Read /proc/self/mounts when /etc/mtab is missing

src/OVAL/probes/fsdev.c

@@ -146,6 +146,19 @@ static int is_local_fs(struct mntent *ment)
 	return 1;
 }
 
+/*
+ * /etc/mtab links to /proc/self/mounts. Some container runtimes,
+ * containerd among them, create containers without the link.
+ */
+static FILE *open_mount_table(void)
+{
+	FILE *fp = setmntent(_PATH_MOUNTED, "r");
+
+	if (fp == NULL && errno == ENOENT)
+		fp = setmntent("/proc/self/mounts", "r");
+	return fp;
+}
+
 #elif defined(OS_AIX)
 static int is_local_fs(struct mntent *ment)
 {
@@ -174,6 +187,11 @@ static int is_local_fs(struct mntent *ment)
 	return 1;
 }
 
+static FILE *open_mount_table(void)
+{
+	return setmntent(_PATH_MOUNTED, "r");
+}
+
 #endif /* OS_AIX */
 
 static fsdev_t *__fsdev_init(fsdev_t *lfs)
@@ -185,7 +203,7 @@ static fsdev_t *__fsdev_init(fsdev_t *lfs)
 	struct mntent *ment;
 	struct stat st;
 
-	fp = setmntent(_PATH_MOUNTED, "r");
+	fp = open_mount_table();
 	if (fp == NULL) {
 		e = errno;
 		free(lfs);

tests/API/probes/test_fsdev_is_local_fs.c

@@ -26,6 +26,16 @@
 
 #include <stdio.h>
 #include <string.h>
+
+#if defined(OS_LINUX)
+#include <mntent.h>
+
+/* __fsdev_init reads the mount table that mtab names. */
+#undef _PATH_MOUNTED
+static const char *mtab;
+#define _PATH_MOUNTED mtab
+#endif
+
 #include "fsdev.h"
 #include "fsdev.c"
 
@@ -63,6 +73,60 @@ static int test_multiple_calls(const char *fake_mtab)
 	return (locals == 3);
 }
 
+#if defined(OS_LINUX)
+/*
+ * read_table returns the devices that __fsdev_init finds in the mount
+ * table at path, in the order of the table, or NULL with errno set.
+ */
+static fsdev_t *read_table(const char *path)
+{
+	fsdev_t *lfs = malloc(sizeof(fsdev_t));
+
+	if (lfs == NULL)
+		return NULL;
+	mtab = path;
+	return __fsdev_init(lfs);
+}
+
+/*
+ * A missing mount table gives the devices of /proc/self/mounts, which proc
+ * holds. fake_mtab gives its own: those of /, /tmp and /home that exist
+ * here, / first.
+ */
+static int test_mount_table(const fsdev_t *proc, const char *fake_mtab)
+{
+	fsdev_t *lfs;
+	struct stat st;
+	int ok;
+
+	lfs = read_table("test_fsdev_is_local_fs.missing");
+	if (lfs == NULL) {
+		fprintf(stderr, "missing mount table: %s\n", strerror(errno));
+		return 0;
+	}
+	ok = lfs->cnt == proc->cnt &&
+		(lfs->cnt == 0 || memcmp(lfs->ids, proc->ids, sizeof(dev_t) * lfs->cnt) == 0);
+	if (!ok)
+		fprintf(stderr, "missing mount table: %u devices, /proc/self/mounts gives %u\n",
+			(unsigned)lfs->cnt, (unsigned)proc->cnt);
+	fsdev_free(lfs);
+	if (!ok)
+		return 0;
+
+	lfs = read_table(fake_mtab);
+	if (lfs == NULL) {
+		fprintf(stderr, "%s: %s\n", fake_mtab, strerror(errno));
+		return 0;
+	}
+	ok = lfs->cnt >= 1 && lfs->cnt <= 3 && stat("/", &st) == 0 && lfs->ids[0] == st.st_dev;
+	if (!ok)
+		fprintf(stderr, "%s: %u devices, want 1 to 3, that of / first\n",
+			fake_mtab, (unsigned)lfs->cnt);
+	fsdev_free(lfs);
+	return ok;
+}
+#endif
+
 int main(int argc, char *argv[])
 {
 	if (!test_single_call()) {
@@ -73,6 +137,19 @@ int main(int argc, char *argv[])
 		fprintf(stderr, "test_multiple_calls has failed\n");
 		return 1;
 	}
+#if defined(OS_LINUX)
+	fsdev_t *proc = read_table("/proc/self/mounts");
+	if (proc == NULL) {
+		fprintf(stderr, "/proc/self/mounts: %s: skipping test_mount_table\n", strerror(errno));
+		return 255;
+	}
+	int ok = test_mount_table(proc, argv[1]);
+	fsdev_free(proc);
+	if (!ok) {
+		fprintf(stderr, "test_mount_table has failed\n");
+		return 1;
+	}
+#endif
 	return 0;
 }