sugar/linux
1
0
Fork 0
Code Activity

perf: Use css_tryget() to avoid propping up css refcount

Browse Source 
An rmdir pushes css's ref count to zero.  However, if the associated
directory is open at the time, the dentry ref count is non-zero.  If
the fd for this directory is then passed into perf_event_open, it
does a css_get().  This bounces the ref count back up from zero.  This
is a problem by itself.  But what makes it turn into a crash is the
fact that we end up doing an extra dput, since we perform a dput
when css_put sees the ref count go down to zero.

css_tryget() does not fall into that trap. So, we use that instead.

Reproduction test-case for the bug:

 #include <unistd.h>
 #include <sys/types.h>
 #include <sys/stat.h>
 #include <fcntl.h>
 #include <linux/unistd.h>
 #include <linux/perf_event.h>
 #include <string.h>
 #include <errno.h>
 #include <stdio.h>

 #define PERF_FLAG_PID_CGROUP    (1U << 2)

 int perf_event_open(struct perf_event_attr *hw_event_uptr,
                     pid_t pid, int cpu, int group_fd, unsigned long flags) {
         return syscall(__NR_perf_event_open,hw_event_uptr, pid, cpu,
                 group_fd, flags);
 }

 /*
  * Directly poke at the perf_event bug, since it's proving hard to repro
  * depending on where in the kernel tree.  what moved?
  */
 int main(int argc, char **argv)
 {
        int fd;
        struct perf_event_attr attr;
        memset(&attr, 0, sizeof(attr));
        attr.exclude_kernel = 1;
        attr.size = sizeof(attr);
        mkdir("/dev/cgroup/perf_event/blah", 0777);
        fd = open("/dev/cgroup/perf_event/blah", O_RDONLY);
        perror("open");
        rmdir("/dev/cgroup/perf_event/blah");
        sleep(2);
        perf_event_open(&attr, fd, 0, -1,  PERF_FLAG_PID_CGROUP);
        perror("perf_event_open");
        close(fd);
        return 0;
 }

Signed-off-by: Salman Qazi <sqazi@google.com>
Signed-off-by: Peter Zijlstra <a.p.zijlstra@chello.nl>
Acked-by: Tejun Heo <tj@kernel.org>
Link: http://lkml.kernel.org/r/20120614223108.1025.2503.stgit@dungbeetle.mtv.corp.google.com
Signed-off-by: Ingo Molnar <mingo@kernel.org>
This commit is contained in:
Salman Qazi
2012-06-14 15:31:09 -07:00
committed by Ingo Molnar
parent 8ed9eac476
commit 9c5da09d26
1 changed files with 7 additions and 3 deletions
Download Patch File Download Diff File Expand all files Collapse all files

10
kernel/events/core.c
View File

@@ -253,9 +253,9 @@ perf_cgroup_match(struct perf_event *event)
return !event->cgrp || event->cgrp == cpuctx->cgrp;
}
static inline void perf_get_cgroup(struct perf_event *event)
static inline bool perf_tryget_cgroup(struct perf_event *event)
{
css_get(&event->cgrp->css);
return css_tryget(&event->cgrp->css);
}
static inline void perf_put_cgroup(struct perf_event *event)
@@ -484,7 +484,11 @@ static inline int perf_cgroup_connect(int fd, struct perf_event *event,
event->cgrp = cgrp;
/* must be done before we fput() the file */
perf_get_cgroup(event);
if (!perf_tryget_cgroup(event)) {
event->cgrp = NULL;
ret = -ENOENT;
goto out;
}
/*
* all events in a group must monitor