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

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Date:   Sun, 20 Dec 2020 14:19:38 -0800
From:   Randy Dunlap <rdunlap@...radead.org>
To:     Matthew Wilcox <willy@...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 12/20/20 1:23 PM, Matthew Wilcox wrote:
> 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.
> 

Hm, OK, that would make more sense than some Huge value (other than -1).

Do you mean something like this?

---
---
 mm/readahead.c |    6 +++++-
 1 file changed, 5 insertions(+), 1 deletion(-)

--- linux-5.10.1.orig/mm/readahead.c
+++ linux-5.10.1/mm/readahead.c
@@ -310,7 +310,11 @@ void force_page_cache_ra(struct readahea
  */
 static unsigned long get_init_ra_size(unsigned long size, unsigned long max)
 {
-	unsigned long newsize = roundup_pow_of_two(size);
+	unsigned long newsize;
+
+	if (!size)
+		size = 32;
+	newsize = roundup_pow_of_two(size);
 
 	if (newsize <= max / 32)
 		newsize = newsize * 4;


Thanks.