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SubjectRe: Bug Report: Dereferencing a bad pointer
Richard,

Your explanation shows why the process is not killed with a SIGSEGV, but
it don't points out why the process hangs !

"Richard B. Johnson" wrote:
>
> On Thu, 8 Nov 2001, David Chandler wrote:
>
> > Benjamin LaHaise wrote:
> >
> > > On Wed, Nov 07, 2001 at 06:23:13PM -0500, David Chandler wrote:
> > > > The following one-line C program, when compiled by gcc 2.96 without
> > > > optimization, should produce a SIGSEGV segmentation fault (on a machine
> > > > with 3 or less gigabytes of virtual memory, at least):
> > > >
> > > > int main() { int k = *(int *)0xc0000000; }
> > > >
>
> This may not necessarily produce a seg-fault! If this virtual
> address is mapped within the current process (.bss .stack, etc.),
> It's perfectly all right to write to it although you probably
> broke malloc() by doing it. The actual value of the number in
> the pointer depends upon PAGE_OFFSET and other kernel variables.
> If you change the kernel, this number may change. It has nothing
> to do with the size of virtual address space, really.
>
> Script started on Thu Nov 8 10:44:03 2001
> # cat >xxx.c
> #include <stdio.h>
> int bss;
> int data = 0x100;
> const char cons[]="X";
>
> main()
> {
> int stack;
>
> printf("main() = %p\n", main);
> printf("stack = %p\n", &stack);
> printf("const = %p\n", cons);
> printf(" data = %p\n", &data);
> printf(" bss = %p\n", &bss);
> return 0;
>
> }
>
> # gcc -o xxx xxx.c
> # ./xxx
> main() = 0x80484cc
> stack = 0xbffff6fc
> const = 0x8048584
> data = 0x80495d4
> bss = 0x80496b8
> # exit
> exit
>
> Script done on Thu Nov 8 10:44:27 2001
>
> All this stuff you "own". You can write to most all of it because
> the kernel has allocated it for you. Whether or not 'const' is
> really read-only is "implementation dependent".
>
> In your case, it looks as though you scribbled over the top of
> your user stack, in some harmless place.
>
> You cannot presume that a program that doesn't seg-fault is
> memory-error free. Protection is in pages, not bytes, and you
> already own a lot of address-space that you may think that
> you don't. FYI, if you allocate a lot of memory using malloc(),
> it sets the break address to acquire more memory. Then if you
> free that memory, it does not necessarily give back the memory.
>
> You may be able to write to freed memory without a seg-fault.
> However, subsequent calls to malloc() may fail because you have
> ticked-off malloc() and it's gonna get even.
>
> Cheers,
> Dick Johnson
>
> Penguin : Linux version 2.4.1 on an i686 machine (799.53 BogoMips).
>
> I was going to compile a list of innovations that could be
> attributed to Microsoft. Once I realized that Ctrl-Alt-Del
> was handled in the BIOS, I found that there aren't any.
>
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