linux-kernel - Re: [PATCH v5 2/3] man/man2/prctl.2, PR_FUTEX_HASH_SET_SLOTS.2const: Document PR_FUTEX_HASH_SET

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Message-ID: <20250821140851.z1FYafVm@linutronix.de>
Date: Thu, 21 Aug 2025 16:08:51 +0200
From: Sebastian Andrzej Siewior <bigeasy@...utronix.de>
To: Alejandro Colomar <alx@...nel.org>
Cc: linux-kernel@...r.kernel.org, linux-man@...r.kernel.org,
	André Almeida <andrealmeid@...lia.com>,
	Darren Hart <dvhart@...radead.org>,
	Davidlohr Bueso <dave@...olabs.net>, Ingo Molnar <mingo@...hat.com>,
	Juri Lelli <juri.lelli@...hat.com>,
	Peter Zijlstra <peterz@...radead.org>,
	Thomas Gleixner <tglx@...utronix.de>,
	Valentin Schneider <vschneid@...hat.com>,
	Waiman Long <longman@...hat.com>
Subject: Re: [PATCH v5 2/3] man/man2/prctl.2, PR_FUTEX_HASH_SET_SLOTS.2const:
 Document PR_FUTEX_HASH_SET_SLOTS

On 2025-08-19 13:19:15 [+0200], Alejandro Colomar wrote:
> Hi Sebastian,
Hi Alejandro,

> > > +.B int prctl(PR_FUTEX_HASH, PR_FUTEX_HASH_SET_SLOTS,
> > > +.BI "          unsigned long " hash_size ", unsigned long " hash_flags ");
> 
> I see that it is implemented in the kernel as
> 
> 	static int
> 	futex_hash_allocate(unsigned int hash_slots, unsigned int flags)
> 	{
> 		...
> 	}
> 
> But PR_FUTEX_HASH is implemented as
> 
> 	int
> 	futex_hash_prctl(unsigned long arg2, unsigned long arg3,
> 	    unsigned long arg4)
> 	{
> 		...
> 	}
> 
> Should we document is as a u_int, or a u_long?  Is that mismatch a bug,
> or is it on purpose?

The prctl() interface is long so I started with that. Internally we keep
it as an int since we don't need it that big and it avoids a hole
otherwise (but then there is a hole towards the end of the structure).
Realistically speaking 1 << 31 is the largest value that can be
specified (atm) and it will very likely lead to ENOMEM. If the user
tries the next higher value, 1 << 32, then the upper bits will be
truncated and the global hash will be requested and this will succeed.

The bug could be based on the argument type 1 << 32 will succeed but
it should not. Using 1 << 25 will request ~2GiB of memory and is a bit
far from sane.
I am bit forth and back between forcing an error for anything > 1 << 30,
or updating the docs to int. But this will be the easiest ;)
Any recommendations based on similar cases?

> Cheers,
> Alex

Sebastian