linux-kernel - Re: [PATCH v2 12/39] x86/ibt,ftrace: Search for __fentry_

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Date:   Wed, 02 Mar 2022 21:55:58 +0530
From:   "Naveen N. Rao" <naveen.n.rao@...ux.ibm.com>
To:     andrew.cooper3@...rix.com, hjl.tools@...il.com,
        joao@...rdrivepizza.com, jpoimboe@...hat.com,
        Peter Zijlstra <peterz@...radead.org>, x86@...nel.org
Cc:     alexei.starovoitov@...il.com, alyssa.milburn@...el.com,
        keescook@...omium.org, linux-kernel@...r.kernel.org,
        mark.rutland@....com, mbenes@...e.cz, mhiramat@...nel.org,
        ndesaulniers@...gle.com, rostedt@...dmis.org,
        samitolvanen@...gle.com
Subject: Re: [PATCH v2 12/39] x86/ibt,ftrace: Search for __fentry__ location

Peter Zijlstra wrote:
> Have ftrace_location() search the symbol for the __fentry__ location
> when it isn't at func+0 and use this for {,un}register_ftrace_direct().
> 
> This avoids a whole bunch of assumptions about __fentry__ being at
> func+0.
> 
> Suggested-by: Steven Rostedt <rostedt@...dmis.org>
> Signed-off-by: Peter Zijlstra (Intel) <peterz@...radead.org>
> ---
>  kernel/trace/ftrace.c |   30 ++++++++++++++++++++++++++++--
>  1 file changed, 28 insertions(+), 2 deletions(-)
> 
> --- a/kernel/trace/ftrace.c
> +++ b/kernel/trace/ftrace.c
> @@ -1578,7 +1578,24 @@ unsigned long ftrace_location_range(unsi
>   */
>  unsigned long ftrace_location(unsigned long ip)
>  {
> -	return ftrace_location_range(ip, ip);
> +	struct dyn_ftrace *rec;
> +	unsigned long offset;
> +	unsigned long size;
> +
> +	rec = lookup_rec(ip, ip);
> +	if (!rec) {
> +		if (!kallsyms_lookup_size_offset(ip, &size, &offset))
> +			goto out;
> +
> +		if (!offset)
> +			rec = lookup_rec(ip - offset, (ip - offset) + size);
> +	}
> +
> +	if (rec)
> +		return rec->ip;
> +
> +out:
> +	return 0;
>  }
>  
>  /**
> @@ -5110,11 +5127,16 @@ int register_ftrace_direct(unsigned long
>  	struct ftrace_func_entry *entry;
>  	struct ftrace_hash *free_hash = NULL;
>  	struct dyn_ftrace *rec;
> -	int ret = -EBUSY;
> +	int ret = -ENODEV;
>  
>  	mutex_lock(&direct_mutex);
>  
> +	ip = ftrace_location(ip);
> +	if (!ip)
> +		goto out_unlock;
> +
>  	/* See if there's a direct function at @ip already */
> +	ret = -EBUSY;
>  	if (ftrace_find_rec_direct(ip))
>  		goto out_unlock;

I think some of the validation at this point can be removed (diff below).

>  
> @@ -5222,6 +5244,10 @@ int unregister_ftrace_direct(unsigned lo
>  
>  	mutex_lock(&direct_mutex);
>  
> +	ip = ftrace_location(ip);
> +	if (!ip)
> +		goto out_unlock;
> +
>  	entry = find_direct_entry(&ip, NULL);
>  	if (!entry)
>  		goto out_unlock;

We should also update modify_ftrace_direct(). An incremental diff below.


- Naveen


---
diff --git a/kernel/trace/ftrace.c b/kernel/trace/ftrace.c
index 65d7553668ca3d..17ce4751a2051a 100644
--- a/kernel/trace/ftrace.c
+++ b/kernel/trace/ftrace.c
@@ -5126,7 +5126,6 @@ int register_ftrace_direct(unsigned long ip, unsigned long addr)
 	struct ftrace_direct_func *direct;
 	struct ftrace_func_entry *entry;
 	struct ftrace_hash *free_hash = NULL;
-	struct dyn_ftrace *rec;
 	int ret = -ENODEV;
 
 	mutex_lock(&direct_mutex);
@@ -5140,26 +5139,6 @@ int register_ftrace_direct(unsigned long ip, unsigned long addr)
 	if (ftrace_find_rec_direct(ip))
 		goto out_unlock;
 
-	ret = -ENODEV;
-	rec = lookup_rec(ip, ip);
-	if (!rec)
-		goto out_unlock;
-
-	/*
-	 * Check if the rec says it has a direct call but we didn't
-	 * find one earlier?
-	 */
-	if (WARN_ON(rec->flags & FTRACE_FL_DIRECT))
-		goto out_unlock;
-
-	/* Make sure the ip points to the exact record */
-	if (ip != rec->ip) {
-		ip = rec->ip;
-		/* Need to check this ip for a direct. */
-		if (ftrace_find_rec_direct(ip))
-			goto out_unlock;
-	}
-
 	ret = -ENOMEM;
 	direct = ftrace_find_direct_func(addr);
 	if (!direct) {
@@ -5380,6 +5359,10 @@ int modify_ftrace_direct(unsigned long ip,
 	mutex_lock(&direct_mutex);
 
 	mutex_lock(&ftrace_lock);
+	ip = ftrace_location(ip);
+	if (!ip)
+		goto out_unlock;
+
 	entry = find_direct_entry(&ip, &rec);
 	if (!entry)
 		goto out_unlock;
-- 
2.35.1