Re: segfault in sysdep/i386/backtrace.h

[Marco Trudel <mtrudel at gmx dot ch>]

Marco Trudel wrote:
> Marco Trudel wrote:
> > Andrew Haley wrote:
> > > Marco Trudel writes:
> > >  >  >  > The segfault happens on reading scan_bytes[x]. I assume that 
> > > there is no  > "pushl %ebp; movl %esp, %ebp" function prologue in 
> > > certain cases and  > thus we go reading protected areas below the 
> > > function.
> > >
> > > Why don't you have a look, and tell us what is there?
> >
> > Because I don't know how and what these hex values mean (how to 
> > interpret them) when doing the backtrace...
>
> Ok, learnt it...
> The problem is that the code assumes that there is always a "pushl %ebp; 
> movl %esp, %ebp" function prologue. But, from [1]: "Note that many 
> compilers can optimize these standard sequences away when not needed 
> (often called "no stackframe generation")".
>
> So, when turning on maximum optimization in microsoft visual c++, there 
> are no longer "pushl %ebp; movl %esp, %ebp" intros and thus we run into 
> trouble (tried it). I don't know if GCC can do that too... Can it?
> I checked a couple of dll's (awt.dll, swt.dll, aBluetoothLib.dll) I had 
> around and they all miss the intro in at least a couple of functions.

If someone is interested:

JNIEXPORT void JNICALL Java_JniTest_fooBar(JNIEnv *env, jclass c)
{
	  printf("hello\n");
}

Turns, with mvc no optimization, into:

10001039:	55                   	push   %ebp
1000103a:	8b ec                	mov    %esp,%ebp
1000103c:	68 60 70 00 10       	push   $0x10007060
10001041:	e8 07 00 00 00       	call   0x1000104d
10001046:	83 c4 04             	add    $0x4,%esp
10001049:	5d                   	pop    %ebp
1000104a:	c2 08 00             	ret    $0x8

And, with mvc with optimiation, into:

10001030:	68 60 70 00 10       	push   $0x10007060
10001035:	e8 06 00 00 00       	call   0x10001040
1000103a:	59                   	pop    %ecx
1000103b:	c2 08 00             	ret    $0x8


Marco



> So, questions:
> - Is this a sjlj-exception only problem? Can DW EH do that better?
> - Is there another way to reliably recognize the start of a function? I 
> assume this only affects native libs since Java compiled apps will 
> always have the intro?!
>
>
> Marco
>
> [1] 
> http://en.wikipedia.org/wiki/X86_calling_conventions#Standard_Exit_and_Entry_Sequences_for_C_Code