[PATCH v2 2/4] selftests/seccomp: Allow syscall nr and ret value to be set separately

From: Kees Cook
Date: Sat Sep 19 2020 - 04:06:58 EST


In preparation for setting syscall nr and ret values separately, refactor
the helpers to take a pointer to a value, so that a NULL can indicate
"do not change this respective value". This is done to keep the regset
read/write happening once and in one code path.

Signed-off-by: Kees Cook <keescook@xxxxxxxxxxxx>
---
tools/testing/selftests/seccomp/seccomp_bpf.c | 59 +++++++++++++++----
1 file changed, 47 insertions(+), 12 deletions(-)

diff --git a/tools/testing/selftests/seccomp/seccomp_bpf.c b/tools/testing/selftests/seccomp/seccomp_bpf.c
index c0311b4c736b..98ce5e8a6398 100644
--- a/tools/testing/selftests/seccomp/seccomp_bpf.c
+++ b/tools/testing/selftests/seccomp/seccomp_bpf.c
@@ -1888,27 +1888,47 @@ int get_syscall(struct __test_metadata *_metadata, pid_t tracee)
}

/* Architecture-specific syscall changing routine. */
-void change_syscall(struct __test_metadata *_metadata,
- pid_t tracee, int syscall, int result)
+void __change_syscall(struct __test_metadata *_metadata,
+ pid_t tracee, long *syscall, long *ret)
{
ARCH_REGS orig, regs;

+ /* Do not get/set registers if we have nothing to do. */
+ if (!syscall && !ret)
+ return;
+
EXPECT_EQ(0, ARCH_GETREGS(regs)) {
return;
}
orig = regs;

- SYSCALL_NUM_SET(regs, syscall);
+ if (syscall)
+ SYSCALL_NUM_SET(regs, *syscall);

- /* If syscall is skipped, change return value. */
- if (syscall == -1)
- SYSCALL_RET_SET(regs, result);
+ if (ret)
+ SYSCALL_RET_SET(regs, *ret);

/* Flush any register changes made. */
if (memcmp(&orig, &regs, sizeof(orig)) != 0)
EXPECT_EQ(0, ARCH_SETREGS(regs));
}

+/* Change only syscall number. */
+void change_syscall_nr(struct __test_metadata *_metadata,
+ pid_t tracee, long syscall)
+{
+ __change_syscall(_metadata, tracee, &syscall, NULL);
+}
+
+/* Change syscall return value (and set syscall number to -1). */
+void change_syscall_ret(struct __test_metadata *_metadata,
+ pid_t tracee, long ret)
+{
+ long syscall = -1;
+
+ __change_syscall(_metadata, tracee, &syscall, &ret);
+}
+
void tracer_seccomp(struct __test_metadata *_metadata, pid_t tracee,
int status, void *args)
{
@@ -1924,17 +1944,17 @@ void tracer_seccomp(struct __test_metadata *_metadata, pid_t tracee,
case 0x1002:
/* change getpid to getppid. */
EXPECT_EQ(__NR_getpid, get_syscall(_metadata, tracee));
- change_syscall(_metadata, tracee, __NR_getppid, 0);
+ change_syscall_nr(_metadata, tracee, __NR_getppid);
break;
case 0x1003:
/* skip gettid with valid return code. */
EXPECT_EQ(__NR_gettid, get_syscall(_metadata, tracee));
- change_syscall(_metadata, tracee, -1, 45000);
+ change_syscall_ret(_metadata, tracee, 45000);
break;
case 0x1004:
/* skip openat with error. */
EXPECT_EQ(__NR_openat, get_syscall(_metadata, tracee));
- change_syscall(_metadata, tracee, -1, -ESRCH);
+ change_syscall_ret(_metadata, tracee, -ESRCH);
break;
case 0x1005:
/* do nothing (allow getppid) */
@@ -1961,6 +1981,8 @@ void tracer_ptrace(struct __test_metadata *_metadata, pid_t tracee,
int ret;
unsigned long msg;
static bool entry;
+ long syscall_nr_val, syscall_ret_val;
+ long *syscall_nr = NULL, *syscall_ret = NULL;
FIXTURE_DATA(TRACE_syscall) *self = args;

/*
@@ -1987,17 +2009,30 @@ void tracer_ptrace(struct __test_metadata *_metadata, pid_t tracee,
else
return;

+ syscall_nr = &syscall_nr_val;
+ syscall_ret = &syscall_ret_val;
+
+ /* Now handle the actual rewriting cases. */
switch (self->syscall_nr) {
case __NR_getpid:
- change_syscall(_metadata, tracee, __NR_getppid, 0);
+ syscall_nr_val = __NR_getppid;
+ /* Never change syscall return for this case. */
+ syscall_ret = NULL;
break;
case __NR_gettid:
- change_syscall(_metadata, tracee, -1, 45000);
+ syscall_nr_val = -1;
+ syscall_ret_val = 45000;
break;
case __NR_openat:
- change_syscall(_metadata, tracee, -1, -ESRCH);
+ syscall_nr_val = -1;
+ syscall_ret_val = -ESRCH;
break;
+ default:
+ /* Unhandled, do nothing. */
+ return;
}
+
+ __change_syscall(_metadata, tracee, syscall_nr, syscall_ret);
}

FIXTURE_VARIANT(TRACE_syscall) {
--
2.25.1