Messages in this thread | | | Date | Tue, 28 Feb 2012 14:52:51 +0100 | From | Ingo Molnar <> | Subject | Re: [PATCH] uprobes/core: handle breakpoint and signal step exception. |
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* Srikar Dronamraju <srikar@linux.vnet.ibm.com> wrote:
> > > Where possible, we check and skip singlestepping the > > > breakpointed instructions. For now we skip single byte as > > > well as few multibyte nop instructions. However this can > > > be extended to other instructions too. > > > > Is this an optimization - allowing a NOP to be inserted for > > easy probe insertion? > > Yes, Its an optimization by which we avoid singlestep > exception.
That would be nice to comment in the code - nowhere in the 'skip' logic is this fact mentioned, and it's really useful information to pretty much anyone reading the code.
It's also a nice optimization, there's no need to obfuscate its existence.
> > > + case DIE_INT3: > > > + /* Run your handler here */ > > > + if (uprobe_bkpt_notifier(regs)) > > > + ret = NOTIFY_STOP; > > > > This comment looks somewhat out of place. > > > > Also, I have not noticed this in the first patch, but 'bkpt' is > > not a standard way to refer to breakpoints: we either use > > 'breakpoint' or 'bp'. > > This is again one of those things that I changed from bp to > bkpt based on LKML feedback. I am okay to go back to bp.
:-/ I can understand it somewhat, 'bp' also means other things.
'hwbp' is a common name - you could use 'swbp' which would pair with that nicely?
> > > +bool arch_uprobe_skip_sstep(struct pt_regs *regs, struct arch_uprobe *auprobe) > > > +{ > > > + int i; > > > + > > > + for (i = 0; i < MAX_UINSN_BYTES; i++) { > > > + if ((auprobe->insn[i] == 0x66)) > > > + continue; > > > + > > > + if (auprobe->insn[i] == 0x90) > > > + return true; > > > + > > > + if (i == (MAX_UINSN_BYTES - 1)) > > > + break; > > > > Looks like the loop could run from 0 to MAX_UINSN_BYTES-2 and > > then this break would be superfluous. > > > > Even if we were to run from 0 to MAX_UINSN_BYTES - 2, we would > have to add extra code to handle 0x66* 0x90 (where 0x90 is > stored at index i == MAX_UINSN_BYTES - 1. So I would like to > keep this code as is.
Ok.
> > > +/* > > > + * uprobe_task: Metadata of a task while it singlesteps. > > > + */ > > > +struct uprobe_task { > > > + unsigned long xol_vaddr; > > > + unsigned long vaddr; > > > > These two fields are never actually used outside of architecture > > code. > > > > Unless there's a good reason to keep them outside I'd > > suggest to move them into struct arch_uprobe_task. This has > > another benefit: we can pass struct arch_uprobe_task to the > > architecture methods, instead of struct uprobe_task. This > > would allow the moving of the struct uprobe_task into > > uprobes.c - no code outside uprobes.c needs to know its > > structure. > > The Xol layer(which is the next patch) uses them in arch > agnostic way. Also vaddr/xol_vaddr are populated/used in arch > agnostic way. We could still move them to arch_uprobe_task but > we will then have to ensure that every other arch defines them > the way uprobes understands.
Correct - and that still isolates the arch code from the core uprobes code.
We could also introduce 'struct generic_arch_uprobe_task' and embedd that inside arch_uprobe via a short field name, to make it easy to access: ->gen.field or so.
You can also leave it as-is for now, I'll reconsider how things look like with the patch following these bits and then make a new suggestion if I see a better way.
> > > +static inline unsigned long get_uprobe_bkpt_addr(struct pt_regs *regs) > > > +{ > > > + return 0; > > > +} > > > > Please use the standard uprobe method naming pattern for > > get_uprobe_bkpt_addr(). > > do you mean uprobe_get_bp_addr ?
Yeah, that sounds good.
> > > +/* > > > + * There could be threads that have hit the breakpoint and are entering the > > > + * notifier code and trying to acquire the uprobes_treelock. The thread > > > + * calling delete_uprobe() that is removing the uprobe from the rb_tree can > > > + * race with these threads and might acquire the uprobes_treelock compared > > > + * to some of the breakpoint hit threads. In such a case, the breakpoint hit > > > + * threads will not find the uprobe. Hence wait till the current breakpoint > > > + * hit threads acquire the uprobes_treelock before the uprobe is removed > > > + * from the rbtree. > > > > Hm, the last sentence does not parse for me. (even if it's > > correct English it might make sense to rephrase it to be clearer > > what is meant.) > > > > Would this be okay with you. > > The current unregistering thread waits till all other threads > that have hit a breakpoint to acquire the uprobes_treelock > before the uprobe is removed from the rbtree.
s/is removed/are removed
?
If yes then indeed this reads better.
> [...] > > If the thread was not in the middle of a uprobe hit then we go > through the regular signal handling. > > Since there is no way this thread can hit a uprobe once a > thread has entered get_signal_to_deliver(kernel code), I dont > see a reason to move it under relock:
Ok, fair enough.
Thanks,
Ingo
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