Re: [PATCH v7 2/6] arm64: ptrace: allow tracer to skip a system call

From: AKASHI Takahiro
Date: Thu Oct 09 2014 - 00:30:13 EST

On 10/08/2014 11:23 PM, Will Deacon wrote:
On Thu, Oct 02, 2014 at 10:46:12AM +0100, AKASHI Takahiro wrote:
If tracer specifies -1 as a syscall number, this traced system call should
be skipped with a value in x0 used as a return value.
This patch implements this semantics, but there is one restriction here:

when syscall(-1) is issued by user, tracer cannot skip this system call
and modify a return value at syscall entry.

In order to ease this flavor, we need to take whatever value x0 has as
a return value, but this might result in a bogus value being returned,
especially when tracer doesn't do anything against this syscall.
So we always return ENOSYS instead, while we still have another chance to
change a return value at syscall exit.

Please also note:
* syscall entry tracing and syscall exit tracing (ftrace tracepoint and
audit) are always executed, if enabled, even when skipping a system call
(that is, -1).
In this way, we can avoid a potential bug where audit_syscall_entry()
might be called without audit_syscall_exit() at the previous system call
being called, that would cause OOPs in audit_syscall_entry().

* syscallno may also be set to -1 if a fatal signal (SIGKILL) is detected
in tracehook_report_syscall_entry(), but since a value set to x0 (ENOSYS)
is not used in this case, we may neglect the case.

Signed-off-by: AKASHI Takahiro <takahiro.akashi@xxxxxxxxxx>
arch/arm64/include/asm/ptrace.h | 8 ++++++++
arch/arm64/kernel/entry.S | 4 ++++
arch/arm64/kernel/ptrace.c | 23 ++++++++++++++++++++++-
3 files changed, 34 insertions(+), 1 deletion(-)

diff --git a/arch/arm64/include/asm/ptrace.h b/arch/arm64/include/asm/ptrace.h
index 41ed9e1..736ebc3 100644
--- a/arch/arm64/include/asm/ptrace.h
+++ b/arch/arm64/include/asm/ptrace.h
@@ -65,6 +65,14 @@
#define COMPAT_PT_TEXT_ADDR 0x10000
#define COMPAT_PT_DATA_ADDR 0x10004
#define COMPAT_PT_TEXT_END_ADDR 0x10008
+ * System call will be skipped if a syscall number is changed to -1
+ * with ptrace(PTRACE_SET_SYSCALL).
+ * Upper 32-bit should be ignored for safe check.
+ */
+#define IS_SKIP_SYSCALL(no) ((int)(no & 0xffffffff) == -1)

I don't think this macro is very useful, especially considering that we
already use ~0UL explicitly in other places. Just move the comment into
syscall_trace_enter and be done with it. I also don't think you need the
mask (the cast is enough).

I remember it was necessary for compat PTRACE_SET_SYSCALL, but
will double-check it anyway.

#ifndef __ASSEMBLY__

/* sizeof(struct user) for AArch32 */
diff --git a/arch/arm64/kernel/entry.S b/arch/arm64/kernel/entry.S
index f0b5e51..b53a1c5 100644
--- a/arch/arm64/kernel/entry.S
+++ b/arch/arm64/kernel/entry.S
@@ -25,6 +25,7 @@
#include <asm/asm-offsets.h>
#include <asm/errno.h>
#include <asm/esr.h>
+#include <asm/ptrace.h>
#include <asm/thread_info.h>
#include <asm/unistd.h>

@@ -671,6 +672,8 @@ ENDPROC(el0_svc)
mov x0, sp
bl syscall_trace_enter
+ cmp w0, #-1 // skip the syscall?
+ b.eq __sys_trace_return_skipped
adr lr, __sys_trace_return // return address
uxtw scno, w0 // syscall number (possibly new)
mov x1, sp // pointer to regs
@@ -685,6 +688,7 @@ __sys_trace:

str x0, [sp] // save returned x0
mov x0, sp
bl syscall_trace_exit
b ret_to_user
diff --git a/arch/arm64/kernel/ptrace.c b/arch/arm64/kernel/ptrace.c
index 2842f9f..6b11c6a 100644
--- a/arch/arm64/kernel/ptrace.c
+++ b/arch/arm64/kernel/ptrace.c
@@ -1126,6 +1126,8 @@ static void tracehook_report_syscall(struct pt_regs *regs,

asmlinkage int syscall_trace_enter(struct pt_regs *regs)
+ unsigned int orig_syscallno = regs->syscallno;
if (test_thread_flag(TIF_SYSCALL_TRACE))
tracehook_report_syscall(regs, PTRACE_SYSCALL_ENTER);

@@ -1133,7 +1135,26 @@ asmlinkage int syscall_trace_enter(struct pt_regs *regs)
trace_sys_enter(regs, regs->syscallno);

audit_syscall_entry(syscall_get_arch(), regs->syscallno,
- regs->orig_x0, regs->regs[1], regs->regs[2], regs->regs[3]);
+ regs->orig_x0, regs->regs[1],
+ regs->regs[2], regs->regs[3]);
+ if (IS_SKIP_SYSCALL(regs->syscallno) &&
+ IS_SKIP_SYSCALL(orig_syscallno)) {
+ /*
+ * For compatibility, we handles user-issued syscall(-1).

Compatibility with what? arch/arm/?

with the case where a process is *not* traced (including audit).

+ *
+ * RESTRICTION: we can't modify a return value here in this
+ * specific case. In order to ease this flavor, we have to
+ * take whatever value x0 has as a return value, but this
+ * might result in a bogus value being returned.

This comment isn't helping me. Are we returning a bogus value or not? If so,
why is that acceptable?

I mean that syscall(-1) always returns -1 with ENOSYS.

Let's think about the case that we didn't have this 'if' statement.
If a debugger catches an user-issued syscall(-1), but let it go without
doing anything (especially changing a value in x0), this syscall will
return an original value in x0, which is the first argument of syscall(-1).
I mentioned this as "bogus."
In this way, a traced process would see a different behavior of syscall(-1).
(On arm, this doesn't happen because syscall(-1) is supposed to raise SIGILL.)
(On x86, this doesn't happen, probably, because syscall arguments are passed
via a stack and we can set a default return value in a register to ENOSYS.)

To avoid this incompatibility, there is no way but to always return -1 in this path
because the kernel doesn't know whether a debugger let x0 unchanged on purpose or not.
This is also the reason why I wanted to have a dedicated ptrace command to set
a return value in skipping a system call.

If we don't care about such erroneous (and exceptional) behaviors, we don't
need this 'if' statement.

Did I make it clear?

+ * NOTE: syscallno may also be set to -1 if fatal signal
+ * is detected in tracehook_report_syscall(ENTRY),
+ * but since a value set to x0 here is not used in this
+ * case, we may neglect the case.
+ */

I think can you remove thise NOTE, it's not very informative.

Also remove descriptions from a commit message.

-Takahiro AKASHI


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