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authorElazar Leibovich <elazar@lightbitslabs.com>2018-12-31 13:58:37 +0200
committerSteven Rostedt (VMware) <rostedt@goodmis.org>2019-05-08 12:15:12 -0400
commitcbe08bcbbe787315c425dde284dcb715cfbf3f39 (patch)
tree963741ea74c1e6fef3f58604ed5ed7b8745303b4 /kernel/trace
parent6fc2171c5c03672bae71d04a0f5fa88cc9c3b4e2 (diff)
tracing: Fix partial reading of trace event's id file
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>
Diffstat (limited to 'kernel/trace')
-rw-r--r--kernel/trace/trace_events.c3
1 files changed, 0 insertions, 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;