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Subject: Re: [PATCH net] powerpc/bpf: fix tail call implementation
From:   David Miller <davem@davemloft.net>
In-Reply-To: <20191101033444.143741-1-edumazet@google.com>
References: <20191101033444.143741-1-edumazet@google.com>
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From: Eric Dumazet <edumazet@google.com>
Date: Thu, 31 Oct 2019 20:34:44 -0700

> 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@google.com>

I am anticipating this will go via the bpf tree.