Re: [PATCH] cc-can-link.sh: check for linking capability in a more robust way

[Masahiro Yamada <masahiroy@xxxxxxxxxx>]

On Sat, Jul 3, 2021 at 8:21 PM Alexey Dobriyan <adobriyan@xxxxxxxxx> wrote:
>
> Compiling "printf("");" doesn't necessarily check for linking capability
> as printf can be optimised for constants strings even at -O0:
>
>         0000000000401106 <main>:
>           401106:       push   rbp
>           401107:       mov    rbp,rsp
>           40110a:       mov    eax,0x0
>           40110f:       pop    rbp
>           401110:       ret
>
> Pass something from the command line to disable optimisations:
>
>         0000000000401126 <main>:
>           401126:       push   rbp
>           401127:       mov    rbp,rsp
>           40112a:       sub    rsp,0x10
>           40112e:       mov    DWORD PTR [rbp-0x4],edi
>           401131:       mov    QWORD PTR [rbp-0x10],rsi
>           401135:       mov    rax,QWORD PTR [rbp-0x10]
>           401139:       add    rax,0x8
>           40113d:       mov    rax,QWORD PTR [rax]
>           401140:       mov    rdi,rax
>           401143:       mov    eax,0x0
>           401148:  ***  call   401030 <printf@plt>
>           40114d:       mov    eax,0x0
>           401152:       leave
>           401153:       ret
>
> Signed-off-by: Alexey Dobriyan <adobriyan@xxxxxxxxx>
> ---
>
>  scripts/cc-can-link.sh |    4 ++--
>  1 file changed, 2 insertions(+), 2 deletions(-)
>
> --- a/scripts/cc-can-link.sh
> +++ b/scripts/cc-can-link.sh
> @@ -3,9 +3,9 @@
>
>  cat << "END" | $@ -x c - -o /dev/null >/dev/null 2>&1
>  #include <stdio.h>
> -int main(void)
> +int main(int argc, char *argv[])
>  {
> -       printf("");
> +       printf(argv[1]);
>         return 0;
>  }
>  END

Ah, right.

But, we should not merge a bad coding example.

argv[1] may contain '%' format string, and
recent GCC versions warn about it.



$ cat test.c
#include <stdio.h>

int main(int argc, char *argv[])
{
    printf(argv[1]);
    return 0;
}
$ gcc -c -o test.o  test.c
test.c: In function ‘main’:
test.c:5:5: warning: format not a string literal and no format
arguments [-Wformat-security]
    5 |     printf(argv[1]);
      |     ^~~~~~




I think replacing printf("") with printf("a")
(or any string you like)
is enough.



-- 
Best Regards
Masahiro Yamada