Date: Tue, 13 Mar 2007 14:04:41 +0100 From: Hartmut Brandt <Hartmut.Brandt@dlr.de> To: Eygene Ryabinkin <rea-fbsd@codelabs.ru> Cc: current@freebsd.org Subject: Re: Bad gcc -O optimization cause core dump. What to do? Message-ID: <45F6A169.9050008@dlr.de> In-Reply-To: <20070313123717.GU58523@codelabs.ru> References: <20070313121106.GA96293@nagual.pp.ru> <20070313123717.GU58523@codelabs.ru>
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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@codelabs : 15:31:43 : ~/xlam
> $ cat 1.s
> .file "1.c"
> .text
> .p2align 2,,3
> .globl main
> .type main, @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, @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
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