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From: Evgenii Shatokhin <eshatokhin@virtuozzo.com>
To: Masami Hiramatsu <mhiramat@kernel.org>,
	Arnaldo Carvalho de Melo <acme@kernel.org>
Cc: Kristen Carlson Accardi <kristen@linux.intel.com>,
	live-patching@vger.kernel.org,
	Peter Zijlstra <peterz@infradead.org>,
	Ingo Molnar <mingo@redhat.com>,
	linux-kernel@vger.kernel.org,
	Konstantin Khorenko <khorenko@virtuozzo.com>
Subject: 'perf probe' and symbols from .text.<something>
Date: Thu, 18 Feb 2021 20:09:17 +0300	[thread overview]
Message-ID: <09257fb8-3ded-07b0-b3cc-55d5431698d8@virtuozzo.com> (raw)

Hi,

It seems, 'perf probe' can only see functions from .text section in the 
kernel modules, but not from .text.unlikely or other .text.* sections.

For example, with kernel 5.11 and nf_conntrack.ko with debug info, 'perf 
probe' succeeds for nf_conntrack_attach() from .text and fails for 
nf_ct_resolve_clash() from .text.unlikely:

------------
# perf probe -v -m nf_conntrack nf_ct_resolve_clash
probe-definition(0): nf_ct_resolve_clash
symbol:nf_ct_resolve_clash file:(null) line:0 offset:0 return:0 lazy:(null)
0 arguments
Failed to get build-id from nf_conntrack.
Cache open error: -1
Open Debuginfo file: 
/lib/modules/5.11.0-test01/kernel/net/netfilter/nf_conntrack.ko
Try to find probe point from debuginfo.
Matched function: nf_ct_resolve_clash [33616]
Probe point found: nf_ct_resolve_clash+0
Found 1 probe_trace_events.
Post processing failed or all events are skipped. (-2)
Probe point 'nf_ct_resolve_clash' not found.
   Error: Failed to add events. Reason: No such file or directory (Code: -2)

# perf probe -v -m nf_conntrack nf_conntrack_attach
probe-definition(0): nf_conntrack_attach
symbol:nf_conntrack_attach file:(null) line:0 offset:0 return:0 lazy:(null)
0 arguments
Failed to get build-id from nf_conntrack.
Cache open error: -1
Open Debuginfo file: 
/lib/modules/5.11.0-test01/kernel/net/netfilter/nf_conntrack.ko
Try to find probe point from debuginfo.
Matched function: nf_conntrack_attach [2c8c3]
Probe point found: nf_conntrack_attach+0
Found 1 probe_trace_events.
Opening /sys/kernel/tracing//kprobe_events write=1
Opening /sys/kernel/tracing//README write=0
Writing event: p:probe/nf_conntrack_attach 
nf_conntrack:nf_conntrack_attach+0
Added new event:
   probe:nf_conntrack_attach (on nf_conntrack_attach in nf_conntrack)
------------

Is there a way to allow probing of functions in .text.<something> ?

Of course, one could place probes using absolute addresses of the 
functions but that would be less convenient.

This also affects many livepatch modules where the kernel code can be 
compiled with -ffunction-sections and each function may end up in a 
separate section .text.<function_name>. 'perf probe' cannot be used 
there, except with the absolute addresses.

Moreover, if FGKASLR patches are merged 
(https://lwn.net/Articles/832434/) and the kernel is built with FGKASLR 
enabled, -ffunction-sections will be used too. 'perf probe' will be 
unable to see the kernel functions then.

Regards,
Evgenii

             reply	other threads:[~2021-02-18 19:04 UTC|newest]

Thread overview: 16+ messages / expand[flat|nested]  mbox.gz  Atom feed  top
2021-02-18 17:09 Evgenii Shatokhin [this message]
2021-02-18 19:35 ` Josh Poimboeuf
2021-02-22 15:05 ` Masami Hiramatsu
2021-02-22 15:12   ` Masami Hiramatsu
2021-02-22 17:51   ` Josh Poimboeuf
2021-02-23  1:23     ` Masami Hiramatsu
2021-02-23  7:36       ` Masami Hiramatsu
2021-02-23 19:45         ` Josh Poimboeuf
2021-02-24  8:00           ` Masami Hiramatsu
2021-02-23  1:48   ` [PATCH] perf-probe: Failback to symbol-base probe for probes on module Masami Hiramatsu
2021-02-23  7:36     ` Masami Hiramatsu
2021-02-23  7:09   ` 'perf probe' and symbols from .text.<something> Masami Hiramatsu
2021-02-23  7:37   ` [PATCH] perf-probe: dso: Add symbols in .text.* subsections to text symbol map in kenrel modules Masami Hiramatsu
2021-02-23 15:02     ` Evgenii Shatokhin
2021-02-23 20:11       ` Arnaldo Carvalho de Melo
2021-02-24  7:47         ` Evgenii Shatokhin

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