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

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Date:   Wed, 24 Aug 2016 17:47:11 +0200
From:   Oleg Nesterov <oleg@...hat.com>
To:     Pratyush Anand <panand@...hat.com>
Cc:     linux-arm-kernel@...ts.infradead.org, linux@....linux.org.uk,
        catalin.marinas@....com, will.deacon@....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

Hi Pratyush,

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.

> 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?

> and we should have a flag in "struct arch_uprobe_task" to handle
> this, right?

Probably yes, because we need to record/restore X86_EFLAGS_TF in case it
was already set by ptrace or something else.

> > However, I agree we can do this later and initial version can use these
> > ptrace helpers.
>
> Yes, I would also like to do that change latter, because these set of patches
> have already been tested heavily with systemtap, so it would be better to go
> with an incremental changes latter on.

Yes, yes, I agree. Let me repeat that this patch looks good to me as initial
version, but obviously I can't really revit it and/or ack.

Oleg.