linux-kernel - Re: [PATCH 0/3] kmemleak report format changes

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Date:   Fri, 28 Apr 2023 18:25:04 +0100
From:   Catalin Marinas <catalin.marinas@....com>
To:     Jim Cromie <jim.cromie@...il.com>
Cc:     linux-mm@...ck.org, linux-kernel@...r.kernel.org,
        Dmitry Vyukov <dvyukov@...gle.com>
Subject: Re: [PATCH 0/3] kmemleak report format changes

On Tue, Apr 25, 2023 at 04:24:43PM -0600, Jim Cromie wrote:
> If format changes are not /sys/** ABI violating, heres 3 minor ones:
> 
> 1st strips "age <increasing>" from output.  This makes the output
> idempotent; unchanging until a new leak is reported.
> 
> 2nd adds the backtrace.checksum to the "backtrace:" line.  This lets a
> user see repeats without actually reading the whole backtrace.  So now
> the backtrace line looks like this:
> 
>   backtrace (ck 603070071):  # also see below
> 
> Q: should ck be spelled crc ? it feels more communicative.

These all would make sense (and 'crc' sounds better) if they were done
from the start. I know there are test scripts out there parsing the
kmemleak sysfs file. I can't tell whether these changes would break
them.

Cc'ing Dmitry, I think syzbot was regularly checking kmemleak (not sure
it still does).

> NB: with ck exposed, it becomes possible to do a "selective clear",
> something like:
> 
>   echo drop 603070071 > /sys/kernel/debug/kmemleak
> 
> The 3rd patch takes __init off of kmemleak_test_init().  This fixes a
> bare-pointer in the 2nd line of the backtrace below, which previously
> looked like:
> 
>     [<00000000ef738764>] 0xffffffffc02350a2
> 
> NB: this happens still/again, after rmmod kmemleak-test.
> 
> unreferenced object 0xffff888005d9ca40 (size 32):
>   comm "modprobe", pid 412, jiffies 4294703300
>   hex dump (first 32 bytes):
>     00 cd d9 05 80 88 ff ff 40 cf d9 05 80 88 ff ff  ........@.......
>     14 a7 c4 f6 7d f9 87 10 00 00 00 00 00 00 00 00  ....}...........
>   backtrace (ck 1354775490):
>     [<000000002c474f61>] kmalloc_trace+0x26/0x90
>     [<00000000b26599c1>] kmemleak_test_init+0x58/0x2d0 [kmemleak_test]
>     [<0000000044d13990>] do_one_initcall+0x43/0x210
>     [<00000000131bc505>] do_init_module+0x4a/0x210
>     [<00000000b2902890>] __do_sys_finit_module+0x93/0xf0
>     [<00000000673fdce2>] do_syscall_64+0x34/0x80
>     [<00000000357a2d80>] entry_SYSCALL_64_after_hwframe+0x46/0xb0

-- 
Catalin