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Re: [RFC][PATCH 2/2] uprobes: single-step out of line
- From: Jim Keniston <jkenisto at us dot ibm dot com>
- To: Ernie Petrides <petrides at redhat dot com>
- Cc: "Frank Ch. Eigler" <fche at redhat dot com>, Linda Wang <lwang at redhat dot com>, systemtap at sources dot redhat dot com
- Date: Thu, 10 May 2007 15:17:44 -0700
- Subject: Re: [RFC][PATCH 2/2] uprobes: single-step out of line
- References: <200705090134.l491YvgW010707@pasta.boston.redhat.com>
On Tue, 2007-05-08 at 21:34 -0400, Ernie Petrides wrote:
> On Monday, 7-May-2007 at 14:2 PDT, Jim Keniston wrote:
>
> > On Fri, 2007-05-04 at 21:09 -0400, Ernie Petrides wrote:
> >
> > > Is there a way to avoid adding the "uprobe_ssol_area" struct into the
> > > "mm_struct"? If so, the uprobes module could be easily back-ported to
> > > kABI-frozen distros of the Linux kernel. If the "mm_struct" ends up
> > > getting changed, that changes the "task_struct" layout, thus breaking
> > > binary compatibility with 3rd-party kernel modules. (We're not allowed
> > > to do this in RHEL distros.)
> >
> >
> > Hmmm. It does indeed change the layout of struct mm_struct. I don't
> > see how it changes the layout of task_struct, since task_struct
> > contains only pointers to mm_structs.
>
> Ah, my mistake. You are correct. But because of how exported symbol
> checksums are generated (recursively traversing all depended-on info),
> all functions taking (task_struct *) arguments would become incompatible.
>
>
>
> > But changing mm_struct itself is bad, right?
>
> Besides the exported symbol versioning issue I've already explained, it
> might also be the case that somewhere there is a global (or auto-class)
> mm_struct. (There are a few in the base kernel, but one might argue
> that there shouldn't be any in 3rd-party modules.) If there were one,
> and somehow the "runt" mm_struct were referenced by a kernel built with
> the uprobes infrastructure changes (expanding the mm_struct), then you
> get fetch a bogus "uprobes_ssol_area" pointer off the end of an old struct.
>
> I'm not sure how plausible this is, but it's something to consider.
>
>
>
> > An obvious alternative is for uprobes to maintain this pointer
> > in one of its own data structures. Currently, when the last uprobe
> > for a process is unregistered, we discard the uprobe_process and
> > uprobe_tasks, and the only thing that remains is the pointer to
> > the uprobe_ssol_area (in mm_context). We need to remember that
> > pointer in case the process is probed again -- we want to reuse the
> > vma. [...]
>
> Originally, I missed the point about reusing the VMA again later
> (following the unregistering of the last probe). So, I guess you
> do have a reasonable need for MM-persistent data.
>
> I'm not sure what the best solution is. Maybe what you've already
> got here is reasonable. I'd need to study mm_struct compatibility
> issues for a while to determine if this would be a deal-breaker in
> terms of the kABI issue. (We have this "#ifndef __GENKSYMS__" hack
> that can sometimes be used to accommodate these sorts of structure
> additions in a RHEL update to avoid the symbol checksum change, but
> it's only viable if there's no true underlying compatibility problem.)
Yes, I'd appreciate it if you confirm the need for a change here, since
the effort/implications for this change are non-trivial.
>
>
> Cheers. -ernie
Thanks.
Jim