From: Kai Tietz <ktietz70@googlemail.com>
To: Bob Breuer <breuerr@mc.net>
Cc: "gcc@gcc.gnu.org" <gcc@gcc.gnu.org>,
Mark Cave-Ayland <mark.cave-ayland@siriusit.co.uk>,
qemu-devel <qemu-devel@nongnu.org>,
Richard Henderson <rth@twiddle.net>
Subject: Re: [Qemu-devel] gcc auto-omit-frame-pointer vs msvc longjmp
Date: Tue, 18 Oct 2011 04:22:00 -0000 [thread overview]
Message-ID: <CAEwic4aADQUtZOmjAYVVrS1cBU=uRNjwAV49f+GtPtro664JDw@mail.gmail.com> (raw)
In-Reply-To: <4E9CAACE.4070804@mc.net>
2011/10/18 Bob Breuer <breuerr@mc.net>:
> Kai Tietz wrote:
>> 2011/10/17 Bob Breuer <breuerr@mc.net>:
>>> Richard Henderson wrote:
>>>> On 10/17/2011 07:09 AM, Bob Breuer wrote:
>>>>> I don't think this is a free/g_free issue. If I use the following
>>>>> patch, then I at least get the openbios messages:
>>>>>
>>>>> diff --git a/cpu-exec.c b/cpu-exec.c
>>>>> index a9fa608..dfbd6ea 100644
>>>>> --- a/cpu-exec.c
>>>>> +++ b/cpu-exec.c
>>>>> @@ -180,6 +180,7 @@ static void cpu_handle_debug_exception(CPUState
>>>>> /* main execution loop */
>>>>>
>>>>> volatile sig_atomic_t exit_request;
>>>>> +register void *ebp asm("ebp");
>>>>>
>>>>> int cpu_exec(CPUState *env)
>>>>> {
>>>>> @@ -233,6 +234,8 @@ int cpu_exec(CPUState *env)
>>>>>
>>>>> /* prepare setjmp context for exception handling */
>>>>> for(;;) {
>>>>> + int dummy = 0;
>>>>> + ebp = &dummy;
>>>> See if
>>>>
>>>> asm("" : : : "ebp");
>>>>
>>>> also solves the problem.
>>> No, that doesn't fix it.
>>>
>>>>> Google finds a mention of longjmp failing with -fomit-frame-pointer:
>>>>> http://lua-users.org/lists/lua-l/2005-02/msg00158.html
>>>>>
>>>>> Looks like gcc 4.6 turns on -fomit-frame-pointer by default.
>>>> Hmm. This is the first I've heard of a longjmp implementation
>>>> failing without a frame pointer. Presumably this is with the
>>>> mingw i.e. msvc libc?
>>> Yeah, mingw from www.mingw.org which I believe uses msvcrt.dll, package
>>> gcc-core-4.6.1-2-mingw32-bin.
>>>
>>>> This is something that could be worked around in gcc, I suppose.
>>>> We recognize longjmp for some things, we could force the use of
>>>> a frame pointer for msvc targets too.
>>>>
>>>> For now it might be best to simply force -fno-omit-frame-pointer
>>>> for mingw host in the configure script.
>>> Here's a testcase that crashes on the longjmp:
>>>
>>> #include <stdio.h>
>>> #include <setjmp.h>
>>>
>>> jmp_buf env;
>>>
>>> int test(void)
>>> {
>>> int i;
>>>
>>> asm("xor %%ebp,%%ebp" ::: "ebp");
>>>
>>> i = setjmp(env);
>>> printf("i = %d\n", i);
>>>
>>> if (i == 0)
>>> longjmp(env, 2);
>>>
>>> return i;
>>> }
>>>
>>> int main(void)
>>> {
>>> return test();
>>> }
>>>
>>> Remove the asm statement to make it not crash. Obviously with
>>> omit-frame-pointer, gcc can shove anything into ebp.
>>>
>>> Bob
>>
>> This crash isn'r related to ebp existing, or not. The issue is the
>> hidden argument of setjmp, which is missing. If you can try the
>> following at top of file after include section.
>>
>> #define setjmp(BUF) _setjmpex((BUF), NULL)
>> int __cdecl __attribute__ ((__nothrow__,__returns_twice__))
>> _setjmp3(jmp_buf _Buf, void *_Ctx);
>> ...
>
> Did you mean _setjmp3 instead of _setjmpex? With _setjmp3, it works
> without the asm, but still crashes if I zero out ebp before the setjmp.
> Aren't the function arguments on the stack anyway?
Yes, I mean _setjmp3 (pasto from headers and missed the second line
prototyping _setjmp3).
I repeat myself here. setjmp() has an hidden arguement, which is
passed on x86 on stack. By not passing this required argument, setjmp
will take a random-value from stack. In your case 'i'. btw if you
would pre-initialize 'i' with zero, I would assume you won't see a
crash, but anyway this is just by chance.
For this I suggest to use here _setjmp3 instead, as here
second-argument is documented as being present.
Btw I tested your code with i686-pc-mingw32 version 4.6.x and 4.7.x
gcc version. With my suggested pattern, I don't see a crash for your
provide test-code with, or without zero-ing ebp.
Kai
next prev parent reply other threads:[~2011-10-17 22:56 UTC|newest]
Thread overview: 21+ messages / expand[flat|nested] mbox.gz Atom feed top
[not found] <4E9BB180.6080506@mc.net>
[not found] ` <4E9C0497.2000605@siriusit.co.uk>
[not found] ` <4E9C3703.3040109@mc.net>
2011-10-17 21:20 ` Richard Henderson
2011-10-17 21:32 ` [Qemu-devel] " Blue Swirl
2011-10-17 22:23 ` Richard Henderson
2011-10-17 22:56 ` Bob Breuer
2011-10-18 2:06 ` Kai Tietz
2011-10-18 4:20 ` Bob Breuer
2011-10-18 4:22 ` Kai Tietz [this message]
2011-10-20 3:26 ` Bob Breuer
2011-10-20 4:18 ` Richard Henderson
2011-10-20 6:08 ` xunxun
2011-10-20 15:01 ` Kai Tietz
2011-10-20 17:12 ` Kai Tietz
2011-10-21 22:47 ` jojelino
2011-10-22 12:30 ` xunxun
2011-10-22 13:18 ` xunxun
2011-10-22 14:21 ` Kai Tietz
2011-10-22 22:59 ` asmwarrior
2011-10-24 20:05 ` [Qemu-devel] " Bob Breuer
2011-10-24 23:25 ` Kai Tietz
2011-10-25 17:00 ` Bob Breuer
2011-10-20 14:16 ` jojelino
Reply instructions:
You may reply publicly to this message via plain-text email
using any one of the following methods:
* Save the following mbox file, import it into your mail client,
and reply-to-all from there: mbox
Avoid top-posting and favor interleaved quoting:
https://en.wikipedia.org/wiki/Posting_style#Interleaved_style
* Reply using the --to, --cc, and --in-reply-to
switches of git-send-email(1):
git send-email \
--in-reply-to='CAEwic4aADQUtZOmjAYVVrS1cBU=uRNjwAV49f+GtPtro664JDw@mail.gmail.com' \
--to=ktietz70@googlemail.com \
--cc=breuerr@mc.net \
--cc=gcc@gcc.gnu.org \
--cc=mark.cave-ayland@siriusit.co.uk \
--cc=qemu-devel@nongnu.org \
--cc=rth@twiddle.net \
/path/to/YOUR_REPLY
https://kernel.org/pub/software/scm/git/docs/git-send-email.html
* If your mail client supports setting the In-Reply-To header
via mailto: links, try the mailto: link
Be sure your reply has a Subject: header at the top and a blank line
before the message body.
This is a public inbox, see mirroring instructions
for how to clone and mirror all data and code used for this inbox;
as well as URLs for read-only IMAP folder(s) and NNTP newsgroup(s).