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Date: Fri, 22 Apr 2022 14:25:05 +0200
From: Matthieu Baerts <matthieu.baerts@...sares.net>
To: "Rafael J. Wysocki" <rafael@...nel.org>,
Mat Martineau <mathew.j.martineau@...ux.intel.com>
Cc: Pavel Machek <pavel@....cz>, Thomas Gleixner <tglx@...utronix.de>,
Ingo Molnar <mingo@...hat.com>, Borislav Petkov <bp@...en8.de>,
Dave Hansen <dave.hansen@...ux.intel.com>,
the arch/x86 maintainers <x86@...nel.org>,
"H. Peter Anvin" <hpa@...or.com>, Chen Yu <yu.c.chen@...el.com>,
Pawan Gupta <pawan.kumar.gupta@...ux.intel.com>,
Ingo Molnar <mingo@...nel.org>,
"Rafael J. Wysocki" <rafael.j.wysocki@...el.com>,
Linux PM <linux-pm@...r.kernel.org>,
Linux Kernel Mailing List <linux-kernel@...r.kernel.org>
Subject: Re: [PATCH mptcp-next] x86/pm: fix false positive kmemleak report in
msr_build_context()
Hi Mat, Rafael,
(oops, please ignore the "mptcp-next" tag I added by reflex in the
subject: this is not related to MPTCP :) )
On 22/04/2022 13:51, Rafael J. Wysocki wrote:
> On Fri, Apr 22, 2022 at 1:21 AM Mat Martineau
> <mathew.j.martineau@...ux.intel.com> wrote:
>>
>> On Thu, 21 Apr 2022, Matthieu Baerts wrote:
>>
>>> Since commit e2a1256b17b1 ("x86/speculation: Restore speculation related MSRs during S3 resume"),
>>> kmemleak reports this issue:
>>>
>>> unreferenced object 0xffff888009cedc00 (size 256):
>>> comm "swapper/0", pid 1, jiffies 4294693823 (age 73.764s)
>>> hex dump (first 32 bytes):
>>> 00 00 00 00 00 00 00 00 48 00 00 00 00 00 00 00 ........H.......
>>> 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 ................
>>> backtrace:
>>> msr_build_context (include/linux/slab.h:621)
>>> pm_check_save_msr (arch/x86/power/cpu.c:520)
>>> do_one_initcall (init/main.c:1298)
>>> kernel_init_freeable (init/main.c:1370)
>>> kernel_init (init/main.c:1504)
>>> ret_from_fork (arch/x86/entry/entry_64.S:304)
>>>
>>> It is easy to reproduce it on my side:
>>>
>>> - boot the VM with a debug kernel config [1]
>>> - wait ~1 minute
>>> - start a kmemleak scan
>>>
>>> It seems kmemleak has an issue with the array allocated in
>>> msr_build_context() and assigned to a pointer in a static structure
>>> (saved_context.saved_msrs->array): there is no leak then.
>>>
>>> It looks like this is a limitation from kmemleak but that's alright,
>>> kmemleak_no_leak() can be used to avoid complaining about that.
>>>
>>> Please note that it looks like this issue is not new, e.g.
>>>
>>> https://lore.kernel.org/all/9f1bb619-c4ee-21c4-a251-870bd4db04fa@lwfinger.net/
>>> https://lore.kernel.org/all/94e48fcd-1dbd-ebd2-4c91-f39941735909@molgen.mpg.de/
>>>
>>> But on my side, msr_build_context() is only used since:
>>>
>>> commit e2a1256b17b1 ("x86/speculation: Restore speculation related MSRs during S3 resume").
>>>
>>> Depending on their CPUs, others have probably the same issue since:
>>>
>>> commit 7a9c2dd08ead ("x86/pm: Introduce quirk framework to save/restore extra MSR registers around suspend/resume"),
>>>
>>> hence the 'Fixes' tag here below to help with the backports. But I
>>> understand if someone says the origin of this issue is more on
>>> kmemleak's side. What is unclear to me is why this issue was not seen by
>>> other people and CIs. Maybe the kernel config [1]?
>>>
>>> [1] https://github.com/multipath-tcp/mptcp_net-next/files/8531660/kmemleak-cpu-sched-bisect.kconfig.txt
>>>
>>
>> Hi Matthieu -
>>
>> It looks like the root cause here is alignment within the packed struct
>> saved_context (from suspend_64.h). Kmemleak only searches for pointers
>> that are aligned, but pahole shows that the saved_msrs struct member and
>> all members after it in the structure are unaligned:
@Mat: Thank you for the analysis and finding the root cause!
>> (gcc 11.2.1, x86_64)
>>
>> struct saved_context {
>> struct pt_regs regs; /* 0 168 */
>> /* --- cacheline 2 boundary (128 bytes) was 40 bytes ago --- */
>> u16 ds; /* 168 2 */
>> u16 es; /* 170 2 */
>> u16 fs; /* 172 2 */
>> u16 gs; /* 174 2 */
>> long unsigned int kernelmode_gs_base; /* 176 8 */
>> long unsigned int usermode_gs_base; /* 184 8 */
>> /* --- cacheline 3 boundary (192 bytes) --- */
>> long unsigned int fs_base; /* 192 8 */
>> long unsigned int cr0; /* 200 8 */
>> long unsigned int cr2; /* 208 8 */
>> long unsigned int cr3; /* 216 8 */
>> long unsigned int cr4; /* 224 8 */
>> u64 misc_enable; /* 232 8 */
>> bool misc_enable_saved; /* 240 1 */
>>
>> /* Note odd offset values for the remainder of this struct vvv */
>>
>> struct saved_msrs saved_msrs; /* 241 16 */
>> /* --- cacheline 4 boundary (256 bytes) was 1 bytes ago --- */
>> long unsigned int efer; /* 257 8 */
>> u16 gdt_pad; /* 265 2 */
>> struct desc_ptr gdt_desc; /* 267 10 */
>> u16 idt_pad; /* 277 2 */
>> struct desc_ptr idt; /* 279 10 */
>> u16 ldt; /* 289 2 */
>> u16 tss; /* 291 2 */
>> long unsigned int tr; /* 293 8 */
>> long unsigned int safety; /* 301 8 */
>> long unsigned int return_address; /* 309 8 */
>>
>> /* size: 317, cachelines: 5, members: 25 */
>> /* last cacheline: 61 bytes */
>> } __attribute__((__packed__));
>>
>> If I move misc_enable_saved to the end of the struct declaration,
>> saved_msrs fits in before the cacheline 4 boundary and the kmemleak
>> warning goes away. The comment above the saved_context declaration says to
>> check wakeup_64.S and __save/__restore_processor_state() if the struct is
>> modified - looks like it's the members before misc_enable that must be
>> carefully placed.
>
> Yes, you can move misc_enable_saved to the end of it safely AFAICS.
@Rafael: thank you for the reply!
Before doing that, is it still needed to keep the "packed" attribute?
This attribute was already there before the first Git commit.
Without it, I no longer have the kmemleak and pahole reports this:
struct saved_context {
(...)
bool misc_enable_saved; /* 240 1 */
/* XXX 7 bytes hole, try to pack */
struct saved_msrs saved_msrs; /* 248 16 */
(...)
/* size: 328, cachelines: 6, members: 25 */
/* sum members: 317, holes: 2, sum holes: 11 */
/* last cacheline: 8 bytes */
};
Everything is still at the same place before 'misc_enable' member.
If it is important to reduce the cachelines, it is still interesting to
move the bool to avoid a whole which costs one cacheline.
>> So far I've only tried this on my local machine, I'll work on getting more
>> thorough validation.
>>
>> Looks like struct saved_context in suspend_32.h has similar odd alignment.
>
> Right, and it can be changed too AFAICS.
Thanks!
Cheers,
Matt
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