linux-kernel - Re: [PATCH v2 3/5] mm: Make PR_MDWE_REFUSE_EXEC

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Date:   Tue, 23 May 2023 14:07:55 +0100
From:   Catalin Marinas <catalin.marinas@....com>
To:     David Hildenbrand <david@...hat.com>
Cc:     Alexey Izbyshev <izbyshev@...ras.ru>,
        Florent Revest <revest@...omium.org>,
        linux-kernel@...r.kernel.org, linux-mm@...ck.org,
        akpm@...ux-foundation.org, anshuman.khandual@....com,
        joey.gouly@....com, mhocko@...e.com, keescook@...omium.org,
        peterx@...hat.com, broonie@...nel.org, szabolcs.nagy@....com,
        kpsingh@...nel.org, gthelen@...gle.com, toiwoton@...il.com
Subject: Re: [PATCH v2 3/5] mm: Make PR_MDWE_REFUSE_EXEC_GAIN an unsigned long

On Tue, May 23, 2023 at 11:12:37AM +0200, David Hildenbrand wrote:
> Also, how is passing "0"s to e.g., PR_GET_THP_DISABLE reliable? We need arg2
> -> arg5 to be 0. But wouldn't the following also just pass a 0 "int" ?
> 
> prctl(PR_GET_THP_DISABLE, 0, 0, 0, 0)
> 
> I'm easily confused by such (va_args) things, so sorry for the dummy
> questions.

Isn't the prctl() prototype in the user headers defined with the first
argument as int while the rest as unsigned long? At least from the man
page:

int prctl(int option, unsigned long arg2, unsigned long arg3,
	  unsigned long arg4, unsigned long arg5);

So there are no va_args tricks (which confuse me as well).

Any int passed to arg[2-5] would be converted by the compiler to an
unsigned long before being passed to the kernel. So I think the change
in this patch is harmless as the conversion is happening anyway.

(well, unless I completely missed what the problem is)

-- 
Catalin