Le 21/04/2021 à 04:17, Jordan Niethe a écrit :
Large branches will go through the plt which includes a stub that loads
a target address from the .branch_lt section. On a relocatable kernel the
targets in .branch_lt have relocations so they will be fixed up for
where the kernel is running by relocate().
For a non-relocatable kernel obviously there are no relocations.
However, until the kernel is moved down to its linked address it is
expected to be able to run where ever it is loaded. For pseries machines
prom_init() is called before running at the linked address.
Certain configs result in a large kernel such as STRICT_KERNEL_RWX
(because of the larger data shift):
config DATA_SHIFT
int "Data shift" if DATA_SHIFT_BOOL
default 24 if STRICT_KERNEL_RWX && PPC64
These large kernels lead to prom_init()'s final call to __start()
generating a plt branch:
bl c000000002000018 <00000078.plt_branch.__start>
This results in the kernel jumping to the linked address of __start,
0xc000000000000000, when really it needs to jump to the
0xc000000000000000 + the runtime address because the kernel is still
running at the load address.
On ppc32 it seems to be different. I can't find plt_branch or lt_branch or
whatever.
Looks like the stubs are placed at the end of .head section, and just after
prom_init:
c0003858 <setup_disp_bat>:
c0003858: 7d 08 02 a6 mflr r8
c000385c: 48 00 d5 35 bl c0010d90 <reloc_offset>
c0003860: 7d 08 03 a6 mtlr r8
c0003864: 3d 03 c2 04 addis r8,r3,-15868
c0003868: 39 08 1d 08 addi r8,r8,7432
c000386c: 2c 08 00 00 cmpwi r8,0
c0003870: 4d 82 00 20 beqlr
c0003874: 81 68 00 00 lwz r11,0(r8)
c0003878: 81 08 00 04 lwz r8,4(r8)
c000387c: 7d 1f 83 a6 mtdbatl 3,r8
c0003880: 7d 7e 83 a6 mtdbatu 3,r11
c0003884: 4e 80 00 20 blr
c0003888: 3d 80 c2 00 lis r12,-15872
c000388c: 39 8c 16 dc addi r12,r12,5852
c0003890: 7d 89 03 a6 mtctr r12
c0003894: 4e 80 04 20 bctr
c0003898: 3d 80 c2 01 lis r12,-15871
c000389c: 39 8c e1 38 addi r12,r12,-7880
c00038a0: 7d 89 03 a6 mtctr r12
c00038a4: 4e 80 04 20 bctr
c00038a8: 3d 80 c2 00 lis r12,-15872
c00038ac: 39 8c 74 d0 addi r12,r12,29904
c00038b0: 7d 89 03 a6 mtctr r12
c00038b4: 4e 80 04 20 bctr
c00038b8: 3d 80 c2 00 lis r12,-15872
c00038bc: 39 8c 73 38 addi r12,r12,29496
c00038c0: 7d 89 03 a6 mtctr r12
c00038c4: 4e 80 04 20 bctr
c00038c8: 3d 80 c2 01 lis r12,-15871
c00038cc: 39 8c 83 6c addi r12,r12,-31892
c00038d0: 7d 89 03 a6 mtctr r12
c00038d4: 4e 80 04 20 bctr
c00038d8: 3d 80 c2 01 lis r12,-15871
c00038dc: 39 8c 8f 08 addi r12,r12,-28920
c00038e0: 7d 89 03 a6 mtctr r12
c00038e4: 4e 80 04 20 bctr
Disassembly of section .text:
c0004000 <Reset_virt>:
c20016dc <prom_init>:
c20016dc: 94 21 ff 50 stwu r1,-176(r1)
c20016e0: 7c 08 02 a6 mflr r0
c20016e4: 42 9f 00 05 bcl 20,4*cr7+so,c20016e8 <prom_init+0xc>
c20016e8: bd c1 00 68 stmw r14,104(r1)
c20016ec: 7f c8 02 a6 mflr r30
c20016f0: 90 01 00 b4 stw r0,180(r1)
c20016f4: 7c bb 2b 78 mr r27,r5
c20016f8: 80 1e ff f0 lwz r0,-16(r30)
....
c20026d4: 4a 00 ed 69 bl c001143c <reloc_got2>
c20026d8: 7f e3 fb 78 mr r3,r31
c20026dc: 7f 24 cb 78 mr r4,r25
c20026e0: 39 20 00 00 li r9,0
c20026e4: 39 00 00 00 li r8,0
c20026e8: 38 e0 00 00 li r7,0
c20026ec: 38 c0 00 00 li r6,0
c20026f0: 38 a0 00 00 li r5,0
c20026f4: 48 00 00 61 bl c2002754 <prom_init+0x1078>
c20026f8: 38 60 00 00 li r3,0
c20026fc: 80 01 00 b4 lwz r0,180(r1)
c2002700: 81 c1 00 68 lwz r14,104(r1)
c2002704: 81 e1 00 6c lwz r15,108(r1)
c2002708: 7c 08 03 a6 mtlr r0
c200270c: 82 01 00 70 lwz r16,112(r1)
c2002710: 82 21 00 74 lwz r17,116(r1)
c2002714: 82 41 00 78 lwz r18,120(r1)
c2002718: 82 61 00 7c lwz r19,124(r1)
c200271c: 82 81 00 80 lwz r20,128(r1)
c2002720: 82 a1 00 84 lwz r21,132(r1)
c2002724: 82 c1 00 88 lwz r22,136(r1)
c2002728: 82 e1 00 8c lwz r23,140(r1)
c200272c: 83 01 00 90 lwz r24,144(r1)
c2002730: 83 21 00 94 lwz r25,148(r1)
c2002734: 83 41 00 98 lwz r26,152(r1)
c2002738: 83 61 00 9c lwz r27,156(r1)
c200273c: 83 81 00 a0 lwz r28,160(r1)
c2002740: 83 a1 00 a4 lwz r29,164(r1)
c2002744: 83 c1 00 a8 lwz r30,168(r1)
c2002748: 83 e1 00 ac lwz r31,172(r1)
c200274c: 38 21 00 b0 addi r1,r1,176
c2002750: 4e 80 00 20 blr
c2002754: 3d 80 c0 00 lis r12,-16384
c2002758: 39 8c 00 0c addi r12,r12,12
c200275c: 7d 89 03 a6 mtctr r12
c2002760: 4e 80 04 20 bctr
Any idea on how the GNU ld does it and how we can alter it, or force generation of dedicated section
like on PPC64 ?
Christophe
