linux-kernel - Re: [PATCH v5 04/17] x86,ftrace: Fix ftrace_regs

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Date:   Wed, 22 Apr 2020 09:33:39 -0400
From:   Steven Rostedt <rostedt@...dmis.org>
To:     Peter Zijlstra <peterz@...radead.org>
Cc:     tglx@...utronix.de, jpoimboe@...hat.com,
        linux-kernel@...r.kernel.org, x86@...nel.org, mhiramat@...nel.org,
        mbenes@...e.cz, jthierry@...hat.com, alexandre.chartre@...cle.com
Subject: Re: [PATCH v5 04/17] x86,ftrace: Fix ftrace_regs_caller() unwind

On Wed, 22 Apr 2020 11:44:46 +0200
Peter Zijlstra <peterz@...radead.org> wrote:

> > Anyway, I added the below patch on top of your patches and it appears
> > to work. Thoughts?  
> 
> So can I push out those patches as is? :-) We can do this on top I
> suppose.

My tests are still running on your series. I want to make sure that nothing
breaks between them. If the tests succeed, then sure, I'm OK building on
top of this series.


> 
> Firstly; your patch isn't objtool clean:
> 
>   arch/x86/kernel/ftrace_64.o: warning: objtool: ftrace_regs_caller()+0x199: sibling call from callable instruction with modified stack frame
> 
> So 10 points deduction for that :-). You got the RET_OFFSET hint on the
> wrong sibling call.

Ah, I just did his patch quickly and made sure that it booted and ran
function tracing and direct calls. I didn't bother caring much about the
objtool annotations.

> 
> Secondly, yes, that ORIG_RAX issue for copies, that _might_ crash and
> burn, but who knows, you're jumping into uninitialized memory afaict. So
> that definitely needs fixing. I'm also not sure of stubbing out the
> test, that's actually more work than poking in the 1 extra ret and also
> gives unexpected behaviour. If anything we should poke an UD2 at the 1:
> location in the copy.

Well, that would crash the system just as well.

We could also just make the jnz into a nop (which would make the trampoline
slightly faster as well).

> 
> Over all, I'm thinking we should keep it as is. If you want to simplify
> the poking we can do something like the below; reading a known retq is
> daft.

I'm actually more concerned about the performance of the created
trampoline, as that is a fast path. I rather get rid of that jump.

I'll play a little more with it while the tests continue.

-- Steve



> 
> ---
> diff --git a/arch/x86/kernel/ftrace.c b/arch/x86/kernel/ftrace.c
> index 867c126ddabe..b334adbacb0e 100644
> --- a/arch/x86/kernel/ftrace.c
> +++ b/arch/x86/kernel/ftrace.c
> @@ -307,8 +307,6 @@ union ftrace_op_code_union {
>  	} __attribute__((packed));
>  };
>  
> -#define RET_SIZE		1
> -
>  static unsigned long
>  create_trampoline(struct ftrace_ops *ops, unsigned int *tramp_size)
>  {
> @@ -319,7 +317,6 @@ create_trampoline(struct ftrace_ops *ops, unsigned int *tramp_size)
>  	unsigned long offset;
>  	unsigned long npages;
>  	unsigned long size;
> -	unsigned long retq;
>  	unsigned long *ptr;
>  	void *trampoline;
>  	void *ip;
> @@ -347,11 +344,11 @@ create_trampoline(struct ftrace_ops *ops, unsigned int *tramp_size)
>  	 * the iret , as well as the address of the ftrace_ops this
>  	 * trampoline is used for.
>  	 */
> -	trampoline = alloc_tramp(size + RET_SIZE + sizeof(void *));
> +	trampoline = alloc_tramp(size + RET_INSN_SIZE + sizeof(void *));
>  	if (!trampoline)
>  		return 0;
>  
> -	*tramp_size = size + RET_SIZE + sizeof(void *);
> +	*tramp_size = size + RET_INSN_SIZE + sizeof(void *);
>  	npages = DIV_ROUND_UP(*tramp_size, PAGE_SIZE);
>  
>  	/* Copy ftrace_caller onto the trampoline memory */
> @@ -359,19 +356,13 @@ create_trampoline(struct ftrace_ops *ops, unsigned int *tramp_size)
>  	if (WARN_ON(ret < 0))
>  		goto fail;
>  
> -	ip = trampoline + size;
> -
>  	/* The trampoline ends with ret(q) */
> -	retq = (unsigned long)ftrace_stub;
> -	ret = probe_kernel_read(ip, (void *)retq, RET_SIZE);
> -	if (WARN_ON(ret < 0))
> -		goto fail;
> +	ip = trampoline + size;
> +	*(u8 *)ip = RET_INSN_OPCODE;
>  
>  	if (ops->flags & FTRACE_OPS_FL_SAVE_REGS) {
>  		ip = trampoline + (ftrace_regs_caller_ret - ftrace_regs_caller);
> -		ret = probe_kernel_read(ip, (void *)retq, RET_SIZE);
> -		if (WARN_ON(ret < 0))
> -			goto fail;
> +		*(u8 *)ip = RET_INSN_OPCODE;
>  	}
>  
>  	/*
> @@ -382,7 +373,7 @@ create_trampoline(struct ftrace_ops *ops, unsigned int *tramp_size)
>  	 * the global function_trace_op variable.
>  	 */
>  
> -	ptr = (unsigned long *)(trampoline + size + RET_SIZE);
> +	ptr = (unsigned long *)(trampoline + size + RET_INSN_SIZE);
>  	*ptr = (unsigned long)ops;
>  
>  	op_offset -= start_offset;