linux-kernel - 'perf probe' and symbols from .text.<something>

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Date:   Thu, 18 Feb 2021 20:09:17 +0300
From:   Evgenii Shatokhin <eshatokhin@...tuozzo.com>
To:     Masami Hiramatsu <mhiramat@...nel.org>,
        Arnaldo Carvalho de Melo <acme@...nel.org>
Cc:     Kristen Carlson Accardi <kristen@...ux.intel.com>,
        live-patching@...r.kernel.org,
        Peter Zijlstra <peterz@...radead.org>,
        Ingo Molnar <mingo@...hat.com>, linux-kernel@...r.kernel.org,
        Konstantin Khorenko <khorenko@...tuozzo.com>
Subject: 'perf probe' and symbols from .text.<something>

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