From: Elazar Leibovich elazar@lightbitslabs.com
When reading only part of the id file, the ppos isn't tracked correctly. This is taken care by simple_read_from_buffer.
Reading a single byte, and then the next byte would result EOF.
While this seems like not a big deal, this breaks abstractions that reads information from files unbuffered. See for example https://github.com/golang/go/issues/29399
This code was mentioned as problematic in commit cd458ba9d5a5 ("tracing: Do not (ab)use trace_seq in event_id_read()")
An example C code that show this bug is:
#include <stdio.h> #include <stdint.h>
#include <sys/types.h> #include <sys/stat.h> #include <fcntl.h> #include <unistd.h>
int main(int argc, char **argv) { if (argc < 2) return 1; int fd = open(argv[1], O_RDONLY); char c; read(fd, &c, 1); printf("First %c\n", c); read(fd, &c, 1); printf("Second %c\n", c); }
Then run with, e.g.
sudo ./a.out /sys/kernel/debug/tracing/events/tcp/tcp_set_state/id
You'll notice you're getting the first character twice, instead of the first two characters in the id file.
Link: http://lkml.kernel.org/r/20181231115837.4932-1-elazar@lightbitslabs.com
Cc: Orit Wasserman orit.was@gmail.com Cc: Oleg Nesterov oleg@redhat.com Cc: Ingo Molnar mingo@redhat.com Cc: stable@vger.kernel.org Fixes: 23725aeeab10b ("ftrace: provide an id file for each event") Signed-off-by: Elazar Leibovich elazar@lightbitslabs.com Signed-off-by: Steven Rostedt (VMware) rostedt@goodmis.org --- kernel/trace/trace_events.c | 3 --- 1 file changed, 3 deletions(-)
diff --git a/kernel/trace/trace_events.c b/kernel/trace/trace_events.c index 81c038ed6cee..0ce3db67f556 100644 --- a/kernel/trace/trace_events.c +++ b/kernel/trace/trace_events.c @@ -1319,9 +1319,6 @@ event_id_read(struct file *filp, char __user *ubuf, size_t cnt, loff_t *ppos) char buf[32]; int len;
- if (*ppos) - return 0; - if (unlikely(!id)) return -ENODEV;