oom: restore old OOM score

restore the original OOM score before running the OCI runtime so that
the container process will inherit the OOM score from the environment
instead of the default value of 0.

Signed-off-by: Giuseppe Scrivano <[email protected]>

src/conmon.c

@@ -43,6 +43,7 @@ int main(int argc, char *argv[])
 	_cleanup_gerror_ GError *err = NULL;
 	char buf[BUF_SIZE];
 	int num_read;
+	int old_oom_score = 0;
 	_cleanup_close_ int dev_null_r_cleanup = -1;
 	_cleanup_close_ int dev_null_w_cleanup = -1;
 	_cleanup_close_ int dummyfd = -1;
@@ -54,7 +55,7 @@ int main(int argc, char *argv[])
 
 	process_cli();
 
-	attempt_oom_adjust("-1000");
+	attempt_oom_adjust(-1000, &old_oom_score);
 
 	/* ignoring SIGPIPE prevents conmon from being spuriously killed */
 	signal(SIGPIPE, SIG_IGN);
@@ -290,7 +291,7 @@ int main(int argc, char *argv[])
 		}
 
 		// We don't want runc to be unkillable so we reset the oom_score_adj back to 0
-		attempt_oom_adjust("0");
+		attempt_oom_adjust(old_oom_score, NULL);
 		execv(g_ptr_array_index(runtime_argv, 0), (char **)runtime_argv->pdata);
 		exit(127);
 	}

src/oom.c

@@ -5,15 +5,23 @@
 #include <string.h>
 #include <unistd.h>
 
-void attempt_oom_adjust(const char *const oom_score)
+void attempt_oom_adjust(int oom_score, int *old_value)
 {
 #ifdef __linux__
-	int oom_score_fd = open("/proc/self/oom_score_adj", O_WRONLY);
+	char fmt_oom_score[16];
+	int oom_score_fd = open("/proc/self/oom_score_adj", O_RDWR);
 	if (oom_score_fd < 0) {
 		ndebugf("failed to open /proc/self/oom_score_adj: %s\n", strerror(errno));
 		return;
 	}
-	if (write(oom_score_fd, oom_score, strlen(oom_score)) < 0) {
+	if (old_value) {
+		if (read(oom_score_fd, fmt_oom_score, sizeof(fmt_oom_score)) < 0) {
+			ndebugf("failed to read from /proc/self/oom_score_adj: %s\n", strerror(errno));
+		}
+		*old_value = atoi(fmt_oom_score);
+	}
+	sprintf(fmt_oom_score, "%d", oom_score);
+	if (write(oom_score_fd, fmt_oom_score, strlen(fmt_oom_score)) < 0) {
 		ndebugf("failed to write to /proc/self/oom_score_adj: %s\n", strerror(errno));
 	}
 	close(oom_score_fd);

src/oom.h

@@ -1,6 +1,6 @@
 #if !defined(OOM_H)
 #define OOM_H
 
-void attempt_oom_adjust(const char *const oom_score);
+void attempt_oom_adjust(int oom_score, int *old_value);
 
 #endif // OOM_H