linux-kernel - Re: [PATCH 5/5] arm64: Add uprobe support

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Date:   Wed, 24 Aug 2016 16:56:50 +0100
From:   Will Deacon <will.deacon@....com>
To:     Oleg Nesterov <oleg@...hat.com>
Cc:     Pratyush Anand <panand@...hat.com>,
        linux-arm-kernel@...ts.infradead.org, linux@....linux.org.uk,
        catalin.marinas@....com, linux-kernel@...r.kernel.org,
        wcohen@...hat.com, dave.long@...aro.org, steve.capper@...aro.org,
        srikar@...ux.vnet.ibm.com, vijaya.kumar@...iumnetworks.com,
        Shi Yang <yang.shi@...aro.org>,
        Andre Przywara <andre.przywara@....com>,
        Ard Biesheuvel <ard.biesheuvel@...aro.org>,
        Ashok Kumar <ashoks@...adcom.com>,
        James Morse <james.morse@....com>,
        Jungseok Lee <jungseoklee85@...il.com>,
        "Kirill A. Shutemov" <kirill.shutemov@...ux.intel.com>,
        Mark Rutland <mark.rutland@....com>,
        Masami Hiramatsu <mhiramat@...nel.org>,
        Robin Murphy <robin.murphy@....com>,
        Sandeepa Prabhu <sandeepa.s.prabhu@...il.com>,
        Shaokun Zhang <zhangshaokun@...ilicon.com>,
        "Suzuki K. Poulose" <suzuki.poulose@....com>,
        Vladimir Murzin <vladimir.murzin@....com>
Subject: Re: [PATCH 5/5] arm64: Add uprobe support

On Wed, Aug 24, 2016 at 05:47:11PM +0200, Oleg Nesterov wrote:
> On 08/24, Pratyush Anand wrote:
> >
> > > I don't think we want user_{enable,disable{_single_step in the long term,
> > > please look at 9bd1190a11c9d2 "uprobes/x86: Do not (ab)use TIF_SINGLESTEP
> > > /user_*_single_step() for single-stepping". it seems that ARM64 sets/clears
> > > TIF_SINGLESTEP. You can also lool at saved_tf logic, probably ARM64 needs
> > > the same.
> >
> > IIUC, then you mean that TIF_SINGLESTEP is a per task flag,
> 
> Yes, and nobody but ptrace should use it, otherwise ptrace/uprobes can confuse
> each other. And uprobes simply doesn't need to set/clear it.

We're already using it for kprobes, hw_breakpoint and kgdb as well as
ptrace, so I'd rather uprobes either followed existing practice, or we
converted everybody off the current code.

In what way do things get confused?

> > while
> > arch_uprobe_pre/post_xol() should enable/disable single stepping using a per
> > uprobe_task,
> 
> I can't really answer since I know nothing about arm. x86 just needs to set
> X86_EFLAGS_TF, I guess arm needs to modify some register too?

We have {user,kernel}_{enable,disable}_single_step for managing the various
registers controlling the single-step state machine on arm64.

Will