Re: Bad gcc -O optimization cause core dump. What to do?

Eygene Ryabinkin wrote:
> Andrey, good day.
>
>   
>> It calls "puts(NULL)" with core dump.
>> It means "printf("%s\n", NULL)" is overoptimized.
>> BTW, things like "printf("1%s\n", NULL)" are not overoptimized.
>>     
>
> Yes, it is in the gcc/builtins.c::expand_builtin_printf(). Currently
> it only handles "%s" and "%c".
>
>   
>> Any ideas? Is it right or needs to be fixed?
>>     
>
> It is definitely not right, since it produces the bad code.
> And there are no compilation-time checks that can say for
> sure will the argument for the "%s" be NULL:
> -----
> $ cat 1.c
> #include <stdio.h>
>
> int main(void)
> {
>         void    *ptr = NULL;
>         func(ptr);
> }
>
> int func(void *ptr)
> {
>         printf("%s\n", ptr);
> }
> :: rea_at_codelabs : 15:31:43 : ~/xlam
> $ cat 1.s
>         .file   "1.c"
>         .text
>         .p2align 2,,3
> .globl main
>         .type   main, _at_function
> main:
>         pushl   %ebp
>         movl    %esp, %ebp
>         subl    $8, %esp
>         andl    $-16, %esp
>         subl    $28, %esp
>         pushl   $0
>         call    func
>         leave
>         ret
>         .size   main, .-main
>         .p2align 2,,3
> .globl func
>         .type   func, _at_function
> func:
>         pushl   %ebp
>         movl    %esp, %ebp
>         subl    $20, %esp
>         pushl   8(%ebp)
>         call    puts
>         leave
>         ret
>         .size   func, .-func
> -----
> The possible way to proceed with this optimization is to have the
> 'puts', but to enable runtime check for the NULL value.
>
> I see the following definition for the fn_puts in builtins.def:
> -----
> DEF_EXT_LIB_BUILTIN    (BUILT_IN_PUTS_UNLOCKED, "puts_unlocked", BT_FN_INT_CONST_STRING, ATTR_NOTHROW_NONNULL_1)
> -----
> The ATTR_NOTHROW_NONNULL_1 makes me think that not all is lost and something
> can be done with the NULL pointer. I am not very familiar with gcc
> internals, but I will try to see if something can be changed.
>   
The behaviour of gcc is correct. Passing NULL to the %s of printf() 
produces undefined behaviour. The compiler is free to do what ever it 
wants including burning your house. Depending on printf to print (null) 
or whatever is just bad programming.

harti