On Tue, Nov 18, 2014 at 01:10:34AM +0000, AKASHI Takahiro wrote:
> If tracer specifies -1 as a syscall number, this traced system call should
> be skipped with a return value specified in x0.
> This patch implements this semantics, but there is one restriction here:
>
> syscall(-1) always return ENOSYS whatever value is stored in x0
> (a return value) at syscall entry.
>
> Normally, with ptrace off, syscall(-1) returns -ENOSYS. With ptrace on,
> however, if a tracer didn't pay any attention to user-issued syscall(-1)
> and just let it go, it would return a value in x0 as in other system call
> cases. This means that this system call might succeed and yet see any bogus
> return value. This should be definitely avoided.
>
> 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().
>
> Signed-off-by: AKASHI Takahiro <takahiro.aka...@linaro.org>
> ---
> arch/arm64/kernel/entry.S | 3 +++
> arch/arm64/kernel/ptrace.c | 18 ++++++++++++++++++
> 2 files changed, 21 insertions(+)
>
> diff --git a/arch/arm64/kernel/entry.S b/arch/arm64/kernel/entry.S
> index 726b910..01118b1 100644
> --- a/arch/arm64/kernel/entry.S
> +++ b/arch/arm64/kernel/entry.S
> @@ -670,6 +670,8 @@ 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
> @@ -684,6 +686,7 @@ __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 8b98781..34b1e85 100644
> --- a/arch/arm64/kernel/ptrace.c
> +++ b/arch/arm64/kernel/ptrace.c
> @@ -1149,6 +1149,8 @@ static void tracehook_report_syscall(struct pt_regs
> *regs,
>
> asmlinkage int syscall_trace_enter(struct pt_regs *regs)
> {
> + int orig_syscallno = regs->syscallno;
> +
> if (test_thread_flag(TIF_SYSCALL_TRACE))
> tracehook_report_syscall(regs, PTRACE_SYSCALL_ENTER);
>
> @@ -1158,6 +1160,22 @@ asmlinkage int syscall_trace_enter(struct pt_regs
> *regs)
> audit_syscall_entry(regs->syscallno, regs->orig_x0, regs->regs[1],
> regs->regs[2], regs->regs[3]);
>
> + if (((int)regs->syscallno == -1) && (orig_syscallno == -1)) {
> + /*
> + * user-issued syscall(-1):
> + * RESTRICTION: We always return ENOSYS whatever value is
> + * stored in x0 (a return value) at this point.
> + * Normally, with ptrace off, syscall(-1) returns -ENOSYS.
> + * With ptrace on, however, if a tracer didn't pay any
> + * attention to user-issued syscall(-1) and just let it go
> + * without a hack here, it would return a value in x0 as in
> + * other system call cases. This means that this system call
> + * might succeed and see any bogus return value.
> + * This should be definitely avoided.
> + */
> + regs->regs[0] = -ENOSYS;
> + }
I'm still really uncomfortable with this, and it doesn't seem to match what
arch/arm/ does either. Doesn't it also prevent a tracer from skipping
syscall(-1)?
Will
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