Messages in this thread | | | Date | Fri, 18 Dec 2009 18:27:47 +0100 | From | Oleg Nesterov <> | Subject | Re: x86: do_debug && PTRACE_SINGLESTEP broken by 08d68323d1f0c34452e614263b212ca556dae47f |
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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; }
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