Date: Fri, 16 Jan 2009 10:33:15 -0800 (PST) From: Nate Eldredge <neldredge@math.ucsd.edu> To: Garrett Cooper <yanefbsd@gmail.com> Cc: Thierry Herbelot <thierry.herbelot@free.fr>, freebsd-hackers@freebsd.org, Christoph Mallon <christoph.mallon@gmx.de> Subject: Re: Confused by segfault with legitimate call to strerror(3) on amd64 / sysctl (3) setting `odd' errno's Message-ID: <Pine.GSO.4.64.0901161023030.18030@zeno.ucsd.edu> In-Reply-To: <7d6fde3d0901161006r79f0cac4yf80c9c5079152b87@mail.gmail.com> References: <7d6fde3d0901160041n55466290l55f737d274a40895@mail.gmail.com> <49705FA2.2020605@gmx.de> <7d6fde3d0901160235o6aa1f096q11c5096b70f3577@mail.gmail.com> <200901161152.53478.thierry.herbelot@free.fr> <7d6fde3d0901161006r79f0cac4yf80c9c5079152b87@mail.gmail.com>
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On Fri, 16 Jan 2009, Garrett Cooper wrote:
> On Fri, Jan 16, 2009 at 2:52 AM, Thierry Herbelot
> <thierry.herbelot@free.fr> wrote:
>> Le Friday 16 January 2009, Garrett Cooper a =E9crit :
>>> On Fri, Jan 16, 2009 at 2:21 AM, Christoph Mallon
>>>
>>> #include <errno.h>
>>> #include <stdio.h>
>>> #include <sys/stat.h>
>>>
>>> int
>>> main()
>>> {
>>>
>>> struct stat sb;
>>>
>>> int o_errno;
>>>
>>> if (stat("/some/file/that/doesn't/exist", &sb) !=3D 0) {
>>> o_errno =3D errno;
>>> printf("Errno: %d\n", errno);
>>> printf("%s\n", strerror(o_errno));
>>> }
>>>
>>> return 0;
>>>
>>> }
>>>
>> with this, it's better on an amd64/ RELENG_7 machine :
>>
>> % diff -ub badfile.c.ori badfile.c
>> --- badfile.c.ori 2009-01-16 11:49:44.778991057 +0100
>> +++ badfile.c 2009-01-16 11:49:03.470465677 +0100
>> @@ -1,6 +1,7 @@
>> #include <errno.h>
>> #include <stdio.h>
>> #include <sys/stat.h>
>> +#include <string.h>
>>
>> int
>> main()
>>
>> Cheers
>>
>> TfH
>
> That's hilarious -- why does it pass though without issue on x86 though?
> -Garrett
As pointed out, when you don't have a declaration for strerror, it's=20
implicitly assumed to return `int'. This "feature" was widely used in the=
=20
early days of C and so continues to be accepted by compilers, and gcc by=20
default doesn't warn about it.
On x86, int and char * are the same size. So even though the compiler=20
thinks strerror is returning an int which is being passed to printf, the=20
code it generates is the same as for a char *. On amd64, int is 32 bits=20
but char * is 64. When the compiler thinks it's using int, it only keeps=
=20
track of the lower 32 bits, and the upper 32 bits get zeroed. So the=20
pointer that printf receives has had its upper 32 bits zeroed, and no=20
longer points where it should. Hence segfault.
Since running on amd64 I've seen a lot of bugs where people carelessly=20
assume (perhaps without noticing) that ints and pointers are practically=20
interchangeable, which works on x86 and the like but breaks on amd64.=20
Variadic functions are special offenders because the compiler can't do=20
much type checking.
Pop quiz: which of the following statements is correct?
#include <stdlib.h>
#include <unistd.h>
execl("/bin/sh", "/bin/sh", 0);
execl("/bin/sh", "/bin/sh", NULL);
--=20
Nate Eldredge
neldredge@math.ucsd.edu
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