encountered interesting segmentation fault on strb w1, [x22,x23] instruction in armv8 assembly code when x23 reaches value (which 635), when i'm not using bl write function.
however, if use bl instruction call write function, segmentation fault not occur; program finishes "... exited code 02" in gdb or echo $? 2.
machine: linux alarm 3.10.65-4-pine64-longsleep #16 smp preempt sun apr 3 10:56:40 cest 2016 aarch64 gnu/linux
gdb -v gnu gdb (gdb) 7.11.1
gcc version 6.1.1 20160707 (gcc)
what causing behavior?
5 .bss 6 n_array: .byte 0 7 .data 8 fmt: .asciz "\t%d" 9 c_array:.ascii "73167176531330624919225119674426574742355349194934" 10 .ascii "96983520312774506326239578318016984801869478851843" 11 .ascii "85861560789112949495459501737958331952853208805511" 12 .ascii "12540698747158523863050715693290963295227443043557" 13 .ascii "66896648950445244523161731856403098711121722383113" 14 .ascii "62229893423380308135336276614282806444486645238749" 15 .ascii "30358907296290491560440772390713810515859307960866" 16 .ascii "70172427121883998797908792274921901699720888093776" 17 .ascii "65727333001053367881220235421809751254540594752243" 18 .ascii "52584907711670556013604839586446706324415722155397" 19 .ascii "53697817977846174064955149290862569321978468622482" 20 .ascii "83972241375657056057490261407972968652414535100474" 21 .ascii "82166370484403199890008895243450658541227588666881" 22 .ascii "16427171479924442928230863465674813919123162824586" 23 .ascii "17866458359124566529476545682848912883142607690042" 24 .ascii "24219022671055626321111109370544217506941658960408" 25 .ascii "07198403850962455444362981230987879927244284909188" 26 .ascii "84580156166097919133875499200524063689912560717606" 27 .ascii "05886116467109405077541002256983155200055935729725" 28 .ascii "71636269561882670428252483600823257530420752963450" 29 .equ len_c_array,.-c_array 30 .text 31 .global main 32 .include "mymac_armv8.s" 33 main: 34 nop 35 mov w19, 0x30 36 ldr x20,=c_array 37 mov x21, #len_c_array 38 sub x21, x21, 1 39 ldr x22,=n_array 40 mov x23, xzr 41 1: 42 ldrb w1, [x20,x23] // ascii char 43 sub w1, w1, w19 // ascii value - 0x30; assumes ascii number chars 44 strb w1, [x22,x23] // store byte n_array element 45 bl write 46 add x23, x23, 1 47 cmp x23, x21 48 b.le 1b 49 _exit 50 write: 51 push2 x29,x30 52 ldr x0,=fmt 53 bl printf 54 pop2 x29, x30 55 ret dmesg [23493.499193] cpu: 0 pid: 18212 comm: ascii_array_con tainted: g o 3.10.65-4-pine64-longsleep #16 [23493.515436] task: ffffffc0764b4ec0 ti: ffffffc073804000 task.ti: ffffffc073804000 [23493.529484] pc @ 0x4005d0 [23493.538487] lr @ 0x7faccae8a4 [23493.547816] pc : [<00000000004005d0>] lr : [<0000007faccae8a4>] pstate: 80000000 [23493.561648] sp : 0000007ffee581a0 [23493.570951] x29: 0000007ffee581a0 x28: 0000000000000000 [23493.582394] x27: 0000000000000000 x26: 0000000000000000 [23493.593717] x25: 0000000000000000 x24: 0000000000000000 [23493.604933] x23: 000000000000027b x22: 0000000000410d85 [23493.616092] x21: 00000000000003e8 x20: 000000000041099c [23493.627122] x19: 0000000000000030 x18: 0000000000030a41 [23493.638002] x17: 0000000000410968 x16: 0000007faccae7c8 [23493.648801] x15: 0000000000000809 x14: 0000000000000000 [23493.659629] x13: 0000000000000000 x12: 0000007face1d000 [23493.670353] x11: 0000000004000000 x10: 0101010101010101 [23493.681037] x9 : 3fffffffffffffff x8 : ffffffffffffffff [23493.691716] x7 : 0000000004000000 x6 : 0000000000000000 [23493.702436] x5 : 0000000000000000 x4 : 0000007ffee581f8 [23493.713174] x3 : 00000000004005b0 x2 : 0000007ffee582e8 [23493.723904] x1 : 0000000000000005 x0 : 0000000000000001 [23493.746914] systemd-coredump[18214]: failed comm, falling command line: no such process [23493.762810] systemd-coredump[18214]: failed exe, ignoring: no such process
the size of bss not sized stated in first comment, , reinforced notlikethat clue: size incorrect, hit mmu boundary after while => segv.
use (as stated in http://web.mit.edu/gnu/doc/html/as_7.html):
n_array: .fill 1000 , 1 , 0 just tried , worked (well, x86 should work arm too):
>objdump -d ascii_array ascii_array_conv.o: file format pe-x86-64 disassembly of section .text: 0000000000000000 <data1>: ... 3e8: 90 nop 3e9: 90 nop 3ea: 90 nop 3eb: 90 nop 3ec: 90 nop 3ed: 90 nop 3ee: 90 nop 3ef: 90 nop 
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