linux-kernel - Re: [PATCH v2 13/39] x86/livepatch: Validate __fentry_

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Date:   Thu, 24 Feb 2022 15:02:03 -0800
From:   Josh Poimboeuf <jpoimboe@...hat.com>
To:     Peter Zijlstra <peterz@...radead.org>
Cc:     x86@...nel.org, joao@...rdrivepizza.com, hjl.tools@...il.com,
        andrew.cooper3@...rix.com, linux-kernel@...r.kernel.org,
        ndesaulniers@...gle.com, keescook@...omium.org,
        samitolvanen@...gle.com, mark.rutland@....com,
        alyssa.milburn@...el.com, mbenes@...e.cz, rostedt@...dmis.org,
        mhiramat@...nel.org, alexei.starovoitov@...il.com
Subject: Re: [PATCH v2 13/39] x86/livepatch: Validate __fentry__ location

On Thu, Feb 24, 2022 at 03:51:51PM +0100, Peter Zijlstra wrote:
> Currently livepatch assumes __fentry__ lives at func+0, which is most
> likely untrue with IBT on. Instead make it use ftrace_location() by
> default which both validates and finds the actual ip if there is any
> in the same symbol.
> 
> Signed-off-by: Peter Zijlstra (Intel) <peterz@...radead.org>
> ---
>  arch/x86/include/asm/livepatch.h |    9 +++++++++
>  kernel/livepatch/patch.c         |    2 +-
>  2 files changed, 10 insertions(+), 1 deletion(-)
> 
> --- a/kernel/livepatch/patch.c
> +++ b/kernel/livepatch/patch.c
> @@ -133,7 +133,7 @@ static void notrace klp_ftrace_handler(u
>  #ifndef klp_get_ftrace_location
>  static unsigned long klp_get_ftrace_location(unsigned long faddr)
>  {
> -	return faddr;
> +	return ftrace_location(faddr);

Now that ftrace is doing the dirty work, I think this means we can get
rid of klp_get_ftrace_location() altogether:

diff --git a/arch/powerpc/include/asm/livepatch.h b/arch/powerpc/include/asm/livepatch.h
index 4fe018cc207b..7b9dcd51af32 100644
--- a/arch/powerpc/include/asm/livepatch.h
+++ b/arch/powerpc/include/asm/livepatch.h
@@ -19,16 +19,6 @@ static inline void klp_arch_set_pc(struct ftrace_regs *fregs, unsigned long ip)
 	regs_set_return_ip(regs, ip);
 }
 
-#define klp_get_ftrace_location klp_get_ftrace_location
-static inline unsigned long klp_get_ftrace_location(unsigned long faddr)
-{
-	/*
-	 * Live patch works only with -mprofile-kernel on PPC. In this case,
-	 * the ftrace location is always within the first 16 bytes.
-	 */
-	return ftrace_location_range(faddr, faddr + 16);
-}
-
 static inline void klp_init_thread_info(struct task_struct *p)
 {
 	/* + 1 to account for STACK_END_MAGIC */
diff --git a/kernel/livepatch/patch.c b/kernel/livepatch/patch.c
index fd295bbbcbc7..ed3464e68bda 100644
--- a/kernel/livepatch/patch.c
+++ b/kernel/livepatch/patch.c
@@ -124,19 +124,6 @@ static void notrace klp_ftrace_handler(unsigned long ip,
 	ftrace_test_recursion_unlock(bit);
 }
 
-/*
- * Convert a function address into the appropriate ftrace location.
- *
- * Usually this is just the address of the function, but on some architectures
- * it's more complicated so allow them to provide a custom behaviour.
- */
-#ifndef klp_get_ftrace_location
-static unsigned long klp_get_ftrace_location(unsigned long faddr)
-{
-	return ftrace_location(faddr);
-}
-#endif
-
 static void klp_unpatch_func(struct klp_func *func)
 {
 	struct klp_ops *ops;
@@ -153,8 +140,7 @@ static void klp_unpatch_func(struct klp_func *func)
 	if (list_is_singular(&ops->func_stack)) {
 		unsigned long ftrace_loc;
 
-		ftrace_loc =
-			klp_get_ftrace_location((unsigned long)func->old_func);
+		ftrace_loc = ftrace_location((unsigned long)func->old_func);
 		if (WARN_ON(!ftrace_loc))
 			return;