netdev - Re: [PATCH net] powerpc/bpf: fix tail call implementation

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Date:   Sat, 2 Nov 2019 01:05:55 +0100
From:   Daniel Borkmann <daniel@...earbox.net>
To:     Eric Dumazet <edumazet@...gle.com>,
        Alexei Starovoitov <ast@...nel.org>
Cc:     "David S . Miller" <davem@...emloft.net>,
        netdev <netdev@...r.kernel.org>,
        Eric Dumazet <eric.dumazet@...il.com>,
        bpf <bpf@...r.kernel.org>,
        "Naveen N. Rao" <naveen.n.rao@...ux.ibm.com>,
        Sandipan Das <sandipan@...ux.ibm.com>,
        Benjamin Herrenschmidt <benh@...nel.crashing.org>,
        Paul Mackerras <paulus@...ba.org>,
        Michael Ellerman <mpe@...erman.id.au>,
        Martin KaFai Lau <kafai@...com>,
        Song Liu <songliubraving@...com>, Yonghong Song <yhs@...com>
Subject: Re: [PATCH net] powerpc/bpf: fix tail call implementation

On 11/1/19 4:34 AM, Eric Dumazet wrote:
> We have seen many crashes on powerpc hosts while loading bpf programs.
> 
> The problem here is that bpf_int_jit_compile() does a first pass
> to compute the program length.
> 
> Then it allocates memory to store the generated program and
> calls bpf_jit_build_body() a second time (and a third time
> later)
> 
> What I have observed is that the second bpf_jit_build_body()
> could end up using few more words than expected.
> 
> If bpf_jit_binary_alloc() put the space for the program
> at the end of the allocated page, we then write on
> a non mapped memory.
> 
> It appears that bpf_jit_emit_tail_call() calls
> bpf_jit_emit_common_epilogue() while ctx->seen might not
> be stable.
> 
> Only after the second pass we can be sure ctx->seen wont be changed.
> 
> Trying to avoid a second pass seems quite complex and probably
> not worth it.
> 
> Fixes: ce0761419faef ("powerpc/bpf: Implement support for tail calls")
> Signed-off-by: Eric Dumazet <edumazet@...gle.com>
> Cc: "Naveen N. Rao" <naveen.n.rao@...ux.ibm.com>
> Cc: Sandipan Das <sandipan@...ux.ibm.com>
> Cc: Benjamin Herrenschmidt <benh@...nel.crashing.org>
> Cc: Paul Mackerras <paulus@...ba.org>
> Cc: Michael Ellerman <mpe@...erman.id.au>
> Cc: Alexei Starovoitov <ast@...nel.org>
> Cc: Daniel Borkmann <daniel@...earbox.net>
> Cc: Martin KaFai Lau <kafai@...com>
> Cc: Song Liu <songliubraving@...com>
> Cc: Yonghong Song <yhs@...com>
> Cc: netdev@...r.kernel.org
> Cc: bpf@...r.kernel.org

Applied, thanks!