I'm trying to set the x86 breakpoint registers to trip on write data.
After they are set, nothing seems to happen when I trigger them.
It's possible I'm not setting them correctly, I tried putting the
virt_to_phys() value in them. And, I tried looking at what KPROBE puts
in them, but it looks like kprobe doesn't use them at all.
In 2.6.11, arch/i386/ has 5 places where it modifies the db regs:
do_debug() in traps.c (??)
do_signal() in signal.c (re-enable them)
fix_processor_context() in cpu.c (reload them)
__switch_to() in process.c (reload them)
cpu_init() in common.c (clears them)
Just FYI: sometime after 2.6.11, the macros get_debugreg() and
set_debugreg() were defined in include/asm-i386/processor.h and set to
be used in the 5 places above. In any case, the functionality seems the
same the above routines. (And the registers names corrected s/db/dr/g )
In any case, setting these registers never seems to do anything. No INT3
or INT1 (is it really supposed to generate an interrupt?) Perhaps I need
to have kgdb setup.
The closest I can get to making anything happen is if I set bit 13 of
DR7 (triggers on the next access of the breakpoint registers) then when
I insmod I get:
root@foxtrot:~/dbregtest# insmod ./dbregtest.ko
Trace/breakpoint trap
root@foxtrot:~/dbregtest#
If I set BR0 with the value from virt_to_phys() I don't get this
trace/breakpoint trap.
Enjoy,
Jeff
// Current documentation for these registers is in Vol 3 Section 15.2:
// Also note: EFLAGS BIT 16 (Resume) section 2.3 disables #DB exceptions
// http://developer.intel.com/design/pentium4/manuals/index_new.htm
static int __init db_reg_test(void)
{
u32 *i;
unsigned int phys_addr;
i = kmalloc( 0x1000, GFP_DMA );
printk("i == 0x%08X\n", (int) i);
phys_addr = virt_to_phys(i);
printk("virt_to_phys(i) == 0x%08X\n", (int) phys_addr);
__asm__ __volatile__( "movl %0, %%dr0\n" : : "r" (phys_addr) );
// clear out the DR6 status register
__asm__ __volatile__( "movl %0, %%dr6\n" : : "r" (0xFFFF0FF0) );
// Enable DR0 as a global breakpoint
__asm__ __volatile__( "movl %0, %%dr7\n" : : "r" (0x00030002) );
// Enables all four BR registers as global breakpoints
__asm__ __volatile__( "movl %0, %%dr7\n" : : "r" (0x333300AA) );
// shouldn't this trigger a breakpoint exception?
i[0] = 0xDEADBEEF;
kfree(i);
// this will correctly trigger a breakpoint
// __asm__ ( "movl %0, %%dr7\n" : : "r" (0x333320AA) );
// __asm__ ( "movl %0, %%dr7\n" : : "r" (0x333300AA) );
return 0;
}
module_init(db_reg_test);
root@foxtrot:~/dbregtest# tail /var/log/kern.log
Jul 5 09:01:16 localhost kernel: i == 0xC046E000
Jul 5 09:01:16 localhost kernel: virt_to_phys(i) == 0x0046E000
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