Bug#915405: gcc-8: Spurious stringop-overflow error when compiling with -O2

[François Trahay <francois.trahay@telecom-sudparis.eu>]

On 05/12/2018 08:29, Matthias Klose wrote:
> Control: tags -1 + moreinfo
>
> On 03.12.18 16:06, François Trahay wrote:
> > Package: gcc-8
> > Version: 8.2.0-9
> > Severity: normal
> >
> > Dear Maintainer,
> > I have a piece of code in a CMake project that fails to compile when building
> > in RelWithDebInfo mode.
> > The problem seems to come from gcc-8 when both -Wall and -O2 options are set:
> >
> > char* copy_string(const char* str) {
> >    int len = strlen(str)+1;
> >    char* dest = malloc(sizeof(char)*len);
> >    if(dest) {
> >      strncpy(dest, str, len);
> >    }
> >    return dest;
> > }
> >
> > gcc reports the following error:
> >
> > test_strncpy.c:7:3: error: ‘strncpy’ specified bound depends on the length of
> > the source argument [-Werror=stringop-overflow=]
> >     strncpy(dest, str, len);
> >     ^~~~~~~~~~~~~~~~~~~~~~~
> > test_strncpy.c:5:13: note: length computed here
> >     int len = strlen(str)+1;
> >               ^~~~~~~~~~~
> > cc1: all warnings being treated as errors
> >
> >
> > In this case, there is no problem in the source code, but -Werror=stringop-
> > overflow still reports an error.
> >
> > Gcc only reports an error when -O2 and -Werror=stringop-overflow (which is
> > enabled by -Wall) are set. Replacing strncpy with memcpy also fixes the
> > problem.
> No, -Werror* is never turned on by -Wall.  Please provide a self-contained
> example and the command line options used to show the warning/error.
My bad, -Wall prints a warning when used with -O2. If -Werror and -O2 
are set, this shows an error.

Here's a self-contained example program.

gcc reports a warning when running:

$ gcc -c test_strncpy.c  -Wall -O2


gcc reports an error when running

$ gcc -c test_strncpy.c  -Werror -O2


The error/warning to do show when compiled with -O1.

#include <string.h>
#include <stdlib.h>

char* copy_string(const char* str) {
  int len = strlen(str);
  char* dest = malloc(sizeof(char)*len+1);
  strncpy(dest, str, len+1);
  return dest;
}