From mboxrd@z Thu Jan 1 00:00:00 1970 Return-Path: Received: (qmail 5787 invoked by alias); 20 Oct 2011 14:48:29 -0000 Received: (qmail 5774 invoked by uid 22791); 20 Oct 2011 14:48:27 -0000 X-SWARE-Spam-Status: No, hits=-1.7 required=5.0 tests=AWL,BAYES_00,DKIM_SIGNED,DKIM_VALID,DKIM_VALID_AU,FREEMAIL_ENVFROM_END_DIGIT,FREEMAIL_FROM,RCVD_IN_DNSWL_LOW,TW_GJ,TW_NX,TW_TJ X-Spam-Check-By: sourceware.org Received: from mail-vx0-f175.google.com (HELO mail-vx0-f175.google.com) (209.85.220.175) by sourceware.org (qpsmtpd/0.43rc1) with ESMTP; Thu, 20 Oct 2011 14:48:12 +0000 Received: by vcbf1 with SMTP id f1so2882555vcb.20 for ; Thu, 20 Oct 2011 07:48:11 -0700 (PDT) MIME-Version: 1.0 Received: by 10.220.74.20 with SMTP id s20mr826093vcj.13.1319122090282; Thu, 20 Oct 2011 07:48:10 -0700 (PDT) Received: by 10.220.181.131 with HTTP; Thu, 20 Oct 2011 07:48:10 -0700 (PDT) In-Reply-To: References: <4E9BB180.6080506@mc.net> <4E9C0497.2000605@siriusit.co.uk> <4E9C3703.3040109@mc.net> <4E9C645A.5060200@twiddle.net> <4E9C9C08.20001@mc.net> <4E9CAACE.4070804@mc.net> <4E9F3BB4.3050604@mc.net> Date: Thu, 20 Oct 2011 15:01:00 -0000 Message-ID: Subject: Re: gcc auto-omit-frame-pointer vs msvc longjmp From: Kai Tietz To: xunxun Cc: Bob Breuer , "gcc@gcc.gnu.org" , Richard Henderson , qemu-devel , Mark Cave-Ayland Content-Type: text/plain; charset=ISO-8859-1 Content-Transfer-Encoding: quoted-printable X-IsSubscribed: yes Mailing-List: contact gcc-help@gcc.gnu.org; run by ezmlm Precedence: bulk List-Id: List-Archive: List-Post: List-Help: Sender: gcc-owner@gcc.gnu.org X-SW-Source: 2011-10/txt/msg00354.txt.bz2 2011/10/20 xunxun : > Hi, all > > I think this issue causes the gdb crash on XP. > You can see the thread: http://sourceware.org/ml/gdb/2011-10/msg00056.html > > My many friends and I can reproduce this crash issue, but no problem on W= in7. > > On Thu, Oct 20, 2011 at 5:05 AM, Bob Breuer wrote: >> Kai Tietz wrote: >>> 2011/10/18 Bob Breuer : >>>> Kai Tietz wrote: >>>>> 2011/10/17 Bob Breuer : >>>>>> Richard Henderson wrote: >>>>>>> On 10/17/2011 07:09 AM, Bob Breuer wrote: >>>>>>>> Google finds a mention of longjmp failing with -fomit-frame-pointe= r: >>>>>>>> http://lua-users.org/lists/lua-l/2005-02/msg00158.html >>>>>>>> >>>>>>>> Looks like gcc 4.6 turns on -fomit-frame-pointer by default. >>>>>>> Hmm. =A0This is the first I've heard of a longjmp implementation >>>>>>> failing without a frame pointer. =A0Presumably this is with the >>>>>>> mingw i.e. msvc libc? >>>>>> Yeah, mingw from www.mingw.org which I believe uses msvcrt.dll, pack= age >>>>>> 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 >>>>>> #include >>>>>> >>>>>> jmp_buf env; >>>>>> >>>>>> int test(void) >>>>>> { >>>>>> =A0int i; >>>>>> >>>>>> =A0asm("xor %%ebp,%%ebp" ::: "ebp"); >>>>>> >>>>>> =A0i =3D setjmp(env); >>>>>> =A0printf("i =3D %d\n", i); >>>>>> >>>>>> =A0if (i =3D=3D 0) >>>>>> =A0 =A0longjmp(env, 2); >>>>>> >>>>>> =A0return i; >>>>>> } >>>>>> >>>>>> int main(void) >>>>>> { >>>>>> =A0return test(); >>>>>> } >>>>>> >>>>>> Remove the asm statement to make it not crash. =A0Obviously 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. =A0If 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? =A0With _setjmp3, it works >>>> without the asm, but still crashes if I zero out ebp before the setjmp. >>>> =A0Aren'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. =A0setjmp() has an hidden arguement, which is >>> passed on x86 on stack. =A0By not passing this required argument, setjmp >>> will take a random-value from stack. =A0In your case 'i'. =A0btw 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. =A0With my suggested pattern, I don't see a crash for your >>> provide test-code with, or without zero-ing ebp. >> >> >> We probably have a difference in build or run environment. =A0I've >> double-checked with another machine and can get the same crash in >> longjmp when running the test executable on both WinXP and Win2k, but >> not on Win7. =A0So it looks like Microsoft may have changed this "featur= e" >> somewhere between WinXP and Win7. >> >> The msvcrt implementation of longjmp (or at least the one I'm looking >> at) does a ebp based access using the saved value of ebp. =A0Here's the >> relevant disassembly of longjmp: >> >> 0x7801e6f3 in longjmpex () from C:\WINNT\system32\msvcrt.dll >> (gdb) disas >> Dump of assembler code for function longjmpex: >> =A0 0x7801e6ef <+0>: =A0 =A0 mov =A0 =A00x4(%esp),%ebx >> =3D> 0x7801e6f3 <+4>: =A0 =A0 mov =A0 =A0(%ebx),%ebp >> ... >> =A0 0x7801e73d <+78>: =A0 =A0call =A0 0x7800bd5e >> ... >> =A0 0x7800bd5e <+56>: =A0 =A0push =A0 %ebx >> =A0 0x7800bd5f <+57>: =A0 =A0push =A0 %ecx >> =A0 0x7800bd60 <+58>: =A0 =A0mov =A0 =A0$0x7803dc64,%ebx >> =3D> 0x7800bd65 <+63>: =A0 =A0mov =A0 =A00x8(%ebp),%ecx >> >> It crashes on the access of 0x8(%ebp). =A0Those are the only 2 places >> where this version of longjmp touches ebp. =A0Is it possible to force a >> stackframe by just adding a suitable attribute to either the setjmp >> function prototype, or the function which calls setjmp? >> >> Bob >> > > > > -- > Best Regards, > xunxun This now makes sense. I use here Vista 64-bit, and Win7 64-bit and I didn't found the issue. But well, it is indeed related to different msvcrt-version. So there might be some need to have for a function using setjmp the frame-pointer enabled. I can confirm this by an older msvcrt.dll version on my 64-bit box, too. So bug can be re-opened. Thanks, Kai