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

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Message-ID: <CAKwvOdkfjyKz6686RzAGjfKMVPriLM8XuNueYyWcd_Sj-WnJbg@mail.gmail.com>
Date: Tue, 19 Dec 2023 08:55:21 -0800
From: Nick Desaulniers <ndesaulniers@...gle.com>
To: Nathan Chancellor <nathan@...nel.org>, Sebastian Andrzej Siewior <bigeasy@...utronix.de>
Cc: kernel test robot <lkp@...el.com>, linux-kernel@...r.kernel.org, netdev@...r.kernel.org, 
	llvm@...ts.linux.dev, oe-kbuild-all@...ts.linux.dev, 
	Boqun Feng <boqun.feng@...il.com>, Daniel Borkmann <daniel@...earbox.net>, 
	Eric Dumazet <edumazet@...gle.com>, Frederic Weisbecker <frederic@...nel.org>, 
	Ingo Molnar <mingo@...hat.com>, Jakub Kicinski <kuba@...nel.org>, Paolo Abeni <pabeni@...hat.com>, 
	Peter Zijlstra <peterz@...radead.org>, Thomas Gleixner <tglx@...utronix.de>, 
	Waiman Long <longman@...hat.com>, Will Deacon <will@...nel.org>, 
	Alexei Starovoitov <ast@...nel.org>, Jesper Dangaard Brouer <hawk@...nel.org>, 
	Jesse Brandeburg <jesse.brandeburg@...el.com>, John Fastabend <john.fastabend@...il.com>, 
	Tony Nguyen <anthony.l.nguyen@...el.com>, bpf@...r.kernel.org, 
	intel-wired-lan@...ts.osuosl.org
Subject: Re: [PATCH net-next 20/24] net: intel: Use nested-BH locking for XDP redirect.

On Mon, Dec 18, 2023 at 4:01 PM Nathan Chancellor <nathan@...nel.org> wrote:
>
> 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@...el.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;
> >                    ^

^ The problematic goto should be replaced with an early return. (and
perhaps a comment that you can't jump over __cleanup variable
initialization).

Otherwise the compiler cannot put the cleanup in the destination basic
block; it would have to split the edges and have all the happy paths
go to a synthesized basic block that runs the cleanup, then jumps to
the original destination.

> >    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
>


-- 
Thanks,
~Nick Desaulniers