linux-kernel - Re: [PATCH v5 5/6] kprobes: Use text_alloc() and text

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Date:   Tue, 28 Jul 2020 20:51:08 +0300
From:   Ard Biesheuvel <ardb@...nel.org>
To:     Masami Hiramatsu <mhiramat@...nel.org>
Cc:     Mike Rapoport <rppt@...nel.org>,
        Jarkko Sakkinen <jarkko.sakkinen@...ux.intel.com>,
        Ingo Molnar <mingo@...nel.org>,
        Linux Kernel Mailing List <linux-kernel@...r.kernel.org>,
        linux-mm@...ck.org, Andi Kleen <ak@...ux.intel.com>,
        Peter Zijlstra <peterz@...radead.org>,
        "Naveen N. Rao" <naveen.n.rao@...ux.ibm.com>,
        Anil S Keshavamurthy <anil.s.keshavamurthy@...el.com>,
        "David S. Miller" <davem@...emloft.net>,
        Jessica Yu <jeyu@...nel.org>
Subject: Re: [PATCH v5 5/6] kprobes: Use text_alloc() and text_free()

On Tue, 28 Jul 2020 at 16:35, Masami Hiramatsu <mhiramat@...nel.org> wrote:
>
> On Tue, 28 Jul 2020 13:56:43 +0300
> Ard Biesheuvel <ardb@...nel.org> wrote:
>
> > On Tue, 28 Jul 2020 at 11:17, Masami Hiramatsu <mhiramat@...nel.org> wrote:
> > > > Masami or Peter should correct me if I am wrong, but it seems to me
> > > > that the way kprobes uses these pages does not require them to be in
> > > > relative branching range of the core kernel on any architecture, given
> > > > that they are populated with individual instruction opcodes that are
> > > > executed in single step mode, and relative branches are emulated (when
> > > > needed)
> > >
> > > Actually, x86 and arm has the "relative branching range" requirements
> > > for the jump optimized kprobes. For the other architectures, I think
> > > we don't need it. Only executable text buffer is needed.
> > >
> >
> > Thanks for the explanation. Today, arm64 uses the definition below.
> >
> > void *alloc_insn_page(void)
> > {
> >   return __vmalloc_node_range(PAGE_SIZE, 1, VMALLOC_START, VMALLOC_END,
> >     GFP_KERNEL, PAGE_KERNEL_ROX, VM_FLUSH_RESET_PERMS,
> >     NUMA_NO_NODE, __builtin_return_address(0));
> > }
> >
> > Do you think we could use that as the generic implementation if we use
> > MODULES_START/_END as the allocation window?
>
> Yes, but for the generic implementation, we don't need to consider the
> relative branching range since we can override it for x86 and arm.
> (and that will be almost same as module_alloc() default code)

Indeed. So having kprobes specific macros that default to
VMALLOC_START/END but can be overridden would be sufficient.

> BTW, is PAGE_KERNEL_ROX flag available generically?
>

Turns out that it is not :-(

> Thank you,
> --
> Masami Hiramatsu <mhiramat@...nel.org>