Date: Tue, 28 Mar 2023 15:52:27 +0300 From: Christos Margiolis <christos@freebsd.org> To: status-updates@freebsdfoundation.org Cc: freebsd-dtrace@freebsd.org, markj@freebsd.org, jrm@freebsd.org Subject: [Development report #6] Improve the kinst DTrace provider Message-ID: <20230328125227.zz35ufmmkocnpfdi@pleb>
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The past few days I've been working on various bug fixes both in kinst
and in libdtrace. Inline function tracing is almost done [1].
libdtrace now parses all loaded kernel modules, instead of just
`kernel`. This makes it compatible with kinst, which also searches all
loaded modules.
After some testing I noticed that it is possible to have both a
non-inline and an inline definition of the same function in a kernel
module. If libdtrace finds such a case, it creates an additional FBT
probe for the non-inline definition:
# dtrace -dn 'kinst::cam_strvis_flag:entry'
kinst::cam_strvis:25,
kinst::cam_strvis_flag:0,
fbt::cam_strvis_flag:entry
{
}
dtrace: description 'kinst::cam_strvis_flag:entry' matched 3
probes
For inline tracing, I implemented the algorithm described here [2]
(markj@ has also implemented it in lib/libproc/proc_sym.c) to make sure
that both the modules' ELF and debug files are up to date (i.e the
module has been built with `DEBUG_FLAGS=-g`), otherwise we might run
into version mismatches between functions. If such a mismatch is found,
libdtrace prints a warning and skips that module.
I wrote a few Kyua tests for kinst and made use of
sys/dev/dtrace/dtrace_test.c (see inline tracing PR).
In my previous email I mentioned that I modified kinst to search for
`push %rbp` anywhere in a function, and skip the function if no `push
%rbp` is found. Since this affects safe-to-trace functions that do
not `push %rbp`, I'm working on an experimental change to exclude only
exception handlers, and not search for `push %rbp` at all [3]. However,
I'm still not sure this is 100% fail-proof, and I will need to do some
more testing to make sure there are no accidental crashes.
[1] https://reviews.freebsd.org/D38825
[2] https://sourceware.org/gdb/onlinedocs/gdb/Separate-Debug-Files.html
[3] https://reviews.freebsd.org/D39229
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