Re: x86: do_debug && PTRACE_SINGLESTEP broken by 08d68323d1f0c34452e614263b212ca556dae47f

Oleg Nesterov
2009-12-18T17:34:13Z

On 12/18, Frederic Weisbecker wrote:
>
> On Fri, Dec 18, 2009 at 01:56:50AM +0100, Oleg Nesterov wrote:
> > Hi.
> >
> > do_debug() is obviously wrong wrt PTRACE_SINGLESTEP/TIF_SINGLESTEP, no?
> >
> > Afaics this was broken by
> >
> > hw-breakpoints: modifying generic debug exception to use thread-specific debug registers
> > commit 08d68323d1f0c34452e614263b212ca556dae47f
> >
> > To verify, the "patch" below fixes the stepping for me, not sure what
> > is the proper fix...
> >
> > Oleg.
> >
> > --- arch/x86/kernel/traps.c~ 2009-12-18 00:20:49.000000000 +0100
> > +++ arch/x86/kernel/traps.c 2009-12-18 01:44:05.000000000 +0100
> > @@ -575,7 +575,7 @@ dotraplinkage void __kprobes do_debug(st
> > regs->flags &= ~X86_EFLAGS_TF;
> > }
> > si_code = get_si_code(tsk->thread.debugreg6);
> > - if (tsk->thread.debugreg6 & (DR_STEP | DR_TRAP_BITS))
> > +// if (tsk->thread.debugreg6 & (DR_STEP | DR_TRAP_BITS))
> > send_sigtrap(tsk, regs, error_code, si_code);
>
>
>
> But I don't understand why it is broken with the check.
> If we are in a singlestep exception, dr6 should have its
> DR_STEP bit set...
>
> Single stepping works well for me, after a quick check on
> gdb. How did you trigger the bug?

Please find the trivial test-case below. It hangs, because
PTRACE_SINGLESTEP doesn't trigger the trap.

(not sure this matters, but I did the testing under kvm)

Oleg.

#include <stdio.h>
#include <unistd.h>
#include <signal.h>
#include <sys/ptrace.h>
#include <sys/wait.h>
#include <assert.h>

int main(void)
{
int pid, status, i;

pid = fork();
if (!pid)
for (;;);

sleep(1);
assert(ptrace(PTRACE_ATTACH, pid, 0,0) == 0);

assert(pid == wait(&status));
assert(WIFSTOPPED(status));

for (i = 0; i < 10; ++i) {
assert(ptrace(PTRACE_SINGLESTEP, pid, 0,0) == 0);

printf("wait %d ...\n", i);
assert(pid == wait(&status));

assert(WIFSTOPPED(status) && WSTOPSIG(status) == SIGTRAP);
}

kill(pid, SIGKILL);
return 0;
}