Re: gdb 7.12.1 does not set zero bit in ARM CPSR register after cmp command during cross debuging

On 2018-04-10 05:58, [email protected] wrote:
> Hello there,
> 
> I have a really strange error and I have no idea how this could
> happen. Maybe you can help me. I am trying to cross debug a binary on
> an ARM926ej-s running Linux 2.6.35 on top of a microvisor (OKL4).
> 
> My testing code looks as follows:
> 
> #include <stdio.h>
> 
> void printout(unsigned int i){
>    if(i == 1){
>      printf("1\n");
>    }else{
>      printf("0\n");
>    }
> }
> 
> int main( int argc, char *argv[] ){
>    unsigned int i=1;
>    printout(i);
>    return 0;
> }
> 
> 
> If I run the binary without gdb on target I get "1" as output. But if
> I run it with gdb, using 'single step' command, I get "0" as output.
> Dissassembly of the if statement looks as follows:
> 
> │0x10418 <printout+16> ldr r3, [r11, #-8] │
> │0x1041c <printout+20> cmp r3, #1 │
> │0x10420 <printout+24> bne 0x10430 <printout+40>
> 
> content of r3 is #1. After executing 0x1041c CPSR is still 0x0 but
> should be 0x40000000 (Zero bit equals 1). Therefore bne does not jump
> to the right address.
> 
> If I step over "printout" function (using next step instruction) I get
> "1" as output. I tried changing the CPSR register value to 0x40000000
> after reaching 0x10420 and now it prints "1". So it looks like while
> single stepping gdb is overwriting the CPSR register value.
> 
> I tried using thumb and arm mode (compile with -marm and -mthumb) but
> this made no difference. I tried different toolchains with the same
> result (toolchains build with crosstools-ng 1.23.0 once for armv5t
> architecture and once for arm296ej-s cpu) but still get the same
> result.
> 
> I also tried the following compiler options:
> 
> * cross-gcc -g -O0 main.c -o main
> * cross-gcc -ggdb -Oo main.c -o main
> * cross-gc -Ogdb main.c -o main
> 
> Unfortunately all with the same result.
> 
> gdbserver is 7.12.1 (rootfs build via buildroot using the self-build
> toolchain). I also tried different toolchains from Linaro project, all
> with the same strange result.
> 
> Any idea how this could happen?

Hi,

Just to let you know that I tried to reproduce it but couldn't.  I tried 
on a Raspberry Pi (3?), with the GDB 8.1 that comes with Arch Linux ARM.

Simon