armv7-m memcpy usage fault

[Martin Velek]

Hello,

the memcpy causes usage fault on Cortex M3 when UNALIGN_TRP is set in CCR.

Test case:
#include <string.h>
int main(void)
{
 memcpy((void *)0x200050d0, (void *)0xa0ffe7b6, 42);
 return 1;
}

Cross ARM Compiler
arm-none-eabi-gcc -mcpu=cortex-m3 -mthumb -mno-unaligned-access -O0 -fmessage-length=0 -fsigned-char -ffunction-sections -fdata-sections -fno-common -Wall -g3 -std=gnu99 -Wa,-adhlns="main.o.lst" -fstack-usage -funwind-tables -MMD -MP -MF"main.d" -MT"main.d" -c -o "main.o" "../main.c"

Cross ARM C Linker
arm-none-eabi-gcc -mcpu=cortex-m3 -mthumb -mno-unaligned-access -O0 -fmessage-length=0 -fsigned-char -ffunction-sections -fdata-sections -fno-common -Wall -g3 -T "rom.ld" -Xlinker --gc-sections -Xlinker --wrap=_free_r -Xlinker --wrap=_malloc_r -Xlinker --wrap=_calloc_r -Xlinker --wrap=_realloc_r -Wl,-Map,"App.map" -o "App.elf" ./main.o ./startup.o ./startup_ARMCM3.o

memcpy asm:
00009902: bne.n 0x9998 <memcpy+160>
00009904: subs r2, #64 ; 0x40
00009906: bcc.n 0x9950 <memcpy+88>
-->>>00009908: ldr r3, [r1, #0] ; ldr from 0xa0ffe7b6, unaligned fault
0000990a: str r3, [r0, #0]
0000990c: ldr r3, [r1, #4]

[Martin Velek (martin-velek)]

I did some inspection and the reason for this error is following:

arm-none-eabi-gcc -mcpu=cortex-m3 by default assumes that the unaligned access is enabled. The macro __ARM_FEATURE_UNALIGNED is set. The source code in newlib machine/arm/memcpy-armv7m.S is built with this macro. The result is weird. It is bug and it is not bug.

I think there should be mentioned in the readme.txt to NOT TO SET UNALIGN_TRP in CCR. Anyway, the GDB with UNALIGN_TRP behaves strange. I was unable to get stack list, debugger reported read from non existent addresses etc. I have disabled UNALIGN_TRP bit and it works now fine.

Best
Martin

[Terry Guo (terry.guo)]

You set UNALIGN_TRP in CCR and then you got UsageFault for unaligned address lately. I think this is correct behavior. About the strange behavior of gdb when you in UsageFault handler, it maybe a bug of gdb, but not related to memcpy function itself in my opinion.

[Joey Ye (jinyun-ye)]

Martin,

This is expected bahavior, as the memcpy in libc was built assuming UNALIGN_TRP unset, which is most of cases.

To make it work in your case, you can either rebuild library with -mno-unaligned-access, or just use libc for armv6-m. To use armv6-m lib you can specify -march=armv6-m in linker option.

Thanks,
Joey