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Load Alignment Exception with Arm Compiler 6.13

Dave Boose

I am working on a low level trusted bootloader (written in C) running on an ARM Cortex-A53 at EL3.  The MMU is not setup and the bootloader is running out of ROM.  We are linking in the CC712 TEE-SBROM library, and running into alignment exceptions when performing a load.  Prior to Arm compiler 6.13, I was able to force strict alignment using the -mno-unaligned-access.  The Alignment Data Abort Exception has returned with the 6.13.   More specifically, the exception happens when trying to initialize a local function variable:

char header[] = CC_SB_CERT_UTIL_PEM_HEADER; // value of pem header ends with "0x0a" == line feed
char footer[] = CC_SB_CERT_UTIL_PEM_FOOTER; // value of pem footer ends with "0x0a" == line feed


The constant string is stored at a 16-bit aligned address.  When it tries to load the address, it generates an alignment exception.

Here's a snippet of the assembly:

# the address assigned (2-byte aligned)
EL3:0x000000000000E640 : ADRP     x8,{pc}+0x5000 ; 0x13640
EL3:0x000000000000E644 : ADD      x8,x8,#0x50a

# * * *
# the load command
EL3:0x000000000000E668 : LDR      x12,[x8,#0]


Using ARM Compiler 6.12, the beginning of the string is placed on a 64-bit boundary.  Any suggestions on how to do this on 6.13?  

here are the compiler flags:

--target=aarch64-arm-none-eabi -mcpu=cortex-a53 -DARM_V8A -D__A53__ -D__64_BIT__ -DNO_MMU -c -DLITTLE__ENDIAN -DHASLONGLONG -Wall -Werror -Wno-self-assign -mno-unaligned-access -m -D__arm64__ -O0 -DDEBUG -g3 



And the Exception:

ESR_EL3: 96000021  FAR_EL3: 1334a, Stack Ptr = 34077024 
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  • 0

    Hi Dave,

    I built an admittedly trivial example, but cannot replicate this behavior:

    #include <stdio.h>
#include <string.h>

volatile char *hello   = "Hello World";
volatile char *goodbye = "Goodbye now";

int main()
{
  printf("The string is: %s\n", hello);
  memcpy(hello, goodbye, 12);
  printf("The string is: %s\n", hello);
  return 0;
}

    Testing with the Cortex-A53 FVP, and setting $SCTLR_EL3.A = 1 to catch unaligned accesses, it does indeed trigger when -mno-unaligned-access is not used.

    I fear that there is a deeper issue here, and you are perhaps simply lucky that the 6.12 build happens to fall on a boundary?

Reply
  • 0

    Hi Dave,

    I built an admittedly trivial example, but cannot replicate this behavior:

    #include <stdio.h>
#include <string.h>

volatile char *hello   = "Hello World";
volatile char *goodbye = "Goodbye now";

int main()
{
  printf("The string is: %s\n", hello);
  memcpy(hello, goodbye, 12);
  printf("The string is: %s\n", hello);
  return 0;
}

    Testing with the Cortex-A53 FVP, and setting $SCTLR_EL3.A = 1 to catch unaligned accesses, it does indeed trigger when -mno-unaligned-access is not used.

    I fear that there is a deeper issue here, and you are perhaps simply lucky that the 6.12 build happens to fall on a boundary?

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