[PATCH] tracing: remove unreachable trace_array_put

[PATCH] tracing: remove unreachable trace_array_put

[Nikita Kiryushin]

There is a trace_array_put() in check result for
nonseekable_open() in tracing_buffers_open(). However,
it would be never executed as nonseekable_open never fails
(by design).

Remove the check and associated unreachable code.

Found by Linux Verification Center (linuxtesting.org) with SVACE.

Fixes: 7b85af630348 ("tracing: Get trace_array ref counts when accessing trace files")
Signed-off-by: Nikita Kiryushin <kiryushin@ancud.ru>
---
 kernel/trace/trace.c | 6 +-----
 1 file changed, 1 insertion(+), 5 deletions(-)

diff --git a/kernel/trace/trace.c b/kernel/trace/trace.c
index 578a49ff5c32..7e480501b509 100644
--- a/kernel/trace/trace.c
+++ b/kernel/trace/trace.c
@@ -7883,11 +7883,7 @@ static int tracing_buffers_open(struct inode *inode, struct file *filp)
 
 	mutex_unlock(&trace_types_lock);
 
-	ret = nonseekable_open(inode, filp);
-	if (ret < 0)
-		trace_array_put(tr);
-
-	return ret;
+	return nonseekable_open(inode, filp);
 }
 
 static __poll_t
-- 
2.34.1

Re: [PATCH] tracing: remove unreachable trace_array_put

[Steven Rostedt]

On Fri, 12 Jul 2024 23:12:58 +0300
Nikita Kiryushin <kiryushin@ancud.ru> wrote:

> There is a trace_array_put() in check result for
> nonseekable_open() in tracing_buffers_open(). However,
> it would be never executed as nonseekable_open never fails
> (by design).
> 
> Remove the check and associated unreachable code.

Then why does it return a value?

If someday it can return a failure, this would then cause a leak. It
doesn't hurt to leave it in.

So NACK.

-- Steve


> 
> Found by Linux Verification Center (linuxtesting.org) with SVACE.
> 
> Fixes: 7b85af630348 ("tracing: Get trace_array ref counts when accessing trace files")
> Signed-off-by: Nikita Kiryushin <kiryushin@ancud.ru>
> ---
>  kernel/trace/trace.c | 6 +-----
>  1 file changed, 1 insertion(+), 5 deletions(-)
> 
> diff --git a/kernel/trace/trace.c b/kernel/trace/trace.c
> index 578a49ff5c32..7e480501b509 100644
> --- a/kernel/trace/trace.c
> +++ b/kernel/trace/trace.c
> @@ -7883,11 +7883,7 @@ static int tracing_buffers_open(struct inode *inode, struct file *filp)
>  
>  	mutex_unlock(&trace_types_lock);
>  
> -	ret = nonseekable_open(inode, filp);
> -	if (ret < 0)
> -		trace_array_put(tr);
> -
> -	return ret;
> +	return nonseekable_open(inode, filp);
>  }
>  
>  static __poll_t

Re: [PATCH] tracing: remove unreachable trace_array_put

[Nikita Kiryushin]

As nonseekable_open() documentation states:
"The function is not supposed to ever fail, the only
reason it returns an 'int' and not 'void' is so that it can be plugged
directly into file_operations structure."

So it seems, that it will not fail anytime as it is not meant to? Otherwise,
there will be a huge problem with leaks in many other parts of code, as
there are plenty of places, where nonseekable_open() is not checked after
resource allocations.

On 7/13/24 02:33, Steven Rostedt wrote:
> Then why does it return a value?
>
> If someday it can return a failure, this would then cause a leak. It
> doesn't hurt to leave it in.
>
> So NACK.
>
> -- Steve
>

Re: [lvc-project] [PATCH] tracing: remove unreachable trace_array_put

[Alexey Khoroshilov]

On 15.07.2024 16:47, Nikita Kiryushin wrote:
> As nonseekable_open() documentation states:
> "The function is not supposed to ever fail, the only
> reason it returns an 'int' and not 'void' is so that it can be plugged
> directly into file_operations structure."
> 
> So it seems, that it will not fail anytime as it is not meant to?
> Otherwise,
> there will be a huge problem with leaks in many other parts of code, as
> there are plenty of places, where nonseekable_open() is not checked after
> resource allocations.

Yes, but there is another possible modification: replacement of call to
nonseekable_open() by a call to some other function that returns error.
Current code is already ready for such modification.

--
Alexey

Re: [lvc-project] [PATCH] tracing: remove unreachable trace_array_put

[Nikita Kiryushin]

On 7/16/24 12:45, Alexey Khoroshilov wrote:
> Yes, but there is another possible modification: replacement of call to
> nonseekable_open() by a call to some other function that returns error.
> Current code is already ready for such modification.

The change of which function is called would change the behavior indeed, but,
TBH, I do not see it as a valid point: If we assume that nonseekable_open() changes to something else in the future, we may assume as well that some other call will be
added later with a risk of resource leaking. This is a thing, that whoever would do
such changes should be careful about.

For me, the code as it is now, is not uniform with the other places that use
nonseekable_open().

Re: [lvc-project] [PATCH] tracing: remove unreachable trace_array_put

[Steven Rostedt]

On Tue, 16 Jul 2024 22:19:05 +0300
Nikita Kiryushin <kiryushin@ancud.ru> wrote:

> On 7/16/24 12:45, Alexey Khoroshilov wrote:
> > Yes, but there is another possible modification: replacement of call to
> > nonseekable_open() by a call to some other function that returns error.
> > Current code is already ready for such modification.  
> 
> The change of which function is called would change the behavior indeed, but,
> TBH, I do not see it as a valid point: If we assume that nonseekable_open() changes to something else in the future, we may assume as well that some other call will be
> added later with a risk of resource leaking. This is a thing, that whoever would do
> such changes should be careful about.
> 
> For me, the code as it is now, is not uniform with the other places that use
> nonseekable_open().

The point is moot. If something returns a value, even if it says it
will never return failure, there's no harm in checking it. If we ignore
the return value, that is a unneeded coupling of design between the
function and its users.

It does no harm in checking the value, so I rather just keep doing so.

-- Steve