Commit 1014c81d authored Nov 28, 2014 by AKASHI Takahiro Committed by Will Deacon Nov 28, 2014

arm64: ptrace: allow tracer to skip a system call



If tracer modifies a syscall number to -1, this traced system call should
be skipped with a return value specified in x0.
This patch implements this semantics.

Please 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.akashi@linaro.org>
[will: fixed up conflict with blr rework]
Signed-off-by: Will Deacon <will.deacon@arm.com>

parent 766a85d7

arch/arm64/kernel/entry.S

+10 −2

Original line number	Diff line number	Diff line
		@@ -669,8 +669,15 @@ ENDPROC(el0_svc)
		* switches, and waiting for our parent to respond.
		*/
		__sys_trace:
		mov x0, sp
		mov w0, #-1 // set default errno for
		cmp scno, x0 // user-issued syscall(-1)
		b.ne 1f
		mov x0, #-ENOSYS
		str x0, [sp, #S_X0]
		1: mov x0, sp
		bl syscall_trace_enter
		cmp w0, #-1 // skip the 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
		@@ -683,7 +690,8 @@ __sys_trace:
		blr x16 // call sys_* routine

		__sys_trace_return:
		str x0, [sp] // save returned x0
		str x0, [sp, #S_X0] // save returned x0
		__sys_trace_return_skipped:
		mov x0, sp
		bl syscall_trace_exit
		b ret_to_user

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