zdtm: add support for LD_PRELOAD tests

This commit adds a `--preload-libfault` option to ZDTM's run command.
This option runs CRIU with LD_PRELOAD to intercept libc functions
such as pread(). This method allows to simulate special cases,
for example, when a successful call to pread() transfers fewer
bytes than requested.

Signed-off-by: Radostin Stoyanov <rstoyanov@fedoraproject.org>

test/libfault/Makefile

@@ -0,0 +1,21 @@
+CC = gcc
+CFLAGS = -c -fPIC -ldl
+
+SRC = libfault.c
+OBJ = $(SRC:.c=.o)
+
+LIB = libfault.so
+
+.PHONY: all clean run
+
+all: $(LIB)
+
+$(LIB): $(OBJ)
+	$(CC) -shared -o $(LIB) $(OBJ)
+
+$(OBJ): $(SRC)
+	$(CC) $(CFLAGS) $<
+
+clean:
+	rm -f $(OBJ) $(LIB)
+

test/libfault/libfault.c

@@ -0,0 +1,31 @@
+#define _GNU_SOURCE
+#include <unistd.h>
+#include <dlfcn.h>
+#include <errno.h>
+
+ssize_t (*original_pread)(int fd, void *buf, size_t count, off_t offset) = NULL;
+
+/**
+ * This function is a wrapper around pread() that is used for testing CRIU's
+ * handling of cases where pread() returns less data than requested.
+ *
+ * pmc_fill() in criu/pagemap.c is a good example of where this can happen.
+ */
+ssize_t pread64(int fd, void *buf, size_t count, off_t offset)
+{
+	if (!original_pread) {
+		original_pread = dlsym(RTLD_NEXT, "pread");
+		if (!original_pread) {
+			errno = EIO;
+			return -1;
+		}
+	}
+
+	/* The following aims to simulate the case when pread() returns less
+	 * data than requested. We need to ensure that CRIU handles such cases. */
+	if (count > 2048) {
+		count -= 1024;
+	}
+
+	return original_pread(fd, buf, count, offset);
+}