linux-kernel - Re: [PATCH] unwind deferred: Annotate struct unwind_cache with __counted

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Message-ID: <20251114150209.27354ddd@pumpkin>
Date: Fri, 14 Nov 2025 15:02:09 +0000
From: David Laight <david.laight.linux@...il.com>
To: Steven Rostedt <rostedt@...dmis.org>
Cc: Thorsten Blum <thorsten.blum@...ux.dev>, Josh Poimboeuf
 <jpoimboe@...nel.org>, Kees Cook <kees@...nel.org>, "Gustavo A. R. Silva"
 <gustavoars@...nel.org>, linux-kernel@...r.kernel.org,
 linux-hardening@...r.kernel.org
Subject: Re: [PATCH] unwind deferred: Annotate struct unwind_cache with
 __counted_by

On Fri, 14 Nov 2025 08:53:32 -0500
Steven Rostedt <rostedt@...dmis.org> wrote:

> On Fri, 14 Nov 2025 08:43:46 -0500
> Steven Rostedt <rostedt@...dmis.org> wrote:
> 
> > I need to add a comment here that entries is not bound by nr_entries.
> > 
> >    https://lore.kernel.org/all/20250730093249.4833be14@gandalf.local.home/
> > 
> > Maybe this?:  
> 
> Or better yet, if this compiles (I haven't tried):
> 
> diff --git a/include/linux/unwind_deferred_types.h b/include/linux/unwind_deferred_types.h
> index 33b62ac25c86..253a69b21e76 100644
> --- a/include/linux/unwind_deferred_types.h
> +++ b/include/linux/unwind_deferred_types.h
> @@ -2,10 +2,14 @@
>  #ifndef _LINUX_UNWIND_USER_DEFERRED_TYPES_H
>  #define _LINUX_UNWIND_USER_DEFERRED_TYPES_H
>  
> +/* Make the cache fit in a 4K page */
> +#define UNWIND_MAX_ENTRIES					\
> +	((SZ_4K - offset_of(struct unwind_cache, entries)) / sizeof(long))
> +
>  struct unwind_cache {
>  	unsigned long		unwind_completed;
>  	unsigned int		nr_entries;
> -	unsigned long		entries[];
> +	unsigned long		entries[UNWIND_MAX_ENTRIES];

That won't compile - I tried it.

You could add __aligned(4096) to force the structure to be padded to 4k,
and then define UNWIND_MAX_ENTRIES in terms of the structure size.

	David

>  };
>  
>  /*
> diff --git a/kernel/unwind/deferred.c b/kernel/unwind/deferred.c
> index dc6040aae3ee..5dfd0ac264d1 100644
> --- a/kernel/unwind/deferred.c
> +++ b/kernel/unwind/deferred.c
> @@ -37,10 +37,6 @@ static inline bool try_assign_cnt(struct unwind_task_info *info, u32 cnt)
>  }
>  #endif
>  
> -/* Make the cache fit in a 4K page */
> -#define UNWIND_MAX_ENTRIES					\
> -	((SZ_4K - sizeof(struct unwind_cache)) / sizeof(long))
> -
>  /* Guards adding to or removing from the list of callbacks */
>  static DEFINE_MUTEX(callback_mutex);
>  static LIST_HEAD(callbacks);
> @@ -118,8 +114,7 @@ int unwind_user_faultable(struct unwind_stacktrace *trace)
>  		return -EINVAL;
>  
>  	if (!info->cache) {
> -		info->cache = kzalloc(struct_size(cache, entries, UNWIND_MAX_ENTRIES),
> -				      GFP_KERNEL);
> +		info->cache = kzalloc(sizeof(*cache), GFP_KERNEL);
>  		if (!info->cache)
>  			return -ENOMEM;
>  	}
> 
> -- Steve
>