm-linux-gnueabihf-objdump -d vmlinux |grep -E bx\\s+pc -c
319

This number does not change when adding --use-blx, so there is probably not
much point in adding it.