Re: [PATCH net-next 20/24] net: intel: Use nested-BH locking for XDP redirect.

[Nathan Chancellor <nathan@kernel.org>]

On Sat, Dec 16, 2023 at 12:53:43PM +0800, kernel test robot wrote:
> Hi Sebastian,
> 
> kernel test robot noticed the following build errors:
> 
> [auto build test ERROR on net-next/main]
> 
> url:    https://github.com/intel-lab-lkp/linux/commits/Sebastian-Andrzej-Siewior/locking-local_lock-Introduce-guard-definition-for-local_lock/20231216-011911
> base:   net-next/main
> patch link:    https://lore.kernel.org/r/20231215171020.687342-21-bigeasy%40linutronix.de
> patch subject: [PATCH net-next 20/24] net: intel: Use nested-BH locking for XDP redirect.
> config: arm-defconfig (https://download.01.org/0day-ci/archive/20231216/202312161212.D5tju5i6-lkp@intel.com/config)
> compiler: clang version 14.0.6 (https://github.com/llvm/llvm-project.git f28c006a5895fc0e329fe15fead81e37457cb1d1)
> reproduce (this is a W=1 build): (https://download.01.org/0day-ci/archive/20231216/202312161212.D5tju5i6-lkp@intel.com/reproduce)
> 
> If you fix the issue in a separate patch/commit (i.e. not just a new version of
> the same patch/commit), kindly add following tags
> | Reported-by: kernel test robot <lkp@intel.com>
> | Closes: https://lore.kernel.org/oe-kbuild-all/202312161212.D5tju5i6-lkp@intel.com/
> 
> All errors (new ones prefixed by >>):
> 
> >> drivers/net/ethernet/intel/igb/igb_main.c:8620:3: error: cannot jump from this goto statement to its label
>                    goto xdp_out;
>                    ^
>    drivers/net/ethernet/intel/igb/igb_main.c:8624:2: note: jump bypasses initialization of variable with __attribute__((cleanup))
>            guard(local_lock_nested_bh)(&bpf_run_lock.redirect_lock);
>            ^
>    include/linux/cleanup.h:142:15: note: expanded from macro 'guard'
>            CLASS(_name, __UNIQUE_ID(guard))
>                         ^
>    include/linux/compiler.h:180:29: note: expanded from macro '__UNIQUE_ID'
>    #define __UNIQUE_ID(prefix) __PASTE(__PASTE(__UNIQUE_ID_, prefix), __COUNTER__)
>                                ^
>    include/linux/compiler_types.h:84:22: note: expanded from macro '__PASTE'
>    #define __PASTE(a,b) ___PASTE(a,b)
>                         ^
>    include/linux/compiler_types.h:83:23: note: expanded from macro '___PASTE'
>    #define ___PASTE(a,b) a##b
>                          ^
>    <scratch space>:52:1: note: expanded from here
>    __UNIQUE_ID_guard753
>    ^
>    1 error generated.

I initially thought that this may have been
https://github.com/ClangBuiltLinux/linux/issues/1886 but asm goto is not
involved here.

This error occurs because jumping over the initialization of a variable
declared with __attribute__((__cleanup__(...))) does not prevent the
clean up function from running as one may expect it to, but could
instead result in the clean up function getting run on uninitialized
memory. A contrived example (see the bottom of the "Output" tabs for the
execution output):

https://godbolt.org/z/9bvGboxvc

While there is a warning from GCC in that example, I don't see one in
the kernel's case. I see there is an open GCC issue around this problem:

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=91951

While it is possible that there may not actually be a problem with how
the kernel uses __attribute__((__cleanup__(...))) and gotos, I think
clang's behavior is reasonable given the potential footguns that this
construct has.

Cheers,
Nathan