[PATCH] ARM: force linker to use PIC veneers
From: Dave.Martin@arm.com (Dave P Martin)
Date: 2015-03-30 12:06:12
On Fri, Mar 27, 2015 at 12:01:03AM +0000, Ard Biesheuvel wrote:
On 26 March 2015 at 14:19, Dave P Martin [off-list ref] wrote:quoted
On Thu, Mar 26, 2015 at 01:05:09PM +0000, Ard Biesheuvel wrote:quoted
On 26 March 2015 at 13:53, Dave P Martin [off-list ref] wrote:
[...]
quoted
THUMB2_KERNEL generates a kernel which is very nearly 100% Thumb code, so BX should be very rare when veneers are not involved.quoted
quoted
Ard, can you should build a couple of !THUMB2_KERNEL/pre-v5 configurations with your change and check, just to be sure?
[...]
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.
Fair enough -- thanks for checking. Cheers ---Dave