linux-kernel - Re: kmemleak memory scanning

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Date:   Tue, 15 Jun 2021 11:15:15 +0100
From:   Catalin Marinas <catalin.marinas@....com>
To:     Rustam Kovhaev <rkovhaev@...il.com>
Cc:     Andrew Morton <akpm@...ux-foundation.org>, linux-mm@...ck.org,
        linux-kernel@...r.kernel.org, dvyukov@...gle.com,
        gregkh@...uxfoundation.org
Subject: Re: kmemleak memory scanning

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(),
early_amd_iommu_init().

> 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.

-- 
Catalin