Re: [PATCH 2/2] cgroups: fix cgroup_event_listener error handling

Greg Thelen
2013-01-07T19:50:22Z

On Mon, Jan 07 2013, Tejun Heo wrote:

> On Fri, Jan 04, 2013 at 01:05:18PM -0800, Greg Thelen wrote:
>> If the <absolute-path-to-control-file> command line parameter cannot
>> be opened, then cgroup_event_listener prints an error message and
>> tries to return an error. However, due to an uninitialized variable
>> the return value was undefined.
>>
>> With this patch such failures always return non-zero error.
>>
>> Compiler warning found this:
>> $ gcc -Wall -O2 cgroup_event_listener.c
>> cgroup_event_listener.c: In function ‘main’:
>> cgroup_event_listener.c:109:2: warning: ‘ret’ may be used uninitialized in this function [-Wuninitialized]
>>
>> Signed-off-by: Greg Thelen <gthelen@google.com>
>> ---
>> tools/cgroup/cgroup_event_listener.c | 2 +-
>> 1 files changed, 1 insertions(+), 1 deletions(-)
>>
>> diff --git a/tools/cgroup/cgroup_event_listener.c b/tools/cgroup/cgroup_event_listener.c
>> index 3e082f9..a70f00c 100644
>> --- a/tools/cgroup/cgroup_event_listener.c
>> +++ b/tools/cgroup/cgroup_event_listener.c
>> @@ -35,7 +35,7 @@ int main(int argc, char **argv)
>> if (cfd == -1) {
>> fprintf(stderr, "Cannot open %s: %s\n", argv[1],
>> strerror(errno));
>> - goto out;
>> + return 1;
>
> Hmm... so, event_control open failure path is broken the same way.
> Can you please fix it together? Please just remove the cleanup path.

Done. Patch below.

---8<---
cgroups: fix cgroup_event_listener error handling

The error handling in cgroup_event_listener.c did not correctly deal
with either an error opening either <control_file> or
cgroup.event_control. Due to an uninitialized variable the program
exit code was undefined if either of these opens failed.

This patch simplifies and corrects cgroup_event_listener.c error
handling by:
1. using err*() rather than printf(),exit()
2. depending on process exit to close open files

With this patch failures always return non-zero error.

Signed-off-by: Greg Thelen <gthelen@google.com>
---
tools/cgroup/cgroup_event_listener.c | 72 ++++++++++-----------------------
1 files changed, 22 insertions(+), 50 deletions(-)

diff --git a/tools/cgroup/cgroup_event_listener.c b/tools/cgroup/cgroup_event_listener.c
index 3e082f9..4eb5507 100644
--- a/tools/cgroup/cgroup_event_listener.c
+++ b/tools/cgroup/cgroup_event_listener.c
@@ -5,6 +5,7 @@
*/

#include <assert.h>
+#include <err.h>
#include <errno.h>
#include <fcntl.h>
#include <libgen.h>
@@ -15,7 +16,7 @@

#include <sys/eventfd.h>

-#define USAGE_STR "Usage: cgroup_event_listener <path-to-control-file> <args>\n"
+#define USAGE_STR "Usage: cgroup_event_listener <path-to-control-file> <args>"

int main(int argc, char **argv)
{
@@ -26,49 +27,33 @@ int main(int argc, char **argv)
char line[LINE_MAX];
int ret;

- if (argc != 3) {
- fputs(USAGE_STR, stderr);
- return 1;
- }
+ if (argc != 3)
+ errx(1, "%s", USAGE_STR);

cfd = open(argv[1], O_RDONLY);
- if (cfd == -1) {
- fprintf(stderr, "Cannot open %s: %s\n", argv[1],
- strerror(errno));
- goto out;
- }
+ if (cfd == -1)
+ err(1, "Cannot open %s", argv[1]);

ret = snprintf(event_control_path, PATH_MAX, "%s/cgroup.event_control",
dirname(argv[1]));
- if (ret >= PATH_MAX) {
- fputs("Path to cgroup.event_control is too long\n", stderr);
- goto out;
- }
+ if (ret >= PATH_MAX)
+ errx(1, "Path to cgroup.event_control is too long");

event_control = open(event_control_path, O_WRONLY);
- if (event_control == -1) {
- fprintf(stderr, "Cannot open %s: %s\n", event_control_path,
- strerror(errno));
- goto out;
- }
+ if (event_control == -1)
+ err(1, "Cannot open %s", event_control_path);

efd = eventfd(0, 0);
- if (efd == -1) {
- perror("eventfd() failed");
- goto out;
- }
+ if (efd == -1)
+ err(1, "eventfd() failed");

ret = snprintf(line, LINE_MAX, "%d %d %s", efd, cfd, argv[2]);
- if (ret >= LINE_MAX) {
- fputs("Arguments string is too long\n", stderr);
- goto out;
- }
+ if (ret >= LINE_MAX)
+ errx(1, "Arguments string is too long");

ret = write(event_control, line, strlen(line) + 1);
- if (ret == -1) {
- perror("Cannot write to cgroup.event_control");
- goto out;
- }
+ if (ret == -1)
+ err(1, "Cannot write to cgroup.event_control");

while (1) {
uint64_t result;
@@ -77,34 +62,21 @@ int main(int argc, char **argv)
if (ret == -1) {
if (errno == EINTR)
continue;
- perror("Cannot read from eventfd");
- break;
+ err(1, "Cannot read from eventfd");
}
assert(ret == sizeof(result));

ret = access(event_control_path, W_OK);
if ((ret == -1) && (errno == ENOENT)) {
- puts("The cgroup seems to have removed.");
- ret = 0;
- break;
- }
-
- if (ret == -1) {
- perror("cgroup.event_control "
- "is not accessible any more");
+ puts("The cgroup seems to have removed.");
break;
}

+ if (ret == -1)
+ err(1, "cgroup.event_control is not accessible any more");
+
printf("%s %s: crossed\n", argv[1], argv[2]);
}

-out:
- if (efd >= 0)
- close(efd);
- if (event_control >= 0)
- close(event_control);
- if (cfd >= 0)
- close(cfd);
-
- return (ret != 0);
+ return 0;
}
--
1.7.7.3