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

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Message-ID: <20251114085332.396aff85@gandalf.local.home>
Date: Fri, 14 Nov 2025 08:53:32 -0500
From: Steven Rostedt <rostedt@...dmis.org>
To: Thorsten Blum <thorsten.blum@...ux.dev>
Cc: 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 08:43:46 -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];
 };
 
 /*
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