Re: [RFC 0/3] Put vdso in ramfs-like filesystem (vdsofs)

Andy Lutomirski
2016-08-26T14:33:12Z

On Fri, Aug 26, 2016 at 4:16 AM, Dmitry Safonov <0x7f454c46@gmail.com> wrote:
> 2016-08-26 2:00 GMT+03:00 H. Peter Anvin <hpa@zytor.com>:
>> On August 25, 2016 3:53:43 PM PDT, Dmitry Safonov <0x7f454c46@gmail.com> wrote:
>>>2016-08-25 23:49 GMT+03:00 H. Peter Anvin <hpa@zytor.com>:
>>>> On August 25, 2016 8:21:07 AM PDT, Dmitry Safonov
>>><dsafonov@virtuozzo.com> wrote:
>>>>>This patches set is cleanly RFC and is not supposed to be applied.
>>>>>Also for RFC time it builds only on x86_64.
>>>>>
>>>>>So, in a mail thread Oleg told that it would be worth to introduce
>>>>>vm_file
>>>>>for vdso mappings as currently uprobes can not be placed on vDSO VMAs
>>>>>[1].
>>>>>In this patches set I introduce in-kernel filesystem for vdso files.
>>>>>After patches vDSO VMA now has inode and is just a private file
>>>>>mapping:
>>>>>7ffcc4b2b000-7ffcc4b2d000 r--p 00000000 00:00 0
>>>>> [vvar]
>>>>>7ffcc4b2d000-7ffcc4b2f000 r-xp 00000000 00:09 18
>>>>> [vdso]
>>>>>
>>>>>Then I introduce interface in uprobe_events to insert uprobes in
>>>vdso.
>>>>>FWIW:
>>>>> [~]# cd kernel/linux
>>>>> [linux]# readelf --syms arch/x86/entry/vdso/vdso64.so
>>>>>Symbol table '.dynsym' contains 11 entries:
>>>>> Num: Value Size Type Bind Vis Ndx Name
>>>>> 0: 0000000000000000 0 NOTYPE LOCAL DEFAULT UND
>>>>> 1: 0000000000000470 0 SECTION LOCAL DEFAULT 8
>>>>>2: 00000000000008d0 885 FUNC WEAK DEFAULT 12
>>>>>clock_gettime@@LINUX_2.6
>>>>>3: 0000000000000c50 472 FUNC GLOBAL DEFAULT 12
>>>>>__vdso_gettimeofday@@LINUX_2.6
>>>>>4: 0000000000000c50 472 FUNC WEAK DEFAULT 12
>>>>>gettimeofday@@LINUX_2.6
>>>>>5: 0000000000000e30 21 FUNC GLOBAL DEFAULT 12
>>>>>__vdso_time@@LINUX_2.6
>>>>> 6: 0000000000000e30 21 FUNC WEAK DEFAULT 12
>>>time@@LINUX_2.6
>>>>>7: 00000000000008d0 885 FUNC GLOBAL DEFAULT 12
>>>>>__vdso_clock_gettime@@LINUX_2.6
>>>>> 8: 0000000000000000 0 OBJECT GLOBAL DEFAULT ABS LINUX_2.6
>>>>>9: 0000000000000e50 41 FUNC GLOBAL DEFAULT 12
>>>>>__vdso_getcpu@@LINUX_2.6
>>>>>10: 0000000000000e50 41 FUNC WEAK DEFAULT 12
>>>>>getcpu@@LINUX_2.6
>>>>> [~]# cd /sys/kernel/debug/tracing/
>>>>> [tracing]# echo 'p:clock_gettime :vdso:/64:0x8d0' > uprobe_events
>>>>> [tracing]# echo 'p:gettimeofday :vdso:/64:0xc50' >> uprobe_events
>>>>> [tracing]# echo 'p:time :vdso:/64:0xe30' >> uprobe_events
>>>>> [tracing]# echo 1 > events/uprobes/enable
>>>>> [tracing]# su test # it has UID=1001
>>>>> [tracing]$ date
>>>>> Thu Aug 25 17:19:29 MSK 2016
>>>>> [tracing]$ exit
>>>>> [tracing]# cat trace
>>>>> # tracer: nop
>>>>> #
>>>>> # entries-in-buffer/entries-written: 175/175 #P:4
>>>>> #
>>>>> # _-----=> irqs-off
>>>>> # / _----=> need-resched
>>>>> # | / _---=> hardirq/softirq
>>>>> # || / _--=> preempt-depth
>>>>> # ||| / delay
>>>>> # TASK-PID CPU# |||| TIMESTAMP FUNCTION
>>>>> # | | | |||| | |
>>>>> bash-11560 [001] d... 316.470236: time:
>>>(0x7ffcacebae30)
>>>>> bash-11560 [001] d... 316.471436: gettimeofday:
>>>(0x7ffcacebac50)
>>>>> bash-11560 [001] d... 316.477550: time:
>>>(0x7ffcacebae30)
>>>>> bash-11560 [001] d... 316.477655: time:
>>>(0x7ffcacebae30)
>>>>> mktemp-11568 [001] d... 316.479589: gettimeofday:
>>>(0x7ffc603f0c50)
>>>>> date-11571 [001] d... 316.481890: clock_gettime:
>>>(0x7ffec9db58d0)
>>>>>[...]
>>>>>
>>>>>If this approach will be decided as fine, I will prepare a better
>>>>>version,
>>>>>fixing the following things:
>>>>>o put vdsofs in generic fs/* dir
>>>>>o support other archs and vdso blobs
>>>>>o remove BUG_ON()'s and UID==1001 check
>>>>>o remove extern's and use headers only
>>>>>o refactor code in create_trace_uprobe()
>>>>>o add some state to (struct trace_uprobe), so i.e., `cat
>>>uprobe_events`
>>>>>will
>>>>> print those uprobes as vdso-based
>>>>>o document this interface in Documentation/trace/uprobetracer.txt
>>>>>o prepare nice patches set?
>>>>>
>>>>>So, opinions? Is it worth to add something like this?
>>>>>
>>>>>[1]: https://lkml.org/lkml/2016/7/12/346
>>>>>
>>>>>Dmitry Safonov (3):
>>>>> x86/vdso: create vdso file, use it for mapping
>>>>> uprobe: drop isdigit() check in create_trace_uprobe
>>>>> uprobe: add vdso support
>>>>>
>>>>>Cc: Oleg Nesterov <oleg@redhat.com>
>>>>>Cc: Al Viro <viro@zeniv.linux.org.uk>
>>>>>Cc: Steven Rostedt <rostedt@goodmis.org>
>>>>>Cc: Andy Lutomirski <luto@amacapital.net>
>>>>>Cc: Thomas Gleixner <tglx@linutronix.de>
>>>>>Cc: Ingo Molnar <mingo@redhat.com>
>>>>>Cc: "H. Peter Anvin" <hpa@zytor.com>
>>>>>Cc: x86@kernel.org
>>>>>Cc: Dmitry Safonov <0x7f454c46@gmail.com>
>>>>>
>>>>>arch/x86/entry/vdso/vma.c | 148
>>>>>++++++++++++++++++++++++++++++++++++++++++--
>>>>> kernel/trace/trace_uprobe.c | 50 +++++++++++----
>>>>> 2 files changed, 180 insertions(+), 18 deletions(-)
>>>>
>>>> I think there is a lot to be said for this idea. However, a private
>>>mapping is definitely wrong for the vvar data; for the vdso code it
>>>could be considered either way I suppose.
>>>
>>>Thanks on your reply.
>>>As you could see, I preserved pure mapping of pfn for vvar:
>>>7ffcc4b2b000-7ffcc4b2d000 r--p 00000000 00:00 0
>>> [vvar]
>>>7ffcc4b2d000-7ffcc4b2f000 r-xp 00000000 00:09 18
>>> [vdso]
>>>(no inode number).
>>>I also think it would be useless to do the same to vvar as it
>>>has just data and there is no point in probing it.
>>
>> Well, it would things like mremap() just work and so on. Let's get rid of special cases if we are.
>
> Well, for RFC it wouldn't move context.vdso pointer on mremap(),
> but as RFC is for x86_64 only, it will work on it.
> Anyway, I don't think it would be hard to fix and make mremap() work on
> other archs on post-RFC.
>
> The only corner-case I see for now is that /proc/self/map_files/<vdso_range>
> will point to [vdso] which is broken link. But one could read this file
> and dump/read vdso blob.
> So, in the other words: if some program assumes that /proc/self/map_files/*
> should always point to correct file, it may be confused. Not sure, maybe
> it would be confused by orphane-file mappings, so having dangling link
> there is just fine.

I don't see anything a priori wrong with having map_files point
somewhere, but it could be worth special casing it for special
mappings to preserve existing behavior (no file at all).

--Andy