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Subject: Re: [PATCH] mm: mmap: show vm_unmapped_area error log
To:     Matthew Wilcox <willy@infradead.org>
Cc:     Andrew Morton <akpm@linux-foundation.org>, walken@google.com,
        bp@suse.de, linux-mm@kvack.org, linux-kernel@vger.kernel.org,
        jaewon31.kim@gmail.com
From:   Jaewon Kim <jaewon31.kim@samsung.com>
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Date:   Mon, 9 Mar 2020 18:12:03 +0900
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On 2020년 03월 08일 21:36, Matthew Wilcox wrote:
> On Sun, Mar 08, 2020 at 06:58:47PM +0900, Jaewon Kim wrote:
>> On 2020년 03월 08일 10:58, Matthew Wilcox wrote:
>>> On Sat, Mar 07, 2020 at 03:47:44PM -0800, Andrew Morton wrote:
>>>> On Fri, 6 Mar 2020 15:16:22 +0900 Jaewon Kim <jaewon31.kim@samsung.com> wrote:
>>>>> Even on 64 bit kernel, the mmap failure can happen for a 32 bit task.
>>>>> Virtual memory space shortage of a task on mmap is reported to userspace
>>>>> as -ENOMEM. It can be confused as physical memory shortage of overall
>>>>> system.
>>> But userspace can trigger this printk.  We don't usually allow printks
>>> under those circumstances, even ratelimited.
>> Hello thank you your comment.
>>
>> Yes, userspace can trigger printk, but this was useful for to know why
>> a userspace task was crashed. There seems to be still many userspace
>> applications which did not check error of mmap and access invalid address.
>>
>> Additionally in my AARCH64 Android environment, display driver tries to
>> get userspace address to map its display memory. The display driver
>> report -ENOMEM from vm_unmapped_area and but also from GPU related
>> address space.
>>
>> Please let me know your comment again if this debug is now allowed
>> in that userspace triggered perspective.
> The scenario that worries us is an attacker being able to fill the log
> files and so also fill (eg) the /var partition.  Once it's full, future
> kernel messages cannot be stored anywhere and so there will be no traces
> of their privilege escalation.
Although up to 10 times within 5 secs is not many, I think those log may remove
other important log in log buffer if it is the system which produces very few log.
In my Android phone device system, there seems to be much more kernel log though.
> Maybe a tracepoint would be a better idea?  Usually they are disabled,
> but they can be enabled by a sysadmin to gain insight into why an
> application is crashing.
In Android phone device system, we cannot get sysadmin permission if it is built
for end user. And it is not easy to reproduce this symptom because it is an user's app.

Anyway let me try pr_devel_ratelimited which is disabled by default. I hope this is
good enough. Additionally I moved the code from mm.h to mmap.c.

--- a/mm/mmap.c
+++ b/mm/mmap.c
@@ -2069,7 +2069,7 @@ unsigned long vm_unmapped_area(struct vm_unmapped_area_info *info)
                addr = unmapped_area(info);
 
        if (IS_ERR_VALUE(addr)) {
-               pr_warn_ratelimited("%s err:%ld total_vm:0x%lx flags:0x%lx len:0x%lx low:0x%lx high:0x%lx mask:0x%lx offset:0x%lx\n",
+               pr_devel_ratelimited("%s err:%ld total_vm:0x%lx flags:0x%lx len:0x%lx low:0x%lx high:0x%lx mask:0x%lx offset:0x%lx\n",
                        __func__, addr, current->mm->total_vm, info->flags,
                        info->length, info->low_limit, info->high_limit,
                        info->align_mask, info->align_offset);
>
>