GCC 9 stack smash protector not generating correct code

Asked by daniel worley on 2020-03-20

gcc-arm-none-eabi-9-2019-q4-major
CFLAGS = --specs=nano.specs --specs=nosys.specs -O0 -mcpu-cortex-m4 -mthumb -fstack-protector-strong
Examined generated code using the debugger and objdump and observed:
000c0704 <main>:
   c0704: b580 push {r7, lr}
   c0706: b0a2 sub sp, #136 ; 0x88
   c0708: af00 add r7, sp, #0
   c070a: 4b17 ldr r3, [pc, #92] ; (c0768 <main+0x64>)
   c070c: 681b ldr r3, [r3, #0]
   c070e: f8c7 3084 str.w r3, [r7, #132] ; 0x84
   c0712: f04f 0300 mov.w r3, #0
   c0716: 2300 movs r3, #0
   c0718: 603b str r3, [r7, #0]
   c071a: e010 b.n c073e <main+0x3a>
...
   c0766: bd80 pop {r7, pc}
   c0768: 000c0c34 This is NOT the address of __stack_chk_guard
   c076c: 400fff00
...
000c0c30 <_global_impure_ptr>:
   c0c30: 00118008
   c0c34: 00118460 From the map file this is the address of __stack_chk_guard
   c0c38: 202a2a2a
   c0c3c: 63617473
   c0c40: 6d73206b
The code sequence at the beginning of main stores the address of __stack_chk_guard on the stack not the value. From the map file:
.bss.__stack_chk_guard 0x0000000000118460 0x4

If I build the same code with arm-none-eabi-gcc-8-2019-q3-update
000c06f4 <main>:
   c06f4: b580 push {r7, lr}
   c06f6: b0a2 sub sp, #136 ; 0x88
   c06f8: af00 add r7, sp, #0
   c06fa: 4b16 ldr r3, [pc, #88] ; (c0754 <main+0x60>)
   c06fc: 681b ldr r3, [r3, #0]
   c06fe: f8c7 3084 str.w r3, [r7, #132] ; 0x84
   c0702: 2300 movs r3, #0
   c0704: 603b str r3, [r7, #0]
   c0706: e010 b.n c072a <main+0x36>
...
   c0752: bd80 pop {r7, pc}
   c0754: 00118460 This is the address of __stack_chk_guard from the map file
   c0758: 400fff00

.bss.__stack_chk_guard 0x0000000000118460 0x4

Am I misunderstanding SSP? I thought the value of the guard variable is saved on functions stack. The GCC 9 code is saving the address of __stack_chk_guard.

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2020-03-20
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