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Date: Fri, 22 Apr 2022 14:32:50 +0200
From: "Rafael J. Wysocki" <rafael@...nel.org>
To: Matthieu Baerts <matthieu.baerts@...sares.net>
Cc: "Rafael J. Wysocki" <rafael@...nel.org>,
Mat Martineau <mathew.j.martineau@...ux.intel.com>,
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()
On Fri, Apr 22, 2022 at 2:25 PM Matthieu Baerts
<matthieu.baerts@...sares.net> wrote:
>
> 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.
It is there because of the RAX-relative accesses in the assembly code.
I'm not sure if correct computation of the offsets in that code can be
guaranteed without it.
> 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.
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