Re: [RFC PATCH 6/6] x86/jump_label,x86/alternatives: Batch jump label transformations

[Steven Rostedt]

On Mon,  8 Oct 2018 14:53:05 +0200
Daniel Bristot de Oliveira <bristot@xxxxxxxxxx> wrote:

> diff --git a/arch/x86/include/asm/jump_label.h b/arch/x86/include/asm/jump_label.h
> index 8c0de4282659..d61c476046fe 100644
> --- a/arch/x86/include/asm/jump_label.h
> +++ b/arch/x86/include/asm/jump_label.h
> @@ -15,6 +15,8 @@
>  #error asm/jump_label.h included on a non-jump-label kernel
>  #endif
>  
> +#define HAVE_JUMP_LABEL_BATCH
> +
>  #define JUMP_LABEL_NOP_SIZE 5
>  
>  #ifdef CONFIG_X86_64
> diff --git a/arch/x86/include/asm/text-patching.h b/arch/x86/include/asm/text-patching.h
> index e85ff65c43c3..a28230f09d72 100644
> --- a/arch/x86/include/asm/text-patching.h
> +++ b/arch/x86/include/asm/text-patching.h
> @@ -18,6 +18,14 @@ static inline void apply_paravirt(struct paravirt_patch_site *start,
>  #define __parainstructions_end	NULL
>  #endif
>  
> +struct text_to_poke {
> +	struct list_head list;
> +	void *opcode;
> +	void *addr;
> +	void *handler;
> +	size_t len;
> +};
> +
>  extern void *text_poke_early(void *addr, const void *opcode, size_t len);
>  
>  /*
> @@ -37,6 +45,7 @@ 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 void text_poke_bp_list(struct list_head *entry_list);
>  extern int after_bootmem;
>  
>  #endif /* _ASM_X86_TEXT_PATCHING_H */
> diff --git a/arch/x86/kernel/alternative.c b/arch/x86/kernel/alternative.c
> index a4c83cb49cd0..3bd502ea4c53 100644
> --- a/arch/x86/kernel/alternative.c
> +++ b/arch/x86/kernel/alternative.c
> @@ -735,9 +735,12 @@ static void do_sync_core(void *info)
>  
>  static bool bp_patching_in_progress;
>  static void *bp_int3_handler, *bp_int3_addr;
> +struct list_head *bp_list;
>  
>  int poke_int3_handler(struct pt_regs *regs)
>  {
> +	void *ip;
> +	struct text_to_poke *tp;
>  	/*
>  	 * Having observed our INT3 instruction, we now must observe
>  	 * bp_patching_in_progress.
> @@ -753,21 +756,38 @@ int poke_int3_handler(struct pt_regs *regs)
>  	if (likely(!bp_patching_in_progress))
>  		return 0;
>  
> -	if (user_mode(regs) || regs->ip != (unsigned long)bp_int3_addr)
> +	if (user_mode(regs))
>  		return 0;
>  
> -	/* set up the specified breakpoint handler */
> -	regs->ip = (unsigned long) bp_int3_handler;
> +	/*
> +	 * Single poke.
> +	 */
> +	if (bp_int3_addr) {
> +		if (regs->ip == (unsigned long) bp_int3_addr) {
> +			regs->ip = (unsigned long) bp_int3_handler;
> +			return 1;
> +		}
> +		return 0;
> +	}
>  
> -	return 1;
> +	/*
> +	 * Batch mode.
> +	 */
> +	ip = (void *) regs->ip - sizeof(unsigned char);
> +	list_for_each_entry(tp, bp_list, list) {
> +		if (ip == tp->addr) {
> +			/* set up the specified breakpoint handler */
> +			regs->ip = (unsigned long) tp->handler;

I hate this loop. Makes hitting the static branch (which is probably in
a fast path, otherwise why use static branches?), do a O(n) loop of
batch updates. You may not have that many, but why not optimize it?

Can we make an array of the handlers, in sorted order, then we simply
do a binary search for the ip involved? Turning this from O(n) to
O(log2(n)).

As Jason mentioned. If you set aside a page for processing batches up
to PAGE / (sizeof tp) then you can also make it sorted and replace the
iteration with a loop.

-- Steve


> +			return 1;
> +		}
> +	}
>  
> +	return 0;
>  }
>  
>  static void text_poke_bp_set_handler(void *addr, void *handler,
>  				     unsigned char int3)
>  {
> -	bp_int3_handler = handler;
> -	bp_int3_addr = (u8 *)addr + sizeof(int3);
>  	text_poke(addr, &int3, sizeof(int3));
>  }
>  
> @@ -812,6 +832,9 @@ void *text_poke_bp(void *addr, const void *opcode, size_t len, void *handler)
>  
>  	lockdep_assert_held(&text_mutex);
>  
> +	bp_int3_handler = handler;
> +	bp_int3_addr = (u8 *)addr + sizeof(int3);
> +
>  	bp_patching_in_progress = true;
>  	/*
>  	 * Corresponding read barrier in int3 notifier for making sure the
> @@ -841,7 +864,53 @@ void *text_poke_bp(void *addr, const void *opcode, size_t len, void *handler)
>  	 * the writing of the new instruction.
>  	 */
>  	bp_patching_in_progress = false;
> -
> +	bp_int3_handler = bp_int3_addr = 0;
>  	return addr;
>  }
>  
> +void text_poke_bp_list(struct list_head *entry_list)
> +{
> +	unsigned char int3 = 0xcc;
> +	int patched_all_but_first = 0;
> +	struct text_to_poke *tp;
> +
> +	bp_list = entry_list;
> +	bp_patching_in_progress = true;
> +	/*
> +	 * Corresponding read barrier in int3 notifier for making sure the
> +	 * in_progress and handler are correctly ordered wrt. patching.
> +	 */
> +	smp_wmb();
> +
> +	list_for_each_entry(tp, entry_list, list)
> +		text_poke_bp_set_handler(tp->addr, tp->handler, int3);
> +
> +	on_each_cpu(do_sync_core, NULL, 1);
> +
> +	list_for_each_entry(tp, entry_list, list) {
> +		if (tp->len - sizeof(int3) > 0) {
> +			patch_all_but_first_byte(tp->addr, tp->opcode, tp->len, int3);
> +			patched_all_but_first++;
> +		}
> +	}
> +
> +	if (patched_all_but_first) {
> +		/*
> +		 * According to Intel, this core syncing is very likely
> +		 * not necessary and we'd be safe even without it. But
> +		 * better safe than sorry (plus there's not only Intel).
> +		 */
> +		on_each_cpu(do_sync_core, NULL, 1);
> +	}
> +
> +	list_for_each_entry(tp, entry_list, list)
> +		patch_first_byte(tp->addr, tp->opcode, int3);
> +
> +	on_each_cpu(do_sync_core, NULL, 1);
> +	/*
> +	 * sync_core() implies an smp_mb() and orders this store against
> +	 * the writing of the new instruction.
> +	 */
> +	bp_list = 0;
> +	bp_patching_in_progress = false;
> +}