netdev - Re: [PATCH bpf-next] bpf, x86, arm64: enable jit by default when not built as always-on

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Message-ID: <20191209143436.GC4401@willie-the-truck>
Date:   Mon, 9 Dec 2019 14:34:37 +0000
From:   Will Deacon <will@...nel.org>
To:     Daniel Borkmann <daniel@...earbox.net>
Cc:     ast@...nel.org, netdev@...r.kernel.org, bpf@...r.kernel.org
Subject: Re: [PATCH bpf-next] bpf, x86, arm64: enable jit by default when not
 built as always-on

On Mon, Dec 09, 2019 at 03:04:42PM +0100, Daniel Borkmann wrote:
> After Spectre 2 fix via 290af86629b2 ("bpf: introduce BPF_JIT_ALWAYS_ON
> config") most major distros use BPF_JIT_ALWAYS_ON configuration these days
> which compiles out the BPF interpreter entirely and always enables the
> JIT. Also given recent fix in e1608f3fa857 ("bpf: Avoid setting bpf insns
> pages read-only when prog is jited"), we additionally avoid fragmenting
> the direct map for the BPF insns pages sitting in the general data heap
> since they are not used during execution. Latter is only needed when run
> through the interpreter.
> 
> Since both x86 and arm64 JITs have seen a lot of exposure over the years,
> are generally most up to date and maintained, there is more downside in
> !BPF_JIT_ALWAYS_ON configurations to have the interpreter enabled by default
> rather than the JIT. Add a ARCH_WANT_DEFAULT_BPF_JIT config which archs can
> use to set the bpf_jit_{enable,kallsyms} to 1. Back in the days the
> bpf_jit_kallsyms knob was set to 0 by default since major distros still
> had /proc/kallsyms addresses exposed to unprivileged user space which is
> not the case anymore. Hence both knobs are set via BPF_JIT_DEFAULT_ON which
> is set to 'y' in case of BPF_JIT_ALWAYS_ON or ARCH_WANT_DEFAULT_BPF_JIT.
> 
> Signed-off-by: Daniel Borkmann <daniel@...earbox.net>
> ---
>  [ Follow-up from https://lore.kernel.org/bpf/20191202200947.GA14353@pc-9.home/,
>    applies to both bpf and bpf-next, but I think going via bpf-next is more
>    appropriate. ]
> 
>  arch/arm64/Kconfig | 1 +
>  arch/x86/Kconfig   | 1 +
>  init/Kconfig       | 6 ++++++
>  kernel/bpf/core.c  | 4 ++--
>  4 files changed, 10 insertions(+), 2 deletions(-)
> 
> diff --git a/arch/arm64/Kconfig b/arch/arm64/Kconfig
> index b1b4476ddb83..29d03459de20 100644
> --- a/arch/arm64/Kconfig
> +++ b/arch/arm64/Kconfig
> @@ -69,6 +69,7 @@ config ARM64
>  	select ARCH_SUPPORTS_INT128 if CC_HAS_INT128 && (GCC_VERSION >= 50000 || CC_IS_CLANG)
>  	select ARCH_SUPPORTS_NUMA_BALANCING
>  	select ARCH_WANT_COMPAT_IPC_PARSE_VERSION if COMPAT
> +	select ARCH_WANT_DEFAULT_BPF_JIT
>  	select ARCH_WANT_DEFAULT_TOPDOWN_MMAP_LAYOUT
>  	select ARCH_WANT_FRAME_POINTERS
>  	select ARCH_WANT_HUGE_PMD_SHARE if ARM64_4K_PAGES || (ARM64_16K_PAGES && !ARM64_VA_BITS_36)
> diff --git a/arch/x86/Kconfig b/arch/x86/Kconfig
> index 5e8949953660..1f6a0388a65f 100644
> --- a/arch/x86/Kconfig
> +++ b/arch/x86/Kconfig
> @@ -93,6 +93,7 @@ config X86
>  	select ARCH_USE_QUEUED_RWLOCKS
>  	select ARCH_USE_QUEUED_SPINLOCKS
>  	select ARCH_WANT_BATCHED_UNMAP_TLB_FLUSH
> +	select ARCH_WANT_DEFAULT_BPF_JIT	if X86_64
>  	select ARCH_WANTS_DYNAMIC_TASK_STRUCT
>  	select ARCH_WANT_HUGE_PMD_SHARE
>  	select ARCH_WANTS_THP_SWAP		if X86_64
> diff --git a/init/Kconfig b/init/Kconfig
> index a34064a031a5..957a5e758e6d 100644
> --- a/init/Kconfig
> +++ b/init/Kconfig
> @@ -1604,6 +1604,9 @@ config BPF_SYSCALL
>  	  Enable the bpf() system call that allows to manipulate eBPF
>  	  programs and maps via file descriptors.
>  
> +config ARCH_WANT_DEFAULT_BPF_JIT
> +	bool
> +
>  config BPF_JIT_ALWAYS_ON
>  	bool "Permanently enable BPF JIT and remove BPF interpreter"
>  	depends on BPF_SYSCALL && HAVE_EBPF_JIT && BPF_JIT
> @@ -1611,6 +1614,9 @@ config BPF_JIT_ALWAYS_ON
>  	  Enables BPF JIT and removes BPF interpreter to avoid
>  	  speculative execution of BPF instructions by the interpreter
>  
> +config BPF_JIT_DEFAULT_ON
> +	def_bool ARCH_WANT_DEFAULT_BPF_JIT || BPF_JIT_ALWAYS_ON
> +

Seems a bit weird to me that this doesn't end up depending on
CONFIG_BPF_JIT, but for the general idea:

Acked-by: Will Deacon <will@...nel.org>

Will