On 2025-01-28 10:36, Masami Hiramatsu (Google) wrote:
Hi,
This introduces relative stacktrace, which records stacktrace entry as
the offset from _stext instead of raw address. User can enable this
format by setting options/relative-stacktrace.
Basically, this does not change anything for users who are using ftrace
with 'trace' text-formatted interface. This changes how each stacktrace
entry address is stored, so users who is using 'trace_pipe_raw' needs
to change how to decode the stacktrace.
Currently, the stacktrace is stored as raw kernel address. Thus, for
decoding the binary trace data, we need to refer the kallsyms. But this
is not useful on the platform which prohibits to access /proc/kallsyms
for security reason. Since KASLR will change the kernel text address,
we can not decode symbols without kallsyms in userspace.
On the other hand, if we record the stacktrace entries in the offset
from _stext, we can use System.map file to decode it. This is also good
for the stacktrace in the persistent ring buffer, because we don't need
to save the kallsyms before crash anymore.
The problem is to decode the address in the modules because it will be
loaded in the different place. To solve this issue, I also introduced
'module_text_offsets' event, which records module's text and init_text
info as the offset from _stext when loading it. User can store this
event in the (another) persistent ring buffer for decoding.
This does not handle the situation where a module is already loaded
before tracing starts. In LTTng we have a statedump facility for this,
where we can iterate on all modules at trace start and dump the relevant
information.
You may want to consider a similar approach for other tracers.
Thanks,
Mathieu
Thank you,
---
Masami Hiramatsu (Google) (3):
tracing: Record stacktrace as the offset from _stext
tracing: Introduce "rel_stack" option
modules: tracing: Add module_text_offsets event
include/trace/events/module.h | 40 ++++++++++++++++++++++++++++++++++++++++
kernel/module/main.c | 1 +
kernel/trace/trace.c | 11 ++++++++++-
kernel/trace/trace.h | 2 ++
kernel/trace/trace_entries.h | 22 ++++++++++++++++++++++
kernel/trace/trace_output.c | 35 +++++++++++++++++++++++++++++++----
6 files changed, 106 insertions(+), 5 deletions(-)
--
Signature
--
Mathieu Desnoyers
EfficiOS Inc.
https://www.efficios.com
