Messages in this thread | | | From | Evgenii Shatokhin <> | Subject | 'perf probe' and symbols from .text.<something> | Date | Thu, 18 Feb 2021 20:09:17 +0300 |
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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
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