arm64: ptrace: allow tracer to skip a system call
Note: This patch is from v6 of Takahiro's proposed
"arm64: add seccomp support" patchset (email@example.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: AKASHI Takahiro <takahiro.akashi <at> linaro.org>
3 files changed