With "bpftool prog tracelog", bpftool prints messages from the trace pipe. To do so, it first needs to find the tracefs mount point to open the pipe. Bpftool looks at a few "default" locations, including /sys/kernel/debug/tracing and /sys/kernel/tracing. Some of these locations, namely /tracing and /trace, are not standard. They are in the list because some users used to hardcode the tracing directory to short names; but we have no compelling reason to look at these locations. If we fail to find the tracefs at the default locations, we have an additional step to find it by parsing /proc/mounts anyway, so it's safe to remove these entries from the list of default locations to check. Additionally, Alexei reports that looking for the tracefs at /sys/kernel/debug/tracing may automatically mount the file system under that location, and generate a kernel log message telling that auto-mounting there is deprecated. To avoid this message, let's swap the order for checking the potential mount points: try /sys/kernel/tracing first, which should be the standard location nowadays. The kernel log message may still appear if the tracefs is not mounted on /sys/kernel/tracing when we run bpftool. Reported-by: Alexei Starovoitov <ast@xxxxxxxxxx> Closes: https://lore.kernel.org/r/CAADnVQLcMi5YQhZKsU4z3S2uVUAGu_62C33G2Zx_ruG3uXa-Ug@xxxxxxxxxxxxxx/ Signed-off-by: Quentin Monnet <qmo@xxxxxxxxxx> --- tools/bpf/bpftool/tracelog.c | 8 +++----- 1 file changed, 3 insertions(+), 5 deletions(-) diff --git a/tools/bpf/bpftool/tracelog.c b/tools/bpf/bpftool/tracelog.c index 31d806e3bdaa..49828a4f60c2 100644 --- a/tools/bpf/bpftool/tracelog.c +++ b/tools/bpf/bpftool/tracelog.c @@ -57,10 +57,8 @@ find_tracefs_mnt_single(unsigned long magic, char *mnt, const char *mntpt) static bool get_tracefs_pipe(char *mnt) { static const char * const known_mnts[] = { - "/sys/kernel/debug/tracing", "/sys/kernel/tracing", - "/tracing", - "/trace", + "/sys/kernel/debug/tracing", }; const char *pipe_name = "/trace_pipe"; const char *fstype = "tracefs"; @@ -96,11 +94,11 @@ static bool get_tracefs_pipe(char *mnt) p_info("could not find tracefs, attempting to mount it now"); /* Most of the time, tracefs is automatically mounted by debugfs at - * /sys/kernel/debug/tracing when we try to access it. If we could not + * /sys/kernel/tracing when we try to access it. If we could not * find it, it is likely that debugfs is not mounted. Let's give one * attempt at mounting just tracefs at /sys/kernel/tracing. */ - strcpy(mnt, known_mnts[1]); + strcpy(mnt, known_mnts[0]); if (mount_tracefs(mnt)) return false; -- 2.43.0
