linux-kernel - Re: [PATCH 1/1] mm: Remove the access_ok() call from gup_fast

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Message-ID: <20250209182422.GK3660748@nvidia.com>
Date: Sun, 9 Feb 2025 14:24:22 -0400
From: Jason Gunthorpe <jgg@...dia.com>
To: David Laight <david.laight.linux@...il.com>
Cc: Andrew Morton <akpm@...ux-foundation.org>,
	Linus Torvalds <torvalds@...ux-foundation.org>, linux-mm@...ck.org,
	linux-kernel@...r.kernel.org, x86@...nel.org,
	Jan Kara <jack@...e.cz>, John Hubbard <jhubbard@...dia.com>,
	Peter Xu <peterx@...hat.com>,
	Dave Hansen <dave.hansen@...ux.intel.com>,
	Andy Lutomirski <luto@...nel.org>,
	Peter Zijlstra <peterz@...radead.org>,
	Thomas Gleixner <tglx@...utronix.de>,
	Ingo Molnar <mingo@...hat.com>, Borislav Petkov <bp@...en8.de>
Subject: Re: [PATCH 1/1] mm: Remove the access_ok() call from
 gup_fast_fallback().

On Sun, Feb 09, 2025 at 05:47:11PM +0000, David Laight wrote:
> Historiaclly the code relied on access_ok() to validate the address range.
> Commit 26f4c328079d7 added an explicit wrap check before access_ok().
> Commit c28b1fc70390d then changed the wrap test to use check_add_overflow().
> Commit 6014bc27561f2 relaxed the checks in x86-64's access_ok() and added
>   an explicit check for TASK_SIZE here to make up for it.
> That left a pointless access_ok() call with its associated 'lfence' that
>   can never actually fail.
> So just delete the test.
> 
> Signed-off-by: David Laight <david.laight.linux@...il.com>
> ---
>  mm/gup.c | 4 +---
>  1 file changed, 1 insertion(+), 3 deletions(-)

Reviewed-by: Jason Gunthorpe <jgg@...dia.com>

I often wonder about about access_ok() calls, if they still do
anything..

Jason