From: Petr Mladek <pmladek@suse.com>
Subject: Re: [PATCH v3 21/27] x86/ftrace: Adapt function tracing for PIE
	support
Date: Thu, 24 May 2018 13:40:24 +0200
Message-ID: <20180524114024.pa67zjipy5qcg4tm@pathway.suse.cz>
References: <20180523195421.180248-1-thgarnie@google.com>
	<20180523195421.180248-22-thgarnie@google.com>
Mime-Version: 1.0
Content-Type: text/plain; charset="us-ascii"
Content-Transfer-Encoding: 7bit
Cc: Kate Stewart <kstewart@linuxfoundation.org>,
	Nicolas Pitre <nicolas.pitre@linaro.org>, x86@kernel.org,
	Sergey Senozhatsky <sergey.senozhatsky.work@gmail.com>,
	kvm@vger.kernel.org, Len Brown <len.brown@intel.com>,
	Peter Zijlstra <peterz@infradead.org>,
	Yonghong Song <yhs@fb.com>, Christopher Li <sparse@chrisli.org>,
	Dave Hansen <dave.hansen@linux.intel.com>,
	Dominik Brodowski <linux@dominikbrodowski.net>,
	linux-kernel@vger.kernel.org,
	Masahiro Yamada <yamada.masahiro@socionext.com>,
	Jan Beulich <JBeulich@suse.com>, Pavel Machek <pavel@ucw.cz>,
	"H . Peter Anvin" <hpa@zytor.com>, kernel-hardening@lists.openwall.com,
	Christoph Lameter <cl@linux.com>,
	Alok Kataria <akataria@vmware.com>, linux-doc@vger.kernel.org,
	linux-arch@vger.kernel.org, Jonathan Corbet <corbet@lwn.net>,
	Herbert Xu <herbert@gondor.apana.org.au>,
	Baoquan He <bhe@redhat.com>, David Woodhouse <dwmw@amazon.
To: Thomas Garnier <thgarnie@google.com>, Steven Rostedt <rostedt@goodmis.org>
Content-Disposition: inline
In-Reply-To: <20180523195421.180248-22-thgarnie@google.com>
Sender: virtualization-bounces@lists.linux-foundation.org
Errors-To: virtualization-bounces@lists.linux-foundation.org

On Wed 2018-05-23 12:54:15, Thomas Garnier wrote:
> When using -fPIE/PIC with function tracing, the compiler generates a
> call through the GOT (call *__fentry__@GOTPCREL). This instruction
> takes 6 bytes instead of 5 on the usual relative call.
> 
> If PIE is enabled, replace the 6th byte of the GOT call by a 1-byte nop
> so ftrace can handle the previous 5-bytes as before.
> 
> Position Independent Executable (PIE) support will allow to extended the
> KASLR randomization range below the -2G memory limit.
> 
> Signed-off-by: Thomas Garnier <thgarnie@google.com>
> ---
>  arch/x86/include/asm/ftrace.h   |  6 +++--
>  arch/x86/include/asm/sections.h |  4 ++++
>  arch/x86/kernel/ftrace.c        | 42 +++++++++++++++++++++++++++++++--
>  3 files changed, 48 insertions(+), 4 deletions(-)
> 
> diff --git a/arch/x86/include/asm/ftrace.h b/arch/x86/include/asm/ftrace.h
> index c18ed65287d5..8f2decce38d8 100644
> --- a/arch/x86/include/asm/ftrace.h
> +++ b/arch/x86/include/asm/ftrace.h
> @@ -25,9 +25,11 @@ extern void __fentry__(void);
>  static inline unsigned long ftrace_call_adjust(unsigned long addr)
>  {
>  	/*
> -	 * addr is the address of the mcount call instruction.
> -	 * recordmcount does the necessary offset calculation.
> +	 * addr is the address of the mcount call instruction. PIE has always a
> +	 * byte added to the start of the function.
>  	 */
> +	if (IS_ENABLED(CONFIG_X86_PIE))
> +		addr -= 1;

This seems to modify the address even for modules that are _not_ compiled with
PIE, see below.

>  	return addr;
>  }
>  
> diff --git a/arch/x86/kernel/ftrace.c b/arch/x86/kernel/ftrace.c
> index 01ebcb6f263e..73b3c30cb7a3 100644
> --- a/arch/x86/kernel/ftrace.c
> +++ b/arch/x86/kernel/ftrace.c
> @@ -135,6 +135,44 @@ ftrace_modify_code_direct(unsigned long ip, unsigned const char *old_code,
>  	return 0;
>  }
>  
> +/* Bytes before call GOT offset */
> +const unsigned char got_call_preinsn[] = { 0xff, 0x15 };
> +
> +static int
> +ftrace_modify_initial_code(unsigned long ip, unsigned const char *old_code,
> +			   unsigned const char *new_code)
> +{
> +	unsigned char replaced[MCOUNT_INSN_SIZE + 1];
> +
> +	ftrace_expected = old_code;
> +
> +	/*
> +	 * If PIE is not enabled or no GOT call was found, default to the
> +	 * original approach to code modification.
> +	 */
> +	if (!IS_ENABLED(CONFIG_X86_PIE) ||
> +	    probe_kernel_read(replaced, (void *)ip, sizeof(replaced)) ||
> +	    memcmp(replaced, got_call_preinsn, sizeof(got_call_preinsn)))
> +		return ftrace_modify_code_direct(ip, old_code, new_code);

And this looks like an attempt to handle modules compiled without
PIE. Does it works with the right ip in that case?

I wonder if a better solution would be to update
scripts/recordmcount.c to store the incremented location into the module.

IMPORTANT: I have only vague picture about how this all works. It is
possible that I am completely wrong. The code might be correct,
especially if you tested this situation.

Best Regards,
Petr