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Salvatore Benedetto reported an issue that when doing syscall tracepoint
tracing the kernel stack is empty. For example, using the following
command line
bpftrace -e 'tracepoint:syscalls:sys_enter_read { print("Kernel Stack\n"); print(kstack()); }'
bpftrace -e 'tracepoint:syscalls:sys_exit_read { print("Kernel Stack\n"); print(kstack()); }'
the output for both commands is
===
Kernel Stack
===
Further analysis shows that pt_regs used for bpf syscall tracepoint
tracing is from the one constructed during user->kernel transition.
The call stack looks like
perf_syscall_enter+0x88/0x7c0
trace_sys_enter+0x41/0x80
syscall_trace_enter+0x100/0x160
do_syscall_64+0x38/0xf0
entry_SYSCALL_64_after_hwframe+0x76/0x7e
The ip address stored in pt_regs is from user space hence no kernel
stack is printed.
To fix the issue, kernel address from pt_regs is required.
In kernel repo, there are already a few cases like this. For example,
in kernel/trace/bpf_trace.c, several perf_fetch_caller_regs(fake_regs_ptr)
instances are used to supply ip address or use ip address to construct
call stack.
Instead of allocate fake_regs in the stack which may consume
a lot of bytes, the function perf_trace_buf_alloc() in
perf_syscall_{enter, exit}() is leveraged to create fake_regs,
which will be passed to perf_call_bpf_{enter,exit}().
For the above bpftrace script, I got the following output with this patch:
for tracepoint:syscalls:sys_enter_read
===
Kernel Stack
syscall_trace_enter+407
syscall_trace_enter+407
do_syscall_64+74
entry_SYSCALL_64_after_hwframe+75
===
and for tracepoint:syscalls:sys_exit_read
===
Kernel Stack
syscall_exit_work+185
syscall_exit_work+185
syscall_exit_to_user_mode+305
do_syscall_64+118
entry_SYSCALL_64_after_hwframe+75
===
Reported-by: Salvatore Benedetto <salvabenedetto@meta.com>
Suggested-by: Andrii Nakryiko <andrii@kernel.org>
Signed-off-by: Yonghong Song <yonghong.song@linux.dev>
Signed-off-by: Andrii Nakryiko <andrii@kernel.org>
Acked-by: Andrii Nakryiko <andrii@kernel.org>
Link: https://lore.kernel.org/bpf/20240910214037.3663272-1-yonghong.song@linux.dev
Linux kernel
============
There are several guides for kernel developers and users. These guides can
be rendered in a number of formats, like HTML and PDF. Please read
Documentation/admin-guide/README.rst first.
In order to build the documentation, use ``make htmldocs`` or
``make pdfdocs``. The formatted documentation can also be read online at:
https://www.kernel.org/doc/html/latest/
There are various text files in the Documentation/ subdirectory,
several of them using the reStructuredText markup notation.
Please read the Documentation/process/changes.rst file, as it contains the
requirements for building and running the kernel, and information about
the problems which may result by upgrading your kernel.