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From: Catalin Marinas <>
To: Rustam Kovhaev <>
Cc: Andrew Morton <>,,,,
Subject: Re: kmemleak memory scanning
Date: Tue, 15 Jun 2021 11:15:15 +0100	[thread overview]
Message-ID: <> (raw)
In-Reply-To: <YMe8ktUsdtwFKHuF@nuc10>

Hi Rustam,

On Mon, Jun 14, 2021 at 01:31:14PM -0700, Rustam Kovhaev wrote:
> a) kmemleak scans struct page (kmemleak.c:1462), but it does not scan
> the actual contents (page_address(page)) of the page.
> if we allocate an object with kmalloc(), then allocate page with
> alloc_page(), and if we put kmalloc pointer somewhere inside that page, 
> kmemleak will report kmalloc pointer as a false positive.
> should we improve kmemleak and make it scan page contents?
> or will this bring too many false negatives?

This is indeed on purpose otherwise (1) we'd get a lot of false
negatives and (2) the scanning would take significantly longer. There
are a lot more pages allocated for user processes than used in the
kernel, we don't need to scan them all.

We do have a few places where we explicitly call kmemleak_alloc():
neigh_hash_alloc(), alloc_page_ext(), blk_mq_alloc_rqs(),

> b) when kmemleak object gets created (kmemleak.c:598) it gets checksum
> of 0, by the time user requests kmemleak "scan" via debugfs the pointer
> will be most likely changed to some value by the kernel and during
> first scan kmemleak won't report the object as orphan even if it did not
> find any reference to it, because it will execute update_checksum() and
> after that will proceed to updating object->count (kmemleak.c:1502).
> and so the user will have to initiate a second "scan" via debugfs and
> only then kmemleak will produce the report.
> should we document this?

That's a mitigation against false positives. Lot's of objects that get
allocated just prior to a memory scan have a tendency to be reported as
leaks before they get referenced via e.g. a list (and the in-object
list_head structure updated). So you'd need to get the checksum
identical in two consecutive scans to report it as a leak.

We should probably document this.


  parent reply	other threads:[~2021-06-15 10:15 UTC|newest]

Thread overview: 11+ messages / expand[flat|nested]  mbox.gz  Atom feed  top
2021-06-14 20:31 kmemleak memory scanning Rustam Kovhaev
2021-06-15  5:15 ` Dmitry Vyukov
2021-06-16 18:25   ` Rustam Kovhaev
2021-06-24 17:36     ` Rustam Kovhaev
2021-06-25 15:01       ` Catalin Marinas
2021-06-25 15:27         ` Rustam Kovhaev
2021-06-25 15:36           ` Dmitry Vyukov
2021-06-15  8:12 ` David Hildenbrand
2021-06-16 18:27   ` Rustam Kovhaev
2021-06-15 10:15 ` Catalin Marinas [this message]
2021-06-16 18:36   ` Rustam Kovhaev

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