[PATCH] ARM: force linker to use PIC veneers

[Ard Biesheuvel]

On 26 March 2015 at 14:19, Dave P Martin <Dave.Martin at arm.com> wrote:
> On Thu, Mar 26, 2015 at 01:05:09PM +0000, Ard Biesheuvel wrote:
>> On 26 March 2015 at 13:53, Dave P Martin <Dave.Martin at arm.com> wrote:
>> > On Thu, Mar 26, 2015 at 12:24:01PM +0000, Ard Biesheuvel wrote:
>> >> On 26 March 2015 at 13:22, Russell King - ARM Linux
>> >> <linux at arm.linux.org.uk> wrote:
>> >> > On Thu, Mar 26, 2015 at 01:20:16PM +0100, Ard Biesheuvel wrote:
>> >
>> > [...]
>> >
>> >> >> 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.
>
>> > 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.