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From: Alexandre Martins <alexandre.martins@netasq.com>
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Cc: Konstantin Belousov <kostikbel@gmail.com>
Subject: Re: Double free() in libc or gdb ?
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On Monday 12 March 2012 18:55:55 Konstantin Belousov wrote:
> On Mon, Mar 12, 2012 at 05:50:33PM +0100, Alexandre Martins wrote:
> > Dear all,
> >=20
> > I'm currently having some trouble with the dynamic loader.
> >=20
> > I have the libc compilled with "MALLOC_DEBUG" flag to detect double fre=
e.
> > When i run this piece of code (attached file) thought GDB, i have this
> > assertion :
> >=20
> > Assertion failed: ((run->regs_mask[elm] & (1U << bit)) =3D=3D 0), funct=
ion
> > arena_run_reg_dalloc, file /usr/src/lib/libc/stdlib/malloc.c, line 2543.
> >=20
> > But when i run the same binary without GDB, no assert.
> >=20
> > I'm very confused. Can you help me to debug that ?
>=20
> There is no attachment.  Put the source somewhere on web.

Sorry, I'll past code here :

first.c:
____________________
#include <stdio.h>

void print_name(void)
{
        printf("I'm " __FILE__ " at line %d\n", __LINE__);
}
____________________

second.c
____________________
#include <stdio.h>

void second_name(void)
{
        printf("I'm " __FILE__ " at line %d\n", __LINE__);
}

void print_name(void)
{
        printf("I'm " __FILE__ " at line %d\n", __LINE__);
}
____________________

main.c
____________________
#include <stdio.h>
#include <dlfcn.h>

extern void print_name(void);

int main(int argc, char *argv[])
{
        void (*second_name)(void);
        void *handle;
        int count =3D 0;

        while(42)
        {
                print_name();
                handle =3D dlopen("second.so", RTLD_NOW);
                print_name();
                if (handle !=3D NULL)
                {
                        second_name =3D dlsym(handle, "second_name");
                        if (second_name !=3D NULL)
                        {
                                printf("second : ");
                                second_name();
                        }
                        dlclose(handle);
                }
                else
                        fprintf(stderr, "Error : %s\n", dlerror());
        }
}
____________________

Compilation and execution :

gcc -shared -O0 -g second.c -o second.so
gcc -shared -O0 -g first.c -o libfirst.so
gcc -O0 -g toto.c -lfirst -L. -o test
export LD_LIBRARY_PATH=3D$PWD
gdb ./test


Thank you for your help
=2D-=20
Alexandre Martins
NETASQ -- We secure IT


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