linux-kernel - Re: [PATCH v2 2/8] x86: return error code in text_poke

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Message-ID: <20131108095323.57cb4993@gandalf.local.home>
Date:	Fri, 8 Nov 2013 09:53:23 -0500
From:	Steven Rostedt <rostedt@...dmis.org>
To:	Petr Mladek <pmladek@...e.cz>
Cc:	Frederic Weisbecker <fweisbec@...il.com>,
	Masami Hiramatsu <masami.hiramatsu.pt@...achi.com>,
	"Paul E. McKenney" <paulmck@...ux.vnet.ibm.com>,
	Jiri Kosina <jkosina@...e.cz>, linux-kernel@...r.kernel.org,
	x86@...nel.org
Subject: Re: [PATCH v2 2/8] x86: return error code in text_poke_bp

On Fri,  8 Nov 2013 10:12:06 +0100
Petr Mladek <pmladek@...e.cz> wrote:

> We would like to use text_poke_bp in the dynamic ftrace which want to
> know about errors. For example, it informs about them in the ftrace log.
> 
> Let's return the error code instead of the address. The address was just copied
> from the first parameter, so it was no extra information. The return value
> has not been used anywhere yet.
> 
> There is a question whether we should recover the original opcode when
> the second or third text_poke_part fails in text_poke_bp. Well, the errors
> were ignored until now. It did not cause any real life problems. There is
> really small chance that the first byte (int3) can be written and the other
> parts of the code can not be modified. It is probably not worth the extra
> complexity.

I agree with Masami. If the first part succeeds, still check the other
parts, and if they don't succeed, then just do a BUG().


> 
> Signed-off-by: Petr Mladek <pmladek@...e.cz>
> ---
>  arch/x86/include/asm/alternative.h |  3 ++-
>  arch/x86/kernel/alternative.c      | 18 +++++++++++++-----
>  2 files changed, 15 insertions(+), 6 deletions(-)
> 
> diff --git a/arch/x86/include/asm/alternative.h b/arch/x86/include/asm/alternative.h
> index 0a3f9c9..f2343d8 100644
> --- a/arch/x86/include/asm/alternative.h
> +++ b/arch/x86/include/asm/alternative.h
> @@ -226,6 +226,7 @@ extern void *text_poke_early(void *addr, const void *opcode, size_t len);
>   */
>  extern void *text_poke(void *addr, const void *opcode, size_t len);
>  extern int poke_int3_handler(struct pt_regs *regs);
> -extern void *text_poke_bp(void *addr, const void *opcode, size_t len, void *handler);
> +extern int text_poke_bp(void *addr, const void *opcode, size_t len,
> +			void *handler);
>  
>  #endif /* _ASM_X86_ALTERNATIVE_H */
> diff --git a/arch/x86/kernel/alternative.c b/arch/x86/kernel/alternative.c
> index 0586dc1..c459e62 100644
> --- a/arch/x86/kernel/alternative.c
> +++ b/arch/x86/kernel/alternative.c
> @@ -673,9 +673,10 @@ int poke_int3_handler(struct pt_regs *regs)
>   *
>   * Note: must be called under text_mutex.
>   */
> -void *text_poke_bp(void *addr, const void *opcode, size_t len, void *handler)
> +int text_poke_bp(void *addr, const void *opcode, size_t len, void *handler)
>  {
>  	unsigned char int3 = 0xcc;
> +	int ret;
>  
>  	bp_int3_handler = handler;
>  	bp_int3_addr = (u8 *)addr + sizeof(int3);
> @@ -687,15 +688,19 @@ void *text_poke_bp(void *addr, const void *opcode, size_t len, void *handler)
>  	 */
>  	smp_wmb();
>  
> -	text_poke_part(addr, &int3, sizeof(int3));
> +	ret = text_poke_part(addr, &int3, sizeof(int3));
> +	if (unlikely(ret))
> +		goto fail;

Ah! you have it return an error code. May need to do that for all users
of text_poke_part().

-- Steve

>  
>  	on_each_cpu(do_sync_core, NULL, 1);
>  
>  	if (len - sizeof(int3) > 0) {
>  		/* patch all but the first byte */
> -		text_poke_part((char *)addr + sizeof(int3),
> +		ret = text_poke_part((char *)addr + sizeof(int3),
>  			       (const char *) opcode + sizeof(int3),
>  			       len - sizeof(int3));
> +		if (unlikely(ret))
> +			goto fail;
>  		/*
>  		 * According to Intel, this core syncing is very likely
>  		 * not necessary and we'd be safe even without it. But
> @@ -705,13 +710,16 @@ void *text_poke_bp(void *addr, const void *opcode, size_t len, void *handler)
>  	}
>  
>  	/* patch the first byte */
> -	text_poke_part(addr, opcode, sizeof(int3));
> +	ret = text_poke_part(addr, opcode, sizeof(int3));
> +	if (unlikely(ret))
> +		goto fail;
>  
>  	on_each_cpu(do_sync_core, NULL, 1);
>  
> +fail:
>  	bp_patching_in_progress = false;
>  	smp_wmb();
>  
> -	return addr;
> +	return ret;
>  }
>  

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