linux-kernel - Re: [PATCH] ftrace: Do not over-allocate ftrace memory

lists.openwall.net		lists / announce owl-users owl-dev john-users john-dev passwdqc-users yescrypt popa3d-users / oss-security kernel-hardening musl sabotage tlsify passwords / crypt-dev xvendor / Bugtraq Full-Disclosure linux-kernel linux-netdev linux-ext4 linux-hardening linux-cve-announce PHC
Open Source and information security mailing list archives

Hash Suite: Windows password security audit tool. GUI, reports in PDF.

[<prev] [next>] [<thread-prev] [day] [month] [year] [list]

Message-ID: <b4ff0141-7012-411c-8ae3-87313a756dd6@roeck-us.net>
Date: Tue, 6 Jan 2026 15:48:09 -0800
From: Guenter Roeck <linux@...ck-us.net>
To: Steven Rostedt <rostedt@...dmis.org>
Cc: Masami Hiramatsu <mhiramat@...nel.org>,
	Mark Rutland <mark.rutland@....com>,
	Mathieu Desnoyers <mathieu.desnoyers@...icios.com>,
	linux-kernel@...r.kernel.org, linux-trace-kernel@...r.kernel.org
Subject: Re: [PATCH] ftrace: Do not over-allocate ftrace memory

Hi Steve,

On Tue, Jan 06, 2026 at 06:38:15PM -0500, Steven Rostedt wrote:
> On Tue, 6 Jan 2026 15:05:25 -0800
> Guenter Roeck <linux@...ck-us.net> wrote:
> 
> > > > 	/*
> > > > 	 * Use ftrace_number_of_pages to determine how many pages were
> > > > 	 * allocated
> > > > 	 */
> > > > 	pages = ftrace_number_of_pages;
> > > > 
> > > > 	start_pg = ftrace_allocate_pages(count);
> > > > 	if (!start_pg)
> > > > 		return -ENOMEM;
> > > > 
> > > > 	/* ftrace_allocate_pages() increments ftrace_number_of_pages */
> > > > 	pages = ftrace_number_of_pages - pages;
> > > >   
> > > 
> > > That might work, assuming that the code updating ftrace_number_of_pages
> > > is (mutex) protected. I don't immediately see that, and the
> > > "mutex_lock(&ftrace_lock);" right after the above code makes me a bit
> > > concerned.
> > >   
> > 
> > One way to avoid the locking problem without potentially risky code changes
> > would be to pass a pointer to pages to ftrace_allocate_pages() and to
> > ftrace_allocate_records(), and to update it from there. I tested that and
> > confirmed that it works.
> 
> I was originally going to suggest that, but when looking at the code, I
> noticed that these variables could be useful. They are only updated on boot
> up, module load, module unload and when module memory is freed.
> 
> But looking into the module code, these updates are done outside of the
> module_mutex. This means these values need to be converted to atomics as
> they are updated without any protection.
> 
> Yeah, better to just get the value from passing in a parameter to both
> ftrace_allocate_pages() and to ftrace_allocate_records().
> 
> Something like:
> 
> 	unsigend long pages = 0;
> 
> 	[..]
> 	start_pg = ftrace_allocate_pages(count, &pages);
> 
> [..]
> 	ftrace_allocate_pages(unsigned long num_to_init, unsigned long *num_pages) {
> 		[..]
> 			cnt = ftrace_allocate_records(pg, num_to_init, num_pages);
> 
> And have ftrace_allocte_records() have:
> 
> 	pages = 1 << order;
> 	*num_pages += pages;
> 	ftrace_number_of_pages += pages;
> 

That is exactly what I tested. With that, I assume I can drop the
code to update pages in ftrace_process_locs(), inclusing the warning
backtraces. I'll send v2 with those changes.

Thanks,
Guenter