selftests/seccomp: Allow syscall nr and ret value to be set separately

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.

Acked-by: Christian Brauner <christian.brauner@ubuntu.com>
Link: https://lore.kernel.org/lkml/20200921075031.j4gruygeugkp2zwd@wittgenstein/
Signed-off-by: Kees Cook <keescook@chromium.org>

diff --git a/tools/testing/selftests/seccomp/seccomp_bpf.c b/tools/testing/selftests/seccomp/seccomp_bpf.c
index c0311b4..98ce5e8 100644 (file)
--- 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) {