Re: [RFC] Proposed manpage additions for ptrace(2)

From: Charles P. Wright
Date: Sat Mar 18 2006 - 15:35:00 EST

Next message: Jesper Juhl: "[PATCH] fix memory leak in mm/slab.c::alloc_kmemlist() (try #2)"
Previous message: Andrew Morton: "Re: [patch 1/2] Validate itimer timeval from userspace"
In reply to: Blaisorblade: "Re: [RFC] Proposed manpage additions for ptrace(2)"
Next in thread: Chuck Ebbert: "Re: [RFC] Proposed manpage additions for ptrace(2)"
Messages sorted by: [ date ] [ thread ] [ subject ] [ author ]

On Fri, 2006-03-17 at 19:46 +0100, Blaisorblade wrote:
> Side note - I'm attaching another (incomplete) patch for ptrace.2 with some
> addition - there is already something worth adding, though I only listed the
> 2.6-only ptrace options since I don't know what they do.
>
> On Thursday 16 March 2006 22:16, Charles P. Wright wrote:
> > On Thu, 2006-03-16 at 15:02 -0500, Daniel Jacobowitz wrote:
> > > > PTRACE_SYSEMU, PTRACE_SYSEMU_SINGLESTEP
> > > > For PTRACE_SYSEMU, continue and stop on entry to
> > > > the next syscall, which will not be executed. For
> > > > PTRACE_SYSEMU_SIN- GLESTEP, so the same but also singlestep if not a
> > > > syscall.
> > >
> > > I think this is right; I had nothing to do with these :-)
> >
> > I didn't have anything to do with it, but this description is correct
> > (if a bit confusing). I think that you should explicitly say (assuming
> > that Paolo does not have any objections):
>
> > PTRACE_SYSEMU only makes sense at a call's exit, not at entry.
>
> I must indeed check the detail but I'm almost totally sure that this point is
> totally wrong; conceptually, I understand what you mean,
> but mixing PTRACE_SYSEMU and PTRACE_SYSCALL has non-obvious results.
Yes, that is probably a better warning that what I was suggesting.

> Indeed, whether a syscall is performed or skipped is decided by the call which
> stops at syscall entry, not by the call which resumes the syscall.
>
> Actually, choosing what to do at resume time would make more sense, but for
> historical reasons this was overlooked (also see at the end of this email).
>
> I've described the exact semantics below, however I feel that mixing the calls
> does not make any sense - we implemented this support mainly for testing - on
> 2.4 hosts we could test UML performance this way, on 2.6 we couldn't and then
> "fixed" the API to be again this strange one.
>
> The exact semantics are these:
>
> remember that PTRACE_SYSEMU is called once per syscall; you attach the
> process, do a PTRACE_SYSEMU on it, and it'll stop at syscall #1 entry, and
> this syscall will not be executed; you look at calls param, perform what you
> want to do, and then resume it.
>
> If you resume it with PTRACE_SYSEMU, it'll stop at next syscall entry, as
> expected, and next syscall will not be executed.
>
> If you resume it with PTRACE_SYSCALL (which made sense only for debugging),
> the only thing which changes is that _next_ syscall will be executed
> normally; then after stopping at syscall #2 exit you can choose to resume
> with PTRACE_SYSEMU. You can do that even at syscall #2 entry, but you get the
> same result.
If you do the PTRACE_SYSEMU at the entry, then it seems that you

> However, I remember I answered to your request to fix this problem with some
> patches to test (I remember I was sure enough of their correctness, for what
> can be seen by code inspection), but got no answer and didn't finish
> anything. Lost the email or the interest?
Actually, I remember that you said that it wasn't very practical to try
and fix SYSEMU because UML already relies on its interface. You
suggested a "checked" version of the call, which I didn't actually lose
interest in. I've included a patch to 2.6.15 (based on your original
patch) that I've been using that adds "PTRACE_CHECKEMU", which I think
has more user-friendly semantics.

The PTRACE_CHECKEMU call makes the emulation decision after
ptrace_notify is called so that the tracing process can examine/update
registers before issuing PTRACE_CHECKEMU (to emulate the call) or
PTRACE_SYSCALL (to let the call go through).

I've also got a patch that allows you to execute the call, but skip the
return. This is useful when you are emulating a subset of calls, and
don't care about the return value of unemulated calls.

> I was also busy so I didn't test them myself (even because reading this code
> and following the exact states causes me a headache).
There is indeed some headache in here. I think particularly for SYSEMU,
because there is a large gap between calling it and the decision that is
made.

Charles

This patch adds support for checked PTRACE_SYSEMU, or PTRACE_CHECKEMU. The key
difference between SYSEMU and CHECKEMU is that the ptrace monitor can decide
whether to emulate a call after examining the registers rather than only before
examining the registers.

This is the interface improvement described here:
http://lkml.org/lkml/2005/7/30/131

Signed-off-by: Gopalan Sivathanu <gopalan@xxxxxxxxxxxxx>
Signed-off-by: Charles P. Wright <cwright@xxxxxxxxxxxxx>

diff -ur linux-2.6.15-vanilla/arch/i386/kernel/entry.S linux-2.6.15-checkemu/arch/i386/kernel/entry.S
--- linux-2.6.15-vanilla/arch/i386/kernel/entry.S 2006-01-02 22:21:10.000000000 -0500
+++ linux-2.6.15-checkemu/arch/i386/kernel/entry.S 2006-02-03 00:46:37.000000000 -0500
@@ -203,7 +203,7 @@
GET_THREAD_INFO(%ebp)

/* Note, _TIF_SECCOMP is bit number 8, and so it needs testw and not testb */
- testw $(_TIF_SYSCALL_EMU|_TIF_SYSCALL_TRACE|_TIF_SECCOMP|_TIF_SYSCALL_AUDIT),TI_flags(%ebp)
+ testw $(_TIF_SYSCALL_CHECKEMU|_TIF_SYSCALL_EMU|_TIF_SYSCALL_TRACE|_TIF_SECCOMP|_TIF_SYSCALL_AUDIT),TI_flags(%ebp)
jnz syscall_trace_entry
cmpl $(nr_syscalls), %eax
jae syscall_badsys
@@ -228,7 +228,7 @@
GET_THREAD_INFO(%ebp)
# system call tracing in operation / emulation
/* Note, _TIF_SECCOMP is bit number 8, and so it needs testw and not testb */
- testw $(_TIF_SYSCALL_EMU|_TIF_SYSCALL_TRACE|_TIF_SECCOMP|_TIF_SYSCALL_AUDIT),TI_flags(%ebp)
+ testw $(_TIF_SYSCALL_CHECKEMU|_TIF_SYSCALL_EMU|_TIF_SYSCALL_TRACE|_TIF_SECCOMP|_TIF_SYSCALL_AUDIT),TI_flags(%ebp)
jnz syscall_trace_entry
cmpl $(nr_syscalls), %eax
jae syscall_badsys
diff -ur linux-2.6.15-vanilla/arch/i386/kernel/ptrace.c linux-2.6.15-checkemu/arch/i386/kernel/ptrace.c
--- linux-2.6.15-vanilla/arch/i386/kernel/ptrace.c 2006-01-02 22:21:10.000000000 -0500
+++ linux-2.6.15-checkemu/arch/i386/kernel/ptrace.c 2006-02-03 00:46:40.000000000 -0500
@@ -475,6 +475,7 @@
break;

case PTRACE_SYSEMU: /* continue and stop at next syscall, which will not be executed */
+ case PTRACE_CHECKEMU: /* like SYSEMU, but allow per-call emulation decisions. */
case PTRACE_SYSCALL: /* continue and stop at next (return from) syscall */
case PTRACE_CONT: /* restart after signal. */
ret = -EIO;
@@ -483,12 +484,19 @@
if (request == PTRACE_SYSEMU) {
set_tsk_thread_flag(child, TIF_SYSCALL_EMU);
clear_tsk_thread_flag(child, TIF_SYSCALL_TRACE);
+ clear_tsk_thread_flag(child, TIF_SYSCALL_CHECKEMU);
} else if (request == PTRACE_SYSCALL) {
set_tsk_thread_flag(child, TIF_SYSCALL_TRACE);
clear_tsk_thread_flag(child, TIF_SYSCALL_EMU);
+ clear_tsk_thread_flag(child, TIF_SYSCALL_CHECKEMU);
+ } else if (request == PTRACE_CHECKEMU) {
+ set_tsk_thread_flag(child, TIF_SYSCALL_CHECKEMU);
+ clear_tsk_thread_flag(child, TIF_SYSCALL_TRACE);
+ clear_tsk_thread_flag(child, TIF_SYSCALL_EMU);
} else {
clear_tsk_thread_flag(child, TIF_SYSCALL_EMU);
clear_tsk_thread_flag(child, TIF_SYSCALL_TRACE);
+ clear_tsk_thread_flag(child, TIF_SYSCALL_CHECKEMU);
}
child->exit_code = data;
/* make sure the single step bit is not set. */
@@ -524,6 +532,7 @@
clear_tsk_thread_flag(child, TIF_SYSCALL_EMU);

clear_tsk_thread_flag(child, TIF_SYSCALL_TRACE);
+ clear_tsk_thread_flag(child, TIF_SYSCALL_CHECKEMU);
set_singlestep(child);
child->exit_code = data;
/* give it a chance to run. */
@@ -655,6 +664,7 @@
int do_syscall_trace(struct pt_regs *regs, int entryexit)
{
int is_sysemu = test_thread_flag(TIF_SYSCALL_EMU);
+ int is_systrace = test_thread_flag(TIF_SYSCALL_TRACE) || test_thread_flag(TIF_SYSCALL_CHECKEMU);
/*
* With TIF_SYSCALL_EMU set we want to ignore TIF_SINGLESTEP for syscall
* interception
@@ -697,7 +707,7 @@
if (is_singlestep)
send_sigtrap(current, regs, 0);

- if (!test_thread_flag(TIF_SYSCALL_TRACE) && !is_sysemu)
+ if (!is_systrace && !is_sysemu)
goto out;

/* the 0x80 provides a way for the tracing parent to distinguish
@@ -705,6 +715,12 @@
/* Note that the debugger could change the result of test_thread_flag!*/
ptrace_notify(SIGTRAP | ((current->ptrace & PT_TRACESYSGOOD) ? 0x80:0));

+ /* The difference between PTRACE_SYSEMU and PTRACE_CHECKEMU is
+ * that PTRACE_CHECKEMU allows you to decide whether to emulate
+ * the call after you've examined the registers. */
+ if (test_thread_flag(TIF_SYSCALL_CHECKEMU))
+ is_sysemu = 1;
+
/*
* this isn't the same as continuing with a signal, but it will do
* for normal use. strace only continues with a signal if the
diff -ur linux-2.6.15-vanilla/include/asm-i386/thread_info.h linux-2.6.15-checkemu/include/asm-i386/thread_info.h
--- linux-2.6.15-vanilla/include/asm-i386/thread_info.h 2006-01-02 22:21:10.000000000 -0500
+++ linux-2.6.15-checkemu/include/asm-i386/thread_info.h 2006-02-03 00:47:15.000000000 -0500
@@ -142,6 +142,7 @@
#define TIF_SYSCALL_EMU 6 /* syscall emulation active */
#define TIF_SYSCALL_AUDIT 7 /* syscall auditing active */
#define TIF_SECCOMP 8 /* secure computing */
+#define TIF_SYSCALL_CHECKEMU 9 /* checked syscall emulation active */
#define TIF_POLLING_NRFLAG 16 /* true if poll_idle() is polling TIF_NEED_RESCHED */
#define TIF_MEMDIE 17

@@ -152,6 +153,7 @@
#define _TIF_SINGLESTEP (1<<TIF_SINGLESTEP)
#define _TIF_IRET (1<<TIF_IRET)
#define _TIF_SYSCALL_EMU (1<<TIF_SYSCALL_EMU)
+#define _TIF_SYSCALL_CHECKEMU (1<<TIF_SYSCALL_CHECKEMU)
#define _TIF_SYSCALL_AUDIT (1<<TIF_SYSCALL_AUDIT)
#define _TIF_SECCOMP (1<<TIF_SECCOMP)
#define _TIF_POLLING_NRFLAG (1<<TIF_POLLING_NRFLAG)
@@ -159,7 +161,7 @@
/* work to do on interrupt/exception return */
#define _TIF_WORK_MASK \
(0x0000FFFF & ~(_TIF_SYSCALL_TRACE|_TIF_SYSCALL_AUDIT|_TIF_SINGLESTEP|\
- _TIF_SECCOMP|_TIF_SYSCALL_EMU))
+ _TIF_SECCOMP|_TIF_SYSCALL_EMU|_TIF_SYSCALL_CHECKEMU))
/* work to do on any return to u-space */
#define _TIF_ALLWORK_MASK (0x0000FFFF & ~_TIF_SECCOMP)

diff -ur linux-2.6.15-vanilla/include/linux/ptrace.h linux-2.6.15-checkemu/include/linux/ptrace.h
--- linux-2.6.15-vanilla/include/linux/ptrace.h 2006-01-02 22:21:10.000000000 -0500
+++ linux-2.6.15-checkemu/include/linux/ptrace.h 2006-02-03 00:47:18.000000000 -0500
@@ -22,6 +22,7 @@
#define PTRACE_SYSCALL 24
#define PTRACE_SYSEMU 31
#define PTRACE_SYSEMU_SINGLESTEP 32
+#define PTRACE_CHECKEMU 33

/* 0x4200-0x4300 are reserved for architecture-independent additions. */
#define PTRACE_SETOPTIONS 0x4200
diff -ur linux-2.6.15-vanilla/kernel/fork.c linux-2.6.15-checkemu/kernel/fork.c
--- linux-2.6.15-vanilla/kernel/fork.c 2006-01-02 22:21:10.000000000 -0500
+++ linux-2.6.15-checkemu/kernel/fork.c 2006-02-03 00:47:11.000000000 -0500
@@ -1016,6 +1016,9 @@
#ifdef TIF_SYSCALL_EMU
clear_tsk_thread_flag(p, TIF_SYSCALL_EMU);
#endif
+#ifdef TIF_SYSCALL_CHECKEMU
+ clear_tsk_thread_flag(p, TIF_SYSCALL_CHECKEMU);
+#endif

/* Our parent execution domain becomes current domain
These must match for thread signalling to apply */

-
To unsubscribe from this list: send the line "unsubscribe linux-kernel" in
the body of a message to majordomo@xxxxxxxxxxxxxxx
More majordomo info at http://vger.kernel.org/majordomo-info.html
Please read the FAQ at http://www.tux.org/lkml/

Next message: Jesper Juhl: "[PATCH] fix memory leak in mm/slab.c::alloc_kmemlist() (try #2)"
Previous message: Andrew Morton: "Re: [patch 1/2] Validate itimer timeval from userspace"
In reply to: Blaisorblade: "Re: [RFC] Proposed manpage additions for ptrace(2)"
Next in thread: Chuck Ebbert: "Re: [RFC] Proposed manpage additions for ptrace(2)"
Messages sorted by: [ date ] [ thread ] [ subject ] [ author ]