Re: [PATCH 3/3] x86/ftrace: Use text_poke()

[Daniel Bristot de Oliveira]

On 27/08/2019 20:06, Peter Zijlstra wrote:
> Move ftrace over to using the generic x86 text_poke functions; this
> avoids having a second/different copy of that code around.
> 
> This also avoids ftrace violating the (new) W^X rule and avoids
> fragmenting the kernel text page-tables, due to no longer having to
> toggle them RW.

I tested this patch, and it works... but it generates more IPIs than the
previous one.

> Cc: Steven Rostedt <rostedt@xxxxxxxxxxx>
> Cc: Daniel Bristot de Oliveira <bristot@xxxxxxxxxx>
> Signed-off-by: Peter Zijlstra (Intel) <peterz@xxxxxxxxxxxxx>
> ---
>  arch/x86/include/asm/ftrace.h |    2 
>  arch/x86/kernel/alternative.c |    4 
>  arch/x86/kernel/ftrace.c      |  630 ++++++------------------------------------
>  arch/x86/kernel/traps.c       |    9 
>  4 files changed, 93 insertions(+), 552 deletions(-)
> 
> --- a/arch/x86/include/asm/ftrace.h
> +++ b/arch/x86/include/asm/ftrace.h

[ ... jumping to the point ...]

> -
>  void ftrace_replace_code(int enable)
>  {
>  	struct ftrace_rec_iter *iter;
>  	struct dyn_ftrace *rec;
> -	const char *report = "adding breakpoints";
> -	int count = 0;
> +	const char *new, *old;
>  	int ret;
>  
>  	for_ftrace_rec_iter(iter) {
>  		rec = ftrace_rec_iter_record(iter);
>  
> -		ret = add_breakpoints(rec, enable);
> -		if (ret)
> -			goto remove_breakpoints;
> -		count++;
> -	}
> -
> -	run_sync();

ftrace was already batching the updates, for instance, causing 3 IPIs to enable
all functions. The text_poke() batching also works. But because of the limited
buffer [ see the reply to the patch 2/3 ], it is flushing the buffer during the
operation, causing more IPIs than the previous code. Using the 5.4-rc1 in a VM,
when enabling the function tracer, I see 250+ intermediate text_poke_finish()
because of a full buffer...

Would this be the case of trying to use a dynamically allocated buffer?

Thoughts?

-- Daniel

> -
> -	report = "updating code";
> -	count = 0;
> -
> -	for_ftrace_rec_iter(iter) {
> -		rec = ftrace_rec_iter_record(iter);
> -
> -		ret = add_update(rec, enable);
> -		if (ret)
> -			goto remove_breakpoints;
> -		count++;
> +		switch (ftrace_test_record(rec, enable)) {
> +		case FTRACE_UPDATE_IGNORE:
> +		default:
> +			continue;
> +
> +		case FTRACE_UPDATE_MAKE_CALL:
> +			old = ftrace_nop_replace();
> +			break;
> +
> +		case FTRACE_UPDATE_MODIFY_CALL:
> +		case FTRACE_UPDATE_MAKE_NOP:
> +			old = ftrace_call_replace(rec->ip, ftrace_get_addr_curr(rec));
> +			break;
> +		};
> +
> +		ret = ftrace_verify_code(rec->ip, old);
> +		if (ret) {
> +			ftrace_bug(ret, rec);
> +			return;
> +		}
>  	}
>  
> -	run_sync();
> -
> -	report = "removing breakpoints";
> -	count = 0;
> -
>  	for_ftrace_rec_iter(iter) {
>  		rec = ftrace_rec_iter_record(iter);
>  
> -		ret = finish_update(rec, enable);
> -		if (ret)
> -			goto remove_breakpoints;
> -		count++;
> -	}
> -
> -	run_sync();
> -
> -	return;
> +		switch (ftrace_test_record(rec, enable)) {
> +		case FTRACE_UPDATE_IGNORE:
> +		default:
> +			continue;
> +
> +		case FTRACE_UPDATE_MAKE_CALL:
> +		case FTRACE_UPDATE_MODIFY_CALL:
> +			new = ftrace_call_replace(rec->ip, ftrace_get_addr_new(rec));
> +			break;
> +
> +		case FTRACE_UPDATE_MAKE_NOP:
> +			new = ftrace_nop_replace();
> +			break;
> +		};
>  
> - remove_breakpoints:
> -	pr_warn("Failed on %s (%d):\n", report, count);
> -	ftrace_bug(ret, rec);
> -	for_ftrace_rec_iter(iter) {
> -		rec = ftrace_rec_iter_record(iter);
> -		/*
> -		 * Breakpoints are handled only when this function is in
> -		 * progress. The system could not work with them.
> -		 */
> -		if (remove_breakpoint(rec))
> -			BUG();
> +		text_poke_queue((void *)rec->ip, new, MCOUNT_INSN_SIZE, NULL);
> +		ftrace_update_record(rec, enable);
>  	}
> -	run_sync();
> -}
> -
> -static int
> -ftrace_modify_code(unsigned long ip, unsigned const char *old_code,
> -		   unsigned const char *new_code)
> -{
> -	int ret;
> -
> -	ret = add_break(ip, old_code);
> -	if (ret)
> -		goto out;
> -
> -	run_sync();
> -
> -	ret = add_update_code(ip, new_code);
> -	if (ret)
> -		goto fail_update;
> -
> -	run_sync();
> -
> -	ret = ftrace_write(ip, new_code, 1);
> -	/*
> -	 * The breakpoint is handled only when this function is in progress.
> -	 * The system could not work if we could not remove it.
> -	 */
> -	BUG_ON(ret);
> - out:
> -	run_sync();
> -	return ret;
> -
> - fail_update:
> -	/* Also here the system could not work with the breakpoint */
> -	if (ftrace_write(ip, old_code, 1))
> -		BUG();
> -	goto out;
> +	text_poke_finish();
>  }
>  
>  void arch_ftrace_update_code(int command)
>  {
> -	/* See comment above by declaration of modifying_ftrace_code */
> -	atomic_inc(&modifying_ftrace_code);
> -
>  	ftrace_modify_all_code(command);
> -
> -	atomic_dec(&modifying_ftrace_code);
>  }
>  
>  int __init ftrace_dyn_arch_init(void)
> @@ -828,11 +394,7 @@ create_trampoline(struct ftrace_ops *ops
>  
>  	set_vm_flush_reset_perms(trampoline);
>  
> -	/*
> -	 * Module allocation needs to be completed by making the page
> -	 * executable. The page is still writable, which is a security hazard,
> -	 * but anyhow ftrace breaks W^X completely.
> -	 */
> +	set_memory_ro((unsigned long)trampoline, npages);
>  	set_memory_x((unsigned long)trampoline, npages);
>  	return (unsigned long)trampoline;
>  fail:
> @@ -859,11 +421,10 @@ static unsigned long calc_trampoline_cal
>  void arch_ftrace_update_trampoline(struct ftrace_ops *ops)
>  {
>  	ftrace_func_t func;
> -	unsigned char *new;
>  	unsigned long offset;
>  	unsigned long ip;
>  	unsigned int size;
> -	int ret, npages;
> +	const char *new;
>  
>  	if (ops->trampoline) {
>  		/*
> @@ -872,14 +433,11 @@ void arch_ftrace_update_trampoline(struc
>  		 */
>  		if (!(ops->flags & FTRACE_OPS_FL_ALLOC_TRAMP))
>  			return;
> -		npages = PAGE_ALIGN(ops->trampoline_size) >> PAGE_SHIFT;
> -		set_memory_rw(ops->trampoline, npages);
>  	} else {
>  		ops->trampoline = create_trampoline(ops, &size);
>  		if (!ops->trampoline)
>  			return;
>  		ops->trampoline_size = size;
> -		npages = PAGE_ALIGN(size) >> PAGE_SHIFT;
>  	}
>  
>  	offset = calc_trampoline_call_offset(ops->flags & FTRACE_OPS_FL_SAVE_REGS);
> @@ -887,15 +445,11 @@ void arch_ftrace_update_trampoline(struc
>  
>  	func = ftrace_ops_get_func(ops);
>  
> -	ftrace_update_func_call = (unsigned long)func;
> -
> +	mutex_lock(&text_mutex);
>  	/* Do a safe modify in case the trampoline is executing */
>  	new = ftrace_call_replace(ip, (unsigned long)func);
> -	ret = update_ftrace_func(ip, new);
> -	set_memory_ro(ops->trampoline, npages);
> -
> -	/* The update should never fail */
> -	WARN_ON(ret);
> +	text_poke_bp((void *)ip, new, MCOUNT_INSN_SIZE, NULL);
> +	mutex_unlock(&text_mutex);
>  }
>  
>  /* Return the address of the function the trampoline calls */
> @@ -981,19 +535,18 @@ void arch_ftrace_trampoline_free(struct
>  #ifdef CONFIG_DYNAMIC_FTRACE
>  extern void ftrace_graph_call(void);
>  
> -static unsigned char *ftrace_jmp_replace(unsigned long ip, unsigned long addr)
> +static const char *ftrace_jmp_replace(unsigned long ip, unsigned long addr)
>  {
> -	return ftrace_text_replace(0xe9, ip, addr);
> +	return ftrace_text_replace(JMP32_INSN_OPCODE, ip, addr);
>  }
>  
>  static int ftrace_mod_jmp(unsigned long ip, void *func)
>  {
> -	unsigned char *new;
> +	const char *new;
>  
> -	ftrace_update_func_call = 0UL;
>  	new = ftrace_jmp_replace(ip, (unsigned long)func);
> -
> -	return update_ftrace_func(ip, new);
> +	text_poke_bp((void *)ip, new, MCOUNT_INSN_SIZE, NULL); // BATCH
> +	return 0;
>  }
>  
>  int ftrace_enable_ftrace_graph_caller(void)
> @@ -1019,10 +572,9 @@ int ftrace_disable_ftrace_graph_caller(v
>  void prepare_ftrace_return(unsigned long self_addr, unsigned long *parent,
>  			   unsigned long frame_pointer)
>  {
> +	unsigned long return_hooker = (unsigned long)&return_to_handler;
>  	unsigned long old;
>  	int faulted;
> -	unsigned long return_hooker = (unsigned long)
> -				&return_to_handler;
>  
>  	/*
>  	 * When resuming from suspend-to-ram, this function can be indirectly
> --- a/arch/x86/kernel/traps.c
> +++ b/arch/x86/kernel/traps.c
> @@ -568,15 +568,6 @@ NOKPROBE_SYMBOL(do_general_protection);
>  
>  dotraplinkage void notrace do_int3(struct pt_regs *regs, long error_code)
>  {
> -#ifdef CONFIG_DYNAMIC_FTRACE
> -	/*
> -	 * ftrace must be first, everything else may cause a recursive crash.
> -	 * See note by declaration of modifying_ftrace_code in ftrace.c
> -	 */
> -	if (unlikely(atomic_read(&modifying_ftrace_code)) &&
> -	    ftrace_int3_handler(regs))
> -		return;
> -#endif
>  	if (poke_int3_handler(regs))
>  		return;
>  
> 
>