linux-kernel - Re: [RFC PATCH 1/2] log2: handle LARGE input to __roundup_pow_of

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Date:   Sun, 20 Dec 2020 21:23:20 +0000
From:   Matthew Wilcox <willy@...radead.org>
To:     Randy Dunlap <rdunlap@...radead.org>
Cc:     linux-kernel@...r.kernel.org, Jens Axboe <axboe@...nel.dk>,
        Andrew Morton <akpm@...ux-foundation.org>,
        Toralf Förster <toralf.foerster@....de>,
        linux-mm@...ck.org
Subject: Re: [RFC PATCH 1/2] log2: handle LARGE input to
 __roundup_pow_of_two()

On Sun, Dec 20, 2020 at 01:10:37PM -0800, Randy Dunlap wrote:
> UBSAN detected a 64-bit shift in log2.h:__roundup_pow_of_two():
>   UBSAN: shift-out-of-bounds in ./include/linux/log2.h:57:13
>   shift exponent 64 is too large for 64-bit type 'long unsigned int'
> 
> This is during a call from mm/readahead.c:ondemand_readahead(),
> get_init_ra_size(), where the 'size' parameter must have been
> extremely large (or "negative").

Actually, I think it was zero, which is the real bug that should be fixed.