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Commit f459f3db authored by AKASHI Takahiro's avatar AKASHI Takahiro Committed by Ruchi Kandoi
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arm64: ptrace: allow tracer to skip a system call 



Note: This patch is from v6 of Takahiro's proposed
"arm64: add seccomp support" patchset (leecam@google.com)

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 enables this semantics, but there is a 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 treat whatever value in x0 as
a return value, but this might result in a bogus value being returned,
especially when tracer doesn't do anything at this syscall.
So we always return ENOSYS instead, while we 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: default avatarAKASHI Takahiro <takahiro.akashi@linaro.org>

Conflicts:
	arch/arm64/kernel/entry.S

Change-Id: Ifcdcdbcb7c8cf97e5b5f1086a1ea4107e1d4f9a8
parent 945fb68f

arch/arm64/include/asm/ptrace.h

+8 −0
Original line number Diff line number Diff line
@@ -65,6 +65,14 @@ 
#define COMPAT_PT_TEXT_ADDR		0x10000

#define COMPAT_PT_DATA_ADDR		0x10004

#define COMPAT_PT_TEXT_END_ADDR		0x10008



/*

 * used to skip a system call when tracer changes its number to -1

 * with ptrace(PTRACE_SET_SYSCALL)

 */

#define RET_SKIP_SYSCALL	-1

#define IS_SKIP_SYSCALL(no)	((int)(no & 0xffffffff) == -1)



#ifndef __ASSEMBLY__



/* sizeof(struct user) for AArch32 */

arch/arm64/kernel/entry.S

+4 −0
Original line number Diff line number Diff line
@@ -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 @@ __sys_trace: 
	mov	x0, sp

	bl	syscall_trace_enter

	adr	lr, __sys_trace_return		// return address

	cmp	w0, #RET_SKIP_SYSCALL		// skip syscall?

	b.eq	__sys_trace_return_skipped

	uxtw	scno, w0			// syscall number (possibly new)

	mov	x1, sp				// pointer to regs

	cmp	scno, sc_nr			// check upper syscall limit
@@ -684,6 +687,7 @@ __sys_trace: 


__sys_trace_return:

	str	x0, [sp]			// save returned x0

__sys_trace_return_skipped:			// x0 already in regs[0]

	mov	x0, sp

	bl	syscall_trace_exit

	b	ret_to_user

arch/arm64/kernel/ptrace.c

+20 −0
Original line number Diff line number Diff line
@@ -1126,12 +1126,32 @@ static void tracehook_report_syscall(struct pt_regs *regs, 


asmlinkage int syscall_trace_enter(struct pt_regs *regs)

{

	unsigned int saved_syscallno = regs->syscallno;



	if (test_thread_flag(TIF_SYSCALL_TRACE))

		tracehook_report_syscall(regs, PTRACE_SYSCALL_ENTER);



	if (test_thread_flag(TIF_SYSCALL_TRACEPOINT))

		trace_sys_enter(regs, regs->syscallno);



	if (IS_SKIP_SYSCALL(regs->syscallno)) {

		/*

		 * RESTRICTION: we can't modify a return value of user

		 * issued syscall(-1) here. In order to ease this flavor,

		 * we need to treat whatever value in x0 as a return value,

		 * but this might result in a bogus value being returned.

		 */

		/*

		 * 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.

		 */

		if (!test_thread_flag(TIF_SYSCALL_TRACE) ||

				(IS_SKIP_SYSCALL(saved_syscallno)))

			regs->regs[0] = -ENOSYS;

	}



	audit_syscall_entry(regs->syscallno, regs->orig_x0, regs->regs[1],

			    regs->regs[2], regs->regs[3]);