[RFC,v1,0/1] Return ENAMESVRLSYMS when func matches several symbols during PMU kprobe creation

Message

Francis Laniel Aug. 23, 2023, 4:14 p.m. UTC

Hi.


In the kernel source code, it exists different functions which share the same
name but which have, of course, different addresses as they can be defined in
different modules:
root@vm-amd64:~# grep ntfs_file_write_iter /proc/kallsyms
ffffffff814ce3c0 t __pfx_ntfs_file_write_iter
ffffffff814ce3d0 t ntfs_file_write_iter
ffffffff814fc8a0 t __pfx_ntfs_file_write_iter
ffffffff814fc8b0 t ntfs_file_write_iter
This can be source of troubles when you create a PMU kprobe for such a function,
as it will only install one for the first address (e.g. 0xffffffff814ce3d0 in
the above).
This could lead to some troubles were BPF based tools does not report any event
because the second function is not called:
root@vm-amd64:/mnt# mount | grep /mnt
/foo.img on /mnt type ntfs3 (rw,relatime,uid=0,gid=0,iocharset=utf8)
# ig is a tool which installs a PMU kprobe on ntfs_file_write_iter().
root@vm-amd64:/mnt# ig trace fsslower -m 0 -f ntfs3 --host &> /tmp/foo &
[1] 207
root@vm-amd64:/mnt# dd if=./foo of=./bar count=3
3+0 records in
3+0 records out
1536 bytes (1.5 kB, 1.5 KiB) copied, 0.00543323 s, 283 kB/s
root@vm-amd64:/mnt# fg
ig trace fsslower -m 0 -f ntfs3 --host &> /tmp/foo
^Croot@vm-amd64:/mnt# more /tmp/foo
RUNTIME.CONTAINERNAME          RUNTIME.CONTAIN… PID              COMM
  T      BYTES     OFFSET        LAT FILE
                                                214              dd
  R        512          0        766 foo
                                                214              dd
  R        512        512          9 foo
                                                214              dd
As you can see in the above, only read events are reported and no write because
the kprobe is installed for the old ntfs_file_write_iter() and not the ntfs3
one.

In this contribution, I modified create_local_trace_kprobe() to test if the
function name given as argument matches several symbols.
If this is the case ENAMESVRLSYMS, a new error code, is returned to indicate the
user to use addr and offs to remove this ambiguity.
So, when the above BPF tool is run, the following error message is printed:
root@vm-amd64:~# ig trace fsslower -m 0 -f ntfs3 --host &> /tmp/foo
root@vm-amd64:~# more /tmp/foo
RUNTIME.CONTAINERNAME          RUNTIME.CONTAIN… PID              COMM
  T      BYTES     OFFSET        LAT FILE
Error: running gadget: running gadget: installing tracer: attaching kprobe: crea
ting perf_kprobe PMU (arch-specific fallback for "ntfs_file_write_iter"): token
ntfs_file_write_iter: opening perf event: errno 134

Note that, this contribution is the conclusion of a previous RFC which intended
to install a PMU kprobe for all matching symbols [1, 2].

If you see any way to improve this contribution, particularly if you have an
idea to add a test for this behavior, please share your feedback.

Francis Laniel (1):
  tracing/kprobes: Return ENAMESVRLSYMS when func matches several symbols

 arch/alpha/include/uapi/asm/errno.h        |  2 ++
 arch/mips/include/uapi/asm/errno.h         |  2 ++
 arch/parisc/include/uapi/asm/errno.h       |  2 ++
 arch/sparc/include/uapi/asm/errno.h        |  2 ++
 include/uapi/asm-generic/errno.h           |  2 ++
 kernel/trace/trace_kprobe.c                | 26 ++++++++++++++++++++++
 tools/arch/alpha/include/uapi/asm/errno.h  |  2 ++
 tools/arch/mips/include/uapi/asm/errno.h   |  2 ++
 tools/arch/parisc/include/uapi/asm/errno.h |  2 ++
 tools/arch/sparc/include/uapi/asm/errno.h  |  2 ++
 tools/include/uapi/asm-generic/errno.h     |  2 ++
 11 files changed, 46 insertions(+)


Best regards and thank you in advance.
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[1]: https://lore.kernel.org/lkml/[e-mail]/
[2]: https://lore.kernel.org/lkml/[e-mail]/
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