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@linaro.org
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) __sys_trace: 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:
__sys_trace_return: str x0, [sp] // save returned x0 +__sys_trace_return_skipped: 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.
Okey. Also remove descriptions from a commit message.
-Takahiro AKASHI
Will