From: Petr Mladek <pmladek@suse.com>
To: Alexander Potapenko <glider@google.com>
Cc: Vegard Nossum <vegard.nossum@oracle.com>,
Sergey Senozhatsky <sergey.senozhatsky@gmail.com>,
Steven Rostedt <rostedt@goodmis.org>,
Dmitry Vyukov <dvyukov@google.com>,
Linux Memory Management List <linux-mm@kvack.org>
Subject: Re: [PATCH RFC v1 05/26] printk_safe: externalize printk_context
Date: Thu, 24 Oct 2019 14:46:34 +0200 [thread overview]
Message-ID: <20191024124634.hzsn74l7q4nl4mti@pathway.suse.cz> (raw)
In-Reply-To: <CAG_fn=WFMHr82PyYNasg9k9YbyXCuJkZbvBwbF1KAsAdokS=NQ@mail.gmail.com>
On Wed 2019-10-23 19:57:39, Alexander Potapenko wrote:
> On Mon, Oct 21, 2019 at 11:09 AM Petr Mladek <pmladek@suse.com> wrote:
> >
> > On Fri 2019-10-18 11:42:43, glider@google.com wrote:
> > > Petr Mladek suggested we use
> > > this_cpu_read(printk_context) & PRINTK_SAFE_CONTEXT_MASK
> > > instead of checking the spinlock status in kmsan_pr_err()
> >
> > I would like to understand why the check is needed.
> >
> > My guess is that it prevents a infinite recursion.
> > Is this the case? It might be possible to debug this using
> > trace_printk().
> This indeed used to prevent infinite recursion, however looks like the
> check isn't needed anymore.
> When I remove it, KMSAN doesn't hang, probably because printk is doing
> a better job at deadlock prevention.
Great.
> What I'm seeing now is that e.g. in the following case:
> ptr = kmalloc(sizeof(int), GFP_KERNEL);
> if (ptr)
> pr_info("true\n");
> else
> pr_info("false\n");
>
> KMSAN detects errors within pr_info(), namely in vprintk_store().
> If I understand correctly, printing from that point causes printk to
> use the per-cpu buffer, which is flushed once we're done with the
> original printing:
>
> [ 58.984971][ T8390] BUG: KMSAN: uninit-value in
> kmsan_handle_vprintk+0xa0/0xf0
> ...
> [ 59.061976][ C0] BUG: KMSAN: uninit-value in vsnprintf+0x3276/0x32b0
> ...
> [ 59.062457][ C0] BUG: KMSAN: uninit-value in format_decode+0x17f/0x1900
> ...
> [ 59.062961][ C0] BUG: KMSAN: uninit-value in format_decode+0x17f/0x1900
Please, do you have an explanation where the uninitialized values come
from? Is it a false positive? Or is there really a bug how the
printk() parameters are passed?
> ...
> [ 59.063338][ C0] Lost 6207 message(s)!
This message comes from report_message_lost(). It means that many
recursive messages did not fit into the limited per-CPU buffers.
> So it turns out we'd better disable reporting under logbuf_lock,
> otherwise these lost messages will confuse the developers.
> Because the first pr_info() isn't called by KMSAN itself, the tool
> still needs a way to know it's running inside printk.
OK. Well, I still would like to understand why the recursive messages
are printed at all. Is it a bug in the printk() code or a false positive?
There should be no recursive messages if we fix the bug or make
some annotations to prevent the false positives.
That said, I am not completely against disabling the recursive
messages. But it would be pity to hide some real bugs.
Best Regards,
Petr
next prev parent reply other threads:[~2019-10-24 12:46 UTC|newest]
Thread overview: 54+ messages / expand[flat|nested] mbox.gz Atom feed top
2019-10-18 9:42 [PATCH RFC v1 00/26] Add KernelMemorySanitizer infrastructure glider
2019-10-18 9:42 ` [PATCH RFC v1 01/26] stackdepot: check depot_index before accessing the stack slab glider
2019-10-18 9:42 ` [PATCH RFC v1 02/26] stackdepot: prevent Clang from optimizing away stackdepot_memcmp() glider
2019-10-18 9:42 ` [PATCH RFC v1 03/26] kasan: stackdepot: move filter_irq_stacks() to stackdepot.c glider
2019-10-18 9:42 ` [PATCH RFC v1 04/26] stackdepot: reserve 5 extra bits in depot_stack_handle_t glider
2019-10-18 9:42 ` [PATCH RFC v1 05/26] printk_safe: externalize printk_context glider
2019-10-21 9:09 ` Petr Mladek
2019-10-23 17:57 ` Alexander Potapenko
2019-10-23 18:00 ` Alexander Potapenko
2019-10-24 12:46 ` Petr Mladek [this message]
2019-10-28 13:09 ` Alexander Potapenko
2019-10-29 12:02 ` Petr Mladek
2019-10-29 12:45 ` Alexander Potapenko
2019-10-18 9:42 ` [PATCH RFC v1 06/26] kasan: compiler.h: rename __no_kasan_or_inline into __no_memory_tool_or_inline glider
2019-10-18 9:42 ` [PATCH RFC v1 07/26] kmsan: add ReST documentation glider
2019-10-18 9:42 ` [PATCH RFC v1 08/26] kmsan: gfp: introduce __GFP_NO_KMSAN_SHADOW glider
2019-10-18 9:42 ` [PATCH RFC v1 09/26] kmsan: introduce __no_sanitize_memory and __SANITIZE_MEMORY__ glider
2019-10-18 9:42 ` [PATCH RFC v1 10/26] kmsan: reduce vmalloc space glider
2019-10-18 9:42 ` [PATCH RFC v1 11/26] kmsan: add KMSAN runtime glider
2019-10-18 9:42 ` [PATCH RFC v1 12/26] kmsan: x86: sync metadata pages on page fault glider
2019-10-18 9:42 ` [PATCH RFC v1 13/26] kmsan: add tests for KMSAN glider
2019-10-18 9:42 ` [PATCH RFC v1 14/26] kmsan: make READ_ONCE_TASK_STACK() return initialized values glider
2019-10-18 9:42 ` [PATCH RFC v1 15/26] kmsan: Kconfig changes to disable options incompatible with KMSAN glider
2019-10-21 14:11 ` Harry Wentland
2019-10-18 9:42 ` [PATCH RFC v1 16/26] kmsan: Changing existing files to enable KMSAN builds glider
2019-10-18 9:42 ` [PATCH RFC v1 17/26] kmsan: disable KMSAN instrumentation for certain kernel parts glider
2019-10-18 9:42 ` [PATCH RFC v1 18/26] kmsan: mm: call KMSAN hooks from SLUB code glider
2019-10-18 13:22 ` Qian Cai
2019-10-18 13:33 ` Alexander Potapenko
2019-10-18 13:41 ` Qian Cai
2019-10-18 13:55 ` Alexander Potapenko
2019-10-18 14:42 ` Qian Cai
2019-10-18 14:54 ` Alexander Potapenko
2019-10-18 15:13 ` Qian Cai
2019-10-18 15:30 ` Alexander Potapenko
2019-10-18 16:08 ` Qian Cai
2019-10-18 9:42 ` [PATCH RFC v1 19/26] kmsan: call KMSAN hooks where needed glider
2019-10-18 15:02 ` Qian Cai
2019-10-29 14:09 ` Alexander Potapenko
2019-10-29 14:56 ` Qian Cai
2019-10-21 9:25 ` Petr Mladek
2019-10-29 13:59 ` Alexander Potapenko
2019-10-18 9:42 ` [PATCH RFC v1 20/26] kmsan: disable instrumentation of certain functions glider
2019-10-18 9:42 ` [PATCH RFC v1 21/26] kmsan: unpoison |tlb| in arch_tlb_gather_mmu() glider
2019-10-18 9:43 ` [PATCH RFC v1 22/26] kmsan: use __msan_memcpy() where possible glider
2019-10-18 9:43 ` [PATCH RFC v1 23/26] kmsan: unpoisoning buffers from devices etc glider
2019-10-18 15:27 ` Christoph Hellwig
2019-10-18 16:22 ` Matthew Wilcox
2019-10-29 14:45 ` Alexander Potapenko
2019-10-30 12:43 ` Alexander Potapenko
2019-10-18 9:43 ` [PATCH RFC v1 24/26] kmsan: hooks for copy_to_user() and friends glider
2019-10-18 9:43 ` [PATCH RFC v1 25/26] kmsan: disable strscpy() optimization under KMSAN glider
2019-10-18 9:43 ` [PATCH RFC v1 26/26] net: kasan: kmsan: support CONFIG_GENERIC_CSUM on x86, enable it for KASAN/KMSAN glider
2019-10-19 3:20 ` Randy Dunlap
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