linux-kernel - Re: [PATCH] unwind: Show that entries of struct unwind_cache is not bound by nr

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Message-ID: <20251217151159.2eee1081@gandalf.local.home>
Date: Wed, 17 Dec 2025 15:11:59 -0500
From: Steven Rostedt <rostedt@...dmis.org>
To: Kees Cook <kees@...nel.org>
Cc: LKML <linux-kernel@...r.kernel.org>, Linux Trace Kernel
 <linux-trace-kernel@...r.kernel.org>, Thorsten Blum
 <thorsten.blum@...ux.dev>, Josh Poimboeuf <jpoimboe@...nel.org>, Peter
 Zijlstra <peterz@...radead.org>, "Gustavo A. R. Silva"
 <gustavoars@...nel.org>, David Laight <david.laight.linux@...il.com>
Subject: Re: [PATCH] unwind: Show that entries of struct unwind_cache is not
 bound by nr_entries

On Mon, 17 Nov 2025 13:28:59 -0800
Kees Cook <kees@...nel.org> wrote:

> struct unwind_cache {
> 	struct_group_tagged(unwind_cache_hdr, hdr,
> 		unsigned long unwind_completed;
> 		unsigned int  nr_entries;
> 	);
> 	unsigned long         entries[(SZ_4K - sizeof(struct unwind_cache_hdr)) / sizeof(long)];
> };

This may help automated tooling, but it is horrendous to read. I value
readability much higher than static analyzers.

Hence, I'm leaving the code as is, and just keep NAKing patches that try to
add __counted_by() to entries.

-- Steve

> 
> #define UNWIND_MAX_ENTRIES ARRAY_SIZE(((struct unwind_cache*)NULL)->entries)
> 
> And this checks out for me:
> 
> UNWIND_MAX_ENTRIES:510
> sizeof(struct unwind_cache):4096
> 
> No hiding things from the compiler, and you can treat "entries" like a
> real array (since it is one now).