[Bug target/85203] New: cmse_nonsecure_caller intrinsic returns incorrect results

[thopre01 at gcc dot gnu.org]

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=85203

            Bug ID: 85203
           Summary: cmse_nonsecure_caller intrinsic returns incorrect
                    results
           Product: gcc
           Version: 7.3.1
            Status: UNCONFIRMED
          Keywords: wrong-code
          Severity: normal
          Priority: P3
         Component: target
          Assignee: thopre01 at gcc dot gnu.org
          Reporter: thopre01 at gcc dot gnu.org
  Target Milestone: ---
            Target: arm-none-eabi

Hi,

The cmse_nonsecure_caller intrinsic for Armv8-M Baseline and Mainline
architecture returns true in almost all cases, ie. compiling and running the
following test with arm-none-eabi-gcc -Os -mcmse -march=armv8-m.main will
return an error:

#include <arm_cmse.h>

int
foo (void)
{
  return cmse_nonsecure_caller ();
}

int
main (void)
{
  /* Return success (0) if main is secure, ie if cmse_nonsecure_caller/foo
     returns false (0).  */
  return foo ();
}

Looking at the implementation of the associated __builtin_cmse_nonsecure_caller
in gcc/config/arm/arm-builtins.c we can see why:

* it performs an add instead of an and to get the lsb of LR
* it does not negate the value to return true when the lsb is 0

This means that except for 0xffffffff it will always return true. I'm currently
testing a patch as I write these lines.