netdev - Re: [PATCH v3 bpf-next 1/5] mm/error_inject: Fix allow_error

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Message-ID: <d6eae293-5427-d5e4-73aa-4df7a493bb89@iogearbox.net>
Date:   Fri, 28 Aug 2020 22:27:25 +0200
From:   Daniel Borkmann <daniel@...earbox.net>
To:     Alexei Starovoitov <alexei.starovoitov@...il.com>,
        davem@...emloft.net
Cc:     josef@...icpanda.com, bpoirier@...e.com, akpm@...ux-foundation.org,
        hannes@...xchg.org, netdev@...r.kernel.org, bpf@...r.kernel.org,
        kernel-team@...com
Subject: Re: [PATCH v3 bpf-next 1/5] mm/error_inject: Fix allow_error_inject
 function signatures.

On 8/28/20 12:01 AM, Alexei Starovoitov wrote:
> From: Alexei Starovoitov <ast@...nel.org>
> 
> 'static' and 'static noinline' function attributes make no guarantees that
> gcc/clang won't optimize them. The compiler may decide to inline 'static'
> function and in such case ALLOW_ERROR_INJECT becomes meaningless. The compiler
> could have inlined __add_to_page_cache_locked() in one callsite and didn't
> inline in another. In such case injecting errors into it would cause
> unpredictable behavior. It's worse with 'static noinline' which won't be
> inlined, but it still can be optimized. Like the compiler may decide to remove
> one argument or constant propagate the value depending on the callsite.
> 
> To avoid such issues make sure that these functions are global noinline.

Back in the days when adding 6bf37e5aa90f ("crypto: crypto_memneq - add equality
testing of memory regions w/o timing leaks") we added noinline, but also an
explicit EXPORT_SYMBOL() to prevent this from being optimized away; I wonder
whether ALLOW_ERROR_INJECT() should have something implicit here too to prevent
from optimization .. otoh we probably don't want to expose every ALLOW_ERROR_INJECT()
function also to modules generically...

Thanks,
Daniel