From mboxrd@z Thu Jan 1 00:00:00 1970 Received: from smtp.kernel.org (aws-us-west-2-korg-mail-1.web.codeaurora.org [10.30.226.201]) (using TLSv1.2 with cipher ECDHE-RSA-AES256-GCM-SHA384 (256/256 bits)) (No client certificate requested) by smtp.subspace.kernel.org (Postfix) with ESMTPS id 13C8A11712 for ; Wed, 20 Mar 2024 03:47:48 +0000 (UTC) Authentication-Results: smtp.subspace.kernel.org; arc=none smtp.client-ip=10.30.226.201 ARC-Seal:i=1; a=rsa-sha256; d=subspace.kernel.org; s=arc-20240116; t=1710906469; cv=none; b=ILyz2YvYP7CfRjjylwViPtOPsg9Y6SgVwTh5ax0EsgcBor6ZMccto2Mk2ynkfr3MKc/CQ9HKyopuzhFhwcxQQpdkEsK712vGmE4k0es1Ld9SGnvpVXizdlX6Nb4mxBBLNpWAfQwi/q0gtJUImdaAPGdP32IQuuTF405HfnpE+ls= ARC-Message-Signature:i=1; a=rsa-sha256; d=subspace.kernel.org; s=arc-20240116; t=1710906469; c=relaxed/simple; bh=+DQQdj/kNsX9HObb+sHPm0lHmfIq8arihGWeKEmlAeU=; h=Date:From:To:Cc:Subject:Message-Id:In-Reply-To:References: Mime-Version:Content-Type; b=jgXP4gC4TENJcgu+thq9fub6ECc2qDtaTkFWsSVWkgtdDsVEl9ufWFeIFpaYi/JnC9bLcHEV1kvQ5G4k/MPalfvyxCv2AQ+5B6OF7Iu52DUYERxtfm6NbE6X6n+arrD/3UeQB3qeACTfFjz4dYkDO6+H4BQ8znMZeTcKCzh4swc= ARC-Authentication-Results:i=1; smtp.subspace.kernel.org; dkim=pass (2048-bit key) header.d=kernel.org header.i=@kernel.org header.b=GxMwANku; arc=none smtp.client-ip=10.30.226.201 Authentication-Results: smtp.subspace.kernel.org; dkim=pass (2048-bit key) header.d=kernel.org header.i=@kernel.org header.b="GxMwANku" Received: by smtp.kernel.org (Postfix) with ESMTPSA id C1C37C433C7; Wed, 20 Mar 2024 03:47:46 +0000 (UTC) DKIM-Signature: v=1; a=rsa-sha256; c=relaxed/simple; d=kernel.org; s=k20201202; t=1710906468; bh=+DQQdj/kNsX9HObb+sHPm0lHmfIq8arihGWeKEmlAeU=; h=Date:From:To:Cc:Subject:In-Reply-To:References:From; b=GxMwANkuqerKzhuMnry93qGDLy9/J9PgAR1JoW5bQvlvNmXFGB8Me3LqjakEGHkSz /6trYSN1D3sP2eBCflz4pDaxpw+hT9Us+YMpCOcv2fGtNzIOAybkFIMofX8gF3Qzbz egS21zaEV7l982pdJBSHwk6E1oaHa19ZpmMIOw+ZQtdsM/EgHWezV+rvdH4G+5JWiw Ov7Wt56IOdlr95LJdylXkPjnd4HjSbyMXsT/TD8IV1w8ZCDXE6nihtmgFROpmQeQ1w F5Z7dp6wKC5ewsdOEf945u7aYZEMoGkxxPJBB9zqdvg3E5uFURfLi3xS6SriP2Kici tWt0h7/qZOL7Q== Date: Wed, 20 Mar 2024 12:47:42 +0900 From: Masami Hiramatsu (Google) To: Andrii Nakryiko Cc: bpf@vger.kernel.org, ast@kernel.org, daniel@iogearbox.net, martin.lau@kernel.org, kernel-team@meta.com, Masami Hiramatsu , Peter Zijlstra Subject: Re: [PATCH bpf-next] bpf: avoid get_kernel_nofault() to fetch kprobe entry IP Message-Id: <20240320124742.5652f47b8f6dfea24cf84ce9@kernel.org> In-Reply-To: <20240319212013.1046779-1-andrii@kernel.org> References: <20240319212013.1046779-1-andrii@kernel.org> X-Mailer: Sylpheed 3.7.0 (GTK+ 2.24.33; x86_64-pc-linux-gnu) Precedence: bulk X-Mailing-List: bpf@vger.kernel.org List-Id: List-Subscribe: List-Unsubscribe: Mime-Version: 1.0 Content-Type: text/plain; charset=US-ASCII Content-Transfer-Encoding: 7bit On Tue, 19 Mar 2024 14:20:13 -0700 Andrii Nakryiko wrote: > get_kernel_nofault() (or, rather, underlying copy_from_kernel_nofault()) > is not free and it does pop up in performance profiles when > kprobes are heavily utilized with CONFIG_X86_KERNEL_IBT=y config. > > Let's avoid using it if we know that fentry_ip - 4 can't cross page > boundary. We do that by masking lowest 12 bits and checking if they are > >= 4, in which case we can do direct memory read. > > Another benefit (and actually what caused a closer look at this part of > code) is that now LBR record is (typically) not wasted on > copy_from_kernel_nofault() call and code, which helps tools like > retsnoop that grab LBR records from inside BPF code in kretprobes. Hmm, we may better to have this function in kprobe side and store a flag which such architecture dependent offset is added. That is more natural. Thanks! > > Cc: Masami Hiramatsu > Cc: Peter Zijlstra > Signed-off-by: Andrii Nakryiko > --- > kernel/trace/bpf_trace.c | 12 +++++++++--- > 1 file changed, 9 insertions(+), 3 deletions(-) > > diff --git a/kernel/trace/bpf_trace.c b/kernel/trace/bpf_trace.c > index 0a5c4efc73c3..f81adabda38c 100644 > --- a/kernel/trace/bpf_trace.c > +++ b/kernel/trace/bpf_trace.c > @@ -1053,9 +1053,15 @@ static unsigned long get_entry_ip(unsigned long fentry_ip) > { > u32 instr; > > - /* Being extra safe in here in case entry ip is on the page-edge. */ > - if (get_kernel_nofault(instr, (u32 *) fentry_ip - 1)) > - return fentry_ip; > + /* We want to be extra safe in case entry ip is on the page edge, > + * but otherwise we need to avoid get_kernel_nofault()'s overhead. > + */ > + if ((fentry_ip & ~PAGE_MASK) < ENDBR_INSN_SIZE) { > + if (get_kernel_nofault(instr, (u32 *)(fentry_ip - ENDBR_INSN_SIZE))) > + return fentry_ip; > + } else { > + instr = *(u32 *)(fentry_ip - ENDBR_INSN_SIZE); > + } > if (is_endbr(instr)) > fentry_ip -= ENDBR_INSN_SIZE; > return fentry_ip; > -- > 2.43.0 > -- Masami Hiramatsu (Google)