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    Subject[PATCH 01/29] perf probe: Try to use symbol table if searching debug info failed
    Date
    A problem can occure in statically linked perf when vmlinux can be found:

    # perf probe --add sys_epoll_pwait
    probe-definition(0): sys_epoll_pwait
    symbol:sys_epoll_pwait file:(null) line:0 offset:0 return:0 lazy:(null)
    0 arguments
    Looking at the vmlinux_path (7 entries long)
    Using /lib/modules/4.2.0-rc1+/build/vmlinux for symbols
    Open Debuginfo file: /lib/modules/4.2.0-rc1+/build/vmlinux
    Try to find probe point from debuginfo.
    Symbol sys_epoll_pwait address found : ffffffff8122bd40
    Matched function: SyS_epoll_pwait
    Failed to get call frame on 0xffffffff8122bd40
    An error occurred in debuginfo analysis (-2).
    Error: Failed to add events. Reason: No such file or directory (Code: -2)

    The reason is caused by libdw that, if libdw is statically linked, it can't
    load libebl_{arch}.so reliable.

    In this case it is still possible to get the address from /proc/kalksyms.
    However, perf tries that only when libdw returns -EBADF.

    This patch gives it another chance to utilize symbol table, even if libdw
    return error code other than -EBADF.

    After applying this patch:

    # perf probe -nv --add sys_epoll_pwait
    probe-definition(0): sys_epoll_pwait
    symbol:sys_epoll_pwait file:(null) line:0 offset:0 return:0 lazy:(null)
    0 arguments
    Looking at the vmlinux_path (7 entries long)
    Using /lib/modules/4.2.0-rc1+/build/vmlinux for symbols
    Open Debuginfo file: /lib/modules/4.2.0-rc1+/build/vmlinux
    Try to find probe point from debuginfo.
    Symbol sys_epoll_pwait address found : ffffffff8122bd40
    Matched function: SyS_epoll_pwait
    Failed to get call frame on 0xffffffff8122bd40
    An error occurred in debuginfo analysis (-2).
    Trying to use symbols.
    Opening /sys/kernel/debug/tracing/kprobe_events write=1
    Added new event:
    Writing event: p:probe/sys_epoll_pwait _text+2276672
    probe:sys_epoll_pwait (on sys_epoll_pwait)

    You can now use it in all perf tools, such as:

    perf record -e probe:sys_epoll_pwait -aR sleep 1

    Although libdw returns an error (Failed to get call frame),
    perf tries symbol table and finally gets correct address.

    Signed-off-by: Wang Nan <wangnan0@huawei.com>
    ---
    tools/perf/util/probe-event.c | 7 ++++---
    1 file changed, 4 insertions(+), 3 deletions(-)

    diff --git a/tools/perf/util/probe-event.c b/tools/perf/util/probe-event.c
    index fe4941a..f07374b 100644
    --- a/tools/perf/util/probe-event.c
    +++ b/tools/perf/util/probe-event.c
    @@ -705,9 +705,10 @@ static int try_to_find_probe_trace_events(struct perf_probe_event *pev,
    }
    /* Error path : ntevs < 0 */
    pr_debug("An error occurred in debuginfo analysis (%d).\n", ntevs);
    - if (ntevs == -EBADF) {
    - pr_warning("Warning: No dwarf info found in the vmlinux - "
    - "please rebuild kernel with CONFIG_DEBUG_INFO=y.\n");
    + if (ntevs < 0) {
    + if (ntevs == -EBADF)
    + pr_warning("Warning: No dwarf info found in the vmlinux - "
    + "please rebuild kernel with CONFIG_DEBUG_INFO=y.\n");
    if (!need_dwarf) {
    pr_debug("Trying to use symbols.\n");
    return 0;
    --
    2.1.0


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