On 11/05, Namhyung Kim wrote:
>
> On Mon, 4 Nov 2013 16:01:12 +0100, Oleg Nesterov wrote:
> > Or the syntax should be "name=probe @file/addr" or something like this.
>
> Okay. Let's call this kind of thing "cross-fetch" (or a better name can
> be suggested).
Yes ;) and I am afraid there was some confusion in our discussion.
I probably confused "probe other binaries" with cross-fetch and vice
versa sometimes.
> This is more complex situation and needs more discussion
> as you said. So let's skip the discussion for now. :)
Agreed.
> > So far I think that trace_uprobes.c should not play games with vma. At all.
>
> Yes, playing with vma is fragile. But otherwise how can we get the
> address from the file+offset in random processes?
Yes, this is not as simple as I thought.
Let me repeat, somehow I completely forgot we need to probe other (libc)
binaries (not only the executable itself) and dump the data from that
binary. That is why I wrongly thought that that ->start_data trick can
work.
OK, I see other emails from you. Perhaps we can rely on instruction_pointer(),
(I'll write more emails on this), But if not, then we probably need tu->inode
and vma games in fetch/get_user_vaddr(). I'd still like to avoid this, if
possible, but I am no longer sure.
> >> > Can't we simply implement get_user_vaddr() as
> >> >
> >> > static void __user *get_user_vaddr(unsigned long addr, struct
> >> > trace_uprobe *tu)
> >> > {
> >> > void __user *vaddr = (void __force __user *)addr;
> >> >
> >> > /* A NULL tu means that we already got the vaddr */
> >> > if (tu)
> >> > vaddr += (current->mm->start_data & PAGE_MASK);
> >> >
> >> > return vaddr;
> >> > }
> >> >
> >> This can only work with the probes fetching data from the executable,
> >> right? But as I said it should support any other binaries too.
> >
> > See above, we can't in general read other binaries.
>
> Okay, I need to clarify my words. I'm not saying about "cross-fetch"
> here, what I wanted to say is adding a probe in some dso and fetch data
> from the dso.
>
> [...snip...]
Yes, sorry for confusion, see above.
Oleg.
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