From: Peter Zijlstra <peterz@infradead.org>
To: Ingo Molnar <mingo@kernel.org>
Cc: Thomas Gleixner <tglx@linutronix.de>,
LKML <linux-kernel@vger.kernel.org>,
x86@kernel.org, Juergen Gross <jgross@suse.com>,
Andi Kleen <ak@linux.intel.com>
Subject: Re: x86/paravirt: Detect over-sized patching bugs in paravirt_patch_call()
Date: Thu, 25 Apr 2019 13:40:04 +0200 [thread overview]
Message-ID: <20190425114004.GN4038@hirez.programming.kicks-ass.net> (raw)
In-Reply-To: <20190425105745.GA29840@gmail.com>
On Thu, Apr 25, 2019 at 12:57:45PM +0200, Ingo Molnar wrote:
> * Peter Zijlstra <peterz@infradead.org> wrote:
> > On Thu, Apr 25, 2019 at 11:50:39AM +0200, Ingo Molnar wrote:
> > > * Peter Zijlstra <peterz@infradead.org> wrote:
> > > > On Thu, Apr 25, 2019 at 11:17:17AM +0200, Ingo Molnar wrote:
> > > > > It basically means that we silently won't do any patching and the kernel
> > > > > will crash later on in mysterious ways, because paravirt patching is
> > > > > usually relied on.
> > > >
> > > > That's OK. The compiler emits an indirect CALL/JMP to the pv_ops
> > > > structure contents. That _should_ stay valid and function correctly at
> > > > all times.
> > >
> > > It might result in a correctly executing kernel in terms of code
> > > generation, but it doesn't result in a viable kernel: some of the places
> > > rely on the patching going through and don't know what to do when it
> > > doesn't and misbehave or crash in interesting ways.
> > >
> > > Guess how I know this. ;-)
> >
> > What sites would that be? It really should work AFAIK.
>
> So for example I tried to increasing the size of one of the struct
> patch_xxl members:
>
> --- a/arch/x86/kernel/paravirt_patch.c
> +++ b/arch/x86/kernel/paravirt_patch.c
> @@ -28,7 +28,7 @@ struct patch_xxl {
> const unsigned char irq_restore_fl[2];
> # ifdef CONFIG_X86_64
> const unsigned char cpu_wbinvd[2];
> - const unsigned char cpu_usergs_sysret64[6];
> + const unsigned char cpu_usergs_sysret64[60];
> const unsigned char cpu_swapgs[3];
> const unsigned char mov64[3];
> # else
So this then fails to patch the immediate; but the compiler emitted:
175: ff 25 00 00 00 00 jmpq *0x0(%rip) # 17b <syscall_return_via_sysret+0x75>
177: R_X86_64_PC32 pv_ops+0xfc
and pv_ops+0xfc is (+4 because of reloc magic):
void (*usergs_sysret64)(void); /* 0x100 0x8 */
which defaults to:
arch/x86/kernel/paravirt.c: .cpu.usergs_sysret64 = native_usergs_sysret64,
which in turn reads like:
0000000000000000 <native_usergs_sysret64>:
0: 0f 01 f8 swapgs
3: 48 0f 07 sysretq
So I _really_ don't understand how:
> Which with the vanilla kernel crashes on boot much, much later:
>
> [ 2.478026] PANIC: double fault, error_code: 0x0
happens.
> But in any case, even if many of the others will work if the patching
> fails silently, is there any case where we'd treat patching failure as an
> acceptable case?
It really should just work. And we need to figure out why it comes
unstuck. Can you print the code when it fails patching?
> BUG_ON() in paravirt kernels is an easily debuggable condition and beats
> the above kinds of symptoms. But I can turn it into a WARN_ON_ONCE() if
> you think that's better?
Not patching should be a performance issue; not a correctness issue, as
per the above. So WARN is the right thing.
next prev parent reply other threads:[~2019-04-25 11:40 UTC|newest]
Thread overview: 28+ messages / expand[flat|nested] mbox.gz Atom feed top
2019-04-24 13:41 [patch 0/3] x86/paravirt: Rework paravirt patching Thomas Gleixner
2019-04-24 13:41 ` [patch 1/3] x86/paravirt: Remove bogus extern declarations Thomas Gleixner
2019-04-25 7:31 ` [tip:x86/paravirt] " tip-bot for Thomas Gleixner
2019-05-24 7:58 ` tip-bot for Thomas Gleixner
2019-04-24 13:41 ` [patch 2/3] x86/paravirt: Unify 32/64 bit patch code Thomas Gleixner
2019-04-25 7:32 ` [tip:x86/paravirt] " tip-bot for Thomas Gleixner
2019-05-24 8:00 ` [tip:x86/paravirt] x86/paravirt: Unify the 32/64 bit paravirt patching code tip-bot for Thomas Gleixner
2019-04-24 13:41 ` [patch 3/3] x86/paravirt: Replace paravirt patch asm magic Thomas Gleixner
2019-04-25 6:52 ` Ingo Molnar
2019-04-25 7:22 ` Thomas Gleixner
2019-04-25 7:46 ` Juergen Gross
2019-04-25 8:10 ` [PATCH] x86/paravirt: Match paravirt patchlet field definition ordering to initialization ordering Ingo Molnar
2019-04-25 9:17 ` [PATCH] x86/paravirt: Detect oversized patching bugs as they happen and BUG_ON() to avoid later crashes Ingo Molnar
2019-04-25 9:21 ` Peter Zijlstra
2019-04-25 9:50 ` x86/paravirt: Detect over-sized patching bugs in paravirt_patch_call() Ingo Molnar
2019-04-25 10:22 ` Peter Zijlstra
2019-04-25 10:57 ` Ingo Molnar
2019-04-25 11:30 ` Juergen Gross
2019-04-25 12:30 ` Juergen Gross
2019-04-25 11:40 ` Peter Zijlstra [this message]
2019-04-25 12:30 ` Peter Zijlstra
2019-05-24 7:59 ` [tip:x86/paravirt] " tip-bot for Ingo Molnar
2019-05-24 7:58 ` [tip:x86/paravirt] x86/paravirt: Detect over-sized patching bugs in paravirt_patch_insns() tip-bot for Ingo Molnar
2019-05-24 8:01 ` [tip:x86/paravirt] x86/paravirt: Match paravirt patchlet field definition ordering to initialization ordering tip-bot for Ingo Molnar
2019-04-25 8:08 ` [patch 3/3] x86/paravirt: Replace paravirt patch asm magic Peter Zijlstra
2019-04-25 8:19 ` Peter Zijlstra
2019-04-25 9:20 ` Ingo Molnar
2019-05-24 8:00 ` [tip:x86/paravirt] x86/paravirt: Replace the " tip-bot for Thomas Gleixner
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