Messages in this thread | | | Date | Mon, 14 Oct 2013 16:55:39 +0200 | From | Oleg Nesterov <> | Subject | Re: [PATCH 5/5] uprobes: Change uprobe_copy_process() to dup xol_area |
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On 10/14, Peter Zijlstra wrote: > > On Sun, Oct 13, 2013 at 09:18:44PM +0200, Oleg Nesterov wrote: > > This finally fixes the serious bug in uretprobes: a forked child > > crashes if the parent called fork() with the pending ret probe. > > ... > > > > Unfortunately, this also means that we can not handle the errors > > properly, we obviously can not abort the already completed fork(). > > So we simply print the warning if GFP_KERNEL allocation (the only > > possible reason) fails. > > Oh cute.. so we could actually ignore this perf_event_mmap() because we > got it for the parent when we inserted the probe, and the perf tools > assume the child mm layout is identical to the parent layout (it doesn't > actually see the VM_DONTCOPY bit). > > So we could add: 'if (vma->vm_mm != current->mm) return;' to > perf_event_mmap() with a very big nasty comment.
Perhaps. I can't really comment, but this is really nasty. I mean, this simply doesn't look good. perf_event_mmap() will be reported or not depending on how/why the task creates xol_area.
> That said; should we hide the XOL vma from perf altogether? That is; it > will greatly obfuscate the perf data to get hits from the XOL table as > we've got no means of mapping it back to an instruction.
Again, I can't really comment. But this creates the special case. OK, xol_area is "special" anon mapping anyway, but still. And of course this needs __install_special_mapping().
So I'd prefer to push these changes as is for now. GFP_KERNEL should "never" fail and we need the fix for stable.
I agree, in the long term we should fix the inability to handle the errors correctly. But this needs more changes and more uprobes hooks. To simplify, suppose that we simply remove perf_event_mmap() from install_special_mapping() (yes, wes, we cant'). Then we should:
1. revert 1/5, it already moves uprobe_copy_process() to the point-of-no-return (for 4-5).
2. uprobe_copy_process() can avoid task_work_add() _and_ it can return an error if dup_utask/dup_xol fails.
3. However, 2. means that we need to handle the potential errors after uprobe_copy_process() suceeds. This means we need, say, uprobe_abort_fork() somewhere near bad_fork_cleanup_mm.
So will you agree with task_work for now?
Oleg.
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