lkdtm: Make lkdtm_do_action() return to avoid tail call optimization

The comments for lkdtm_do_action() explicitly call out that it
shouldn't be inlined because we want it to show up in stack
crawls. However, at least with some compilers / options it's still
vanishing due to tail call optimization. Let's add a return value to
the function to make it harder for the compiler to do tail call
optimization here.

Now that we have a return value, we can actually use it in the
callers, which is a minor improvement in the code.

Signed-off-by: Douglas Anderson <dianders@chromium.org>
Link: https://lore.kernel.org/r/20240122164935.1.I345e485f36babad76370c59659a706723750d950@changeid
Signed-off-by: Kees Cook <keescook@chromium.org>

diff --git a/drivers/misc/lkdtm/core.c b/drivers/misc/lkdtm/core.c
index 0772e4a4757e9f47a32abb6a76dc613261b2a642..5732fd59a227d05723dfa5a2843ecbf43c803ace 100644 (file)
--- a/drivers/misc/lkdtm/core.c
+++ b/drivers/misc/lkdtm/core.c
@@ -153,12 +153,17 @@ static const struct crashtype *find_crashtype(const char *name)
 /*
  * This is forced noinline just so it distinctly shows up in the stackdump
  * which makes validation of expected lkdtm crashes easier.
+ *
+ * NOTE: having a valid return value helps prevent the compiler from doing
+ * tail call optimizations and taking this out of the stack trace.
  */
-static noinline void lkdtm_do_action(const struct crashtype *crashtype)
+static noinline int lkdtm_do_action(const struct crashtype *crashtype)
 {
        if (WARN_ON(!crashtype || !crashtype->func))
-               return;
+               return -EINVAL;
        crashtype->func();
+
+       return 0;
 }
 
 static int lkdtm_register_cpoint(struct crashpoint *crashpoint,
@@ -167,10 +172,8 @@ static int lkdtm_register_cpoint(struct crashpoint *crashpoint,
        int ret;
 
        /* If this doesn't have a symbol, just call immediately. */
-       if (!crashpoint->kprobe.symbol_name) {
-               lkdtm_do_action(crashtype);
-               return 0;
-       }
+       if (!crashpoint->kprobe.symbol_name)
+               return lkdtm_do_action(crashtype);
 
        if (lkdtm_kprobe != NULL)
                unregister_kprobe(lkdtm_kprobe);
@@ -216,7 +219,7 @@ static int lkdtm_kprobe_handler(struct kprobe *kp, struct pt_regs *regs)
        spin_unlock_irqrestore(&crash_count_lock, flags);
 
        if (do_it)
-               lkdtm_do_action(lkdtm_crashtype);
+               return lkdtm_do_action(lkdtm_crashtype);
 
        return 0;
 }
@@ -303,6 +306,7 @@ static ssize_t direct_entry(struct file *f, const char __user *user_buf,
 {
        const struct crashtype *crashtype;
        char *buf;
+       int err;
 
        if (count >= PAGE_SIZE)
                return -EINVAL;
@@ -326,9 +330,11 @@ static ssize_t direct_entry(struct file *f, const char __user *user_buf,
                return -EINVAL;
 
        pr_info("Performing direct entry %s\n", crashtype->name);
-       lkdtm_do_action(crashtype);
+       err = lkdtm_do_action(crashtype);
        *off += count;
 
+       if (err)
+               return err;
        return count;
 }