linux-kernel - Re: [PATCH 2/4] random: vDSO: Don't use PAGE_SIZE and PAGE

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Message-ID: <55a7d3ba-b384-4fb0-8fbb-e05ddf0d1fb8@csgroup.eu>
Date: Tue, 27 Aug 2024 10:16:18 +0200
From: Christophe Leroy <christophe.leroy@...roup.eu>
To: "Jason A. Donenfeld" <Jason@...c4.com>
Cc: Theodore Ts'o <tytso@....edu>, Arnd Bergmann <arnd@...db.de>,
 Andy Lutomirski <luto@...nel.org>, Thomas Gleixner <tglx@...utronix.de>,
 Vincenzo Frascino <vincenzo.frascino@....com>, Ingo Molnar
 <mingo@...hat.com>, Borislav Petkov <bp@...en8.de>,
 Dave Hansen <dave.hansen@...ux.intel.com>, x86@...nel.org,
 "H. Peter Anvin" <hpa@...or.com>, linux-kernel@...r.kernel.org,
 linuxppc-dev@...ts.ozlabs.org, linux-arch@...r.kernel.org
Subject: Re: [PATCH 2/4] random: vDSO: Don't use PAGE_SIZE and PAGE_MASK

Le 27/08/2024 à 09:49, Jason A. Donenfeld a écrit :
> On Tue, Aug 27, 2024 at 09:31:48AM +0200, Christophe Leroy wrote:
>> -	ssize_t ret = min_t(size_t, INT_MAX & PAGE_MASK /* = MAX_RW_COUNT */, len);
>> +	const unsigned long page_size = 1UL << CONFIG_PAGE_SHIFT;
>> +	const unsigned long page_mask = ~(page_size - 1);
>> +	ssize_t ret = min_t(size_t, INT_MAX & page_mask /* = MAX_RW_COUNT */, len);
> 
> I'm really not a fan of making the code less idiomatic...

Ok, I have another idea, let's give it a try.

> 
>> An easy solution would be to define PAGE_SIZE and PAGE_MASK in vDSO
>> when they do not exist already, but this can be misleading.
> 
> Why would what tglx and I suggested be misleading? That seems pretty
> normal... Are you worried they might mismatch somehow? (If so, why?)

All architectures have their own definition, they are all based on 
CONFIG_PAGE_SHIFT and should give the same value but with some 
subtleties. The best would be to have an asm-generic definition of 
PAGE_SIZE and PAGE_MASK that all architectures use, but that's another 
level of work.

tglx or yourself suggested to put in a one of the vdso headers instead 
of directly in getrandom.c. This is too fragile because PAGE_SIZE might 
be absent in that header but arrive in getrandom.c through another header.

Christophe