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>
diff --git a/arch/arm64/kernel/entry.S b/arch/arm64/kernel/entry.S
index 99c8d13..fd4fa37 100644
--- a/arch/arm64/kernel/entry.S
+++ b/arch/arm64/kernel/entry.S
@@ -669,8 +669,15 @@
* 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 @@
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