linux-kernel - Re: [PATCH] fs: buffer: set the expression type to unsigned long in folio_create

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Message-ID: <5002a830-6a66-82fc-19e3-ec1e907eeb49@omp.ru>
Date: Tue, 16 Jul 2024 18:41:49 +0300
From: Sergey Shtylyov <s.shtylyov@....ru>
To: Roman Smirnov <r.smirnov@....ru>, Alexander Viro
	<viro@...iv.linux.org.uk>, Christian Brauner <brauner@...nel.org>, Jan Kara
	<jack@...e.cz>
CC: <linux-fsdevel@...r.kernel.org>, <linux-kernel@...r.kernel.org>,
	<lvc-project@...uxtesting.org>
Subject: Re: [PATCH] fs: buffer: set the expression type to unsigned long in
 folio_create_buffers()

On 7/16/24 12:01 PM, Roman Smirnov wrote:

> Shift without specifying the type casts the expression to int,

   You mean the result of the shift? Or what expression?

> which is then passed as an unsigned long argument. It is necessary

   And here we'll have at least one potential problem (that you neglected
to describe): with 1 << 31, that 1 will land in a sign bit and then, when
it's implicitly cast to *unsigned long*, the 32-bit value will be sign-
extended to 64-bit on 64-bit arches) and then we'll have an incorrect size
(0xffffffff80000000) passed to create_empty_buffers()...

> to use 1UL instead.

   Perphas was worth noting that using 1UL saves us 1 movsx instruction on
x86_64...

> Found by Linux Verification Center (linuxtesting.org) with Svace.
> 
> Signed-off-by: Roman Smirnov <r.smirnov@....ru>
[...]

MBR, Sergey