In commit edb6538da3df ("lkdtm/bugs: Adjust lkdtm_HUNG_TASK() to avoid
tail call optimization") we marked lkdtm_HUNG_TASK() as
__noreturn. The compiler gets unhappy if it thinks a __noreturn
function might return, so there's a BUG_ON(1) at the end. Any human
can see that the function won't return and the compiler can figure
that out too. Except when it can't.

The MIPS architecture defines HAVE_ARCH_BUG_ON and defines its own
version of BUG_ON(). The MIPS version of BUG_ON() is not a macro but
is instead an inline function. Apparently this prevents the compiler
from realizing that the condition to BUG_ON() is constant and that the
function will never return.

Let's change the BUG_ON(1) to just BUG(), which it should have been to
begin with. The only reason I used BUG_ON(1) to begin with was because
I was used to using WARN_ON(1) when writing test code and WARN() and
BUG() are oddly inconsistent in this manner. :-/

Fixes: edb6538da3df ("lkdtm/bugs: Adjust lkdtm_HUNG_TASK() to avoid tail call optimization")
Signed-off-by: Douglas Anderson <dianders@chromium.org>
---

 drivers/misc/lkdtm/bugs.c | 2 +-
 1 file changed, 1 insertion(+), 1 deletion(-)

diff --git a/drivers/misc/lkdtm/bugs.c b/drivers/misc/lkdtm/bugs.c
index d1222d3eda2f..b92767d6bdd2 100644
--- a/drivers/misc/lkdtm/bugs.c
+++ b/drivers/misc/lkdtm/bugs.c
@@ -298,7 +298,7 @@ static void __noreturn lkdtm_HUNG_TASK(void)
 {
 	set_current_state(TASK_UNINTERRUPTIBLE);
 	schedule();
-	BUG_ON(1);
+	BUG();
 }
 
 static volatile unsigned int huge = INT_MAX - 2;
-- 
2.43.0.429.g432eaa2c6b-goog

On Fri, 26 Jan 2024 07:28:53 -0800, Douglas Anderson wrote:
> In commit edb6538da3df ("lkdtm/bugs: Adjust lkdtm_HUNG_TASK() to avoid
> tail call optimization") we marked lkdtm_HUNG_TASK() as
> __noreturn. The compiler gets unhappy if it thinks a __noreturn
> function might return, so there's a BUG_ON(1) at the end. Any human
> can see that the function won't return and the compiler can figure
> that out too. Except when it can't.
> 
> [...]

Applied to for-next/hardening, thanks!

[1/1] lkdtm/bugs: In lkdtm_HUNG_TASK() use BUG(), not BUG_ON(1)
      https://git.kernel.org/kees/c/34b82a2fb747

Take care,

-- 
Kees Cook

Hi,

On Fri, Jan 26, 2024 at 7:29 AM Douglas Anderson <dianders@chromium.org> wrote:
>
> In commit edb6538da3df ("lkdtm/bugs: Adjust lkdtm_HUNG_TASK() to avoid
> tail call optimization") we marked lkdtm_HUNG_TASK() as
> __noreturn. The compiler gets unhappy if it thinks a __noreturn
> function might return, so there's a BUG_ON(1) at the end. Any human
> can see that the function won't return and the compiler can figure
> that out too. Except when it can't.
>
> The MIPS architecture defines HAVE_ARCH_BUG_ON and defines its own
> version of BUG_ON(). The MIPS version of BUG_ON() is not a macro but
> is instead an inline function. Apparently this prevents the compiler
> from realizing that the condition to BUG_ON() is constant and that the
> function will never return.
>
> Let's change the BUG_ON(1) to just BUG(), which it should have been to
> begin with. The only reason I used BUG_ON(1) to begin with was because
> I was used to using WARN_ON(1) when writing test code and WARN() and
> BUG() are oddly inconsistent in this manner. :-/
>
> Fixes: edb6538da3df ("lkdtm/bugs: Adjust lkdtm_HUNG_TASK() to avoid tail call optimization")
> Signed-off-by: Douglas Anderson <dianders@chromium.org>
> ---
>
>  drivers/misc/lkdtm/bugs.c | 2 +-
>  1 file changed, 1 insertion(+), 1 deletion(-)

Ugh. I just realized I forgot these. Kees: hopefully you can add them...

Reported-by: kernel test robot <lkp@intel.com>
Closes: https://lore.kernel.org/oe-kbuild-all/202401262204.wUFKRYZF-lkp@intel.com/

-Doug