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

[Will Deacon]

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 at 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).

> +
>  #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/?

> +		 *
> +		 * 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?

> +		 * 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.

Will