X-Git-Url: http://drtracing.org/?a=blobdiff_plain;f=liblttng-ust-fd%2Flttng-ust-fd.c;fp=liblttng-ust-fd%2Flttng-ust-fd.c;h=513e757a659e6d0bd15e4ea4ad5cabef98c4c798;hb=c668ba47b997cb8bc75771f4f06bb61161d25f31;hp=4818e02ed81436a524d1f39c4af59594e538b230;hpb=f6a5ff0060ed12e71f5c7b046417d74ce90f3ae6;p=deliverable%2Flttng-ust.git

diff --git a/liblttng-ust-fd/lttng-ust-fd.c b/liblttng-ust-fd/lttng-ust-fd.c
index 4818e02e..513e757a 100644
--- a/liblttng-ust-fd/lttng-ust-fd.c
+++ b/liblttng-ust-fd/lttng-ust-fd.c
@@ -24,66 +24,141 @@
 #include <unistd.h>
 #include <ust-fd.h>
 #include <dlfcn.h>
+#include <errno.h>
 
 #include <helper.h>
 #include "usterr-signal-safe.h"
 
-static int (*__lttng_ust_fd_plibc_close)(int fd);
-static int (*__lttng_ust_fd_plibc_fclose)(FILE *stream);
+#define LTTNG_UST_DLSYM_FAILED_PTR 0x1
 
+static int (*__lttng_ust_fd_plibc_close)(int fd) = NULL;
+static int (*__lttng_ust_fd_plibc_fclose)(FILE *stream) = NULL;
+
+/*
+ * Use dlsym to find the original libc close() symbol and store it in
+ * __lttng_ust_fd_plibc_close.
+ */
 static
-int _lttng_ust_fd_libc_close(int fd)
+void *_lttng_ust_fd_init_plibc_close(void)
 {
-	if (!__lttng_ust_fd_plibc_close) {
+	if (__lttng_ust_fd_plibc_close == NULL) {
 		__lttng_ust_fd_plibc_close = dlsym(RTLD_NEXT, "close");
-		if (!__lttng_ust_fd_plibc_close) {
+
+		if (__lttng_ust_fd_plibc_close == NULL) {
+			__lttng_ust_fd_plibc_close = (void *) LTTNG_UST_DLSYM_FAILED_PTR;
 			fprintf(stderr, "%s\n", dlerror());
-			return -1;
 		}
 	}
-	return lttng_ust_safe_close_fd(fd, __lttng_ust_fd_plibc_close);
+
+	return __lttng_ust_fd_plibc_close;
 }
 
+/*
+ * Use dlsym to find the original libc fclose() symbol and store it in
+ * __lttng_ust_fd_plibc_fclose.
+ */
 static
-int _lttng_ust_fd_libc_fclose(FILE *stream)
+void *_lttng_ust_fd_init_plibc_fclose(void)
 {
-	if (!__lttng_ust_fd_plibc_fclose) {
+	if (__lttng_ust_fd_plibc_fclose == NULL) {
 		__lttng_ust_fd_plibc_fclose = dlsym(RTLD_NEXT, "fclose");
-		if (!__lttng_ust_fd_plibc_fclose) {
+
+		if (__lttng_ust_fd_plibc_fclose == NULL) {
+			__lttng_ust_fd_plibc_fclose = (void *) LTTNG_UST_DLSYM_FAILED_PTR;
 			fprintf(stderr, "%s\n", dlerror());
-			return -1;
 		}
 	}
-	return lttng_ust_safe_fclose_stream(stream,
-			__lttng_ust_fd_plibc_fclose);
+
+	return __lttng_ust_fd_plibc_fclose;
+}
+
+static
+void _lttng_ust_fd_ctor(void)
+	__attribute__((constructor));
+static
+void _lttng_ust_fd_ctor(void)
+{
+	lttng_ust_common_ctor();
+
+	/*
+	 * Initialize the function pointers to the original libc symbols in the
+	 * constructor since close() has to stay async-signal-safe and as such,
+	 * we can't call dlsym() in the override functions.
+	 */
+	(void) _lttng_ust_fd_init_plibc_close();
+	(void) _lttng_ust_fd_init_plibc_fclose();
 }
 
+/*
+ * Override the libc close() symbol with our own, allowing applications to
+ * close arbitrary file descriptors. If the fd is owned by lttng-ust, return
+ * -1, errno=EBADF instead of closing it.
+ *
+ * If dlsym failed to find the original libc close() symbol, return -1,
+ * errno=ENOSYS.
+ *
+ * There is a short window before the library constructor has executed where
+ * this wrapper could call dlsym() and thus not be async-signal-safe.
+ */
 int close(int fd)
 {
-	return _lttng_ust_fd_libc_close(fd);
+	/*
+	 * We can't retry dlsym here since close is async-signal-safe.
+	 */
+	if (_lttng_ust_fd_init_plibc_close() == (void *) LTTNG_UST_DLSYM_FAILED_PTR) {
+		errno = ENOSYS;
+		return -1;
+	}
+
+	return lttng_ust_safe_close_fd(fd, __lttng_ust_fd_plibc_close);
 }
 
 /*
- * Note: fcloseall() is not an issue because it fcloses only the
- * streams it knows about, which differs from the problems caused by
- * gnulib close_stdout(), which does an explicit fclose(stdout).
+ * Override the libc fclose() symbol with our own, allowing applications to
+ * close arbitrary streams. If the fd is owned by lttng-ust, return -1,
+ * errno=EBADF instead of closing it.
+ *
+ * If dlsym failed to find the original libc close() symbol, return -1,
+ * errno=ENOSYS.
+ *
+ * There is a short window before the library constructor has executed where
+ * this wrapper could call dlsym() and thus not be async-signal-safe.
+ *
+ * Note: fcloseall() is not an issue because it closes only the streams it
+ * knows about, which differs from the problems caused by gnulib
+ * close_stdout(), which does an explicit fclose(stdout).
  */
 int fclose(FILE *stream)
 {
-	return _lttng_ust_fd_libc_fclose(stream);
+	if (_lttng_ust_fd_init_plibc_fclose() == (void *) LTTNG_UST_DLSYM_FAILED_PTR) {
+		errno = ENOSYS;
+		return -1;
+	}
+
+	return lttng_ust_safe_fclose_stream(stream,
+			__lttng_ust_fd_plibc_fclose);
 }
 
 #if defined(__sun__) || defined(__FreeBSD__)
 /* Solaris and FreeBSD. */
 void closefrom(int lowfd)
 {
-	(void) lttng_ust_safe_closefrom(lowfd, __lttng_ust_fd_plibc_close);
+	if (_lttng_ust_fd_init_plibc_close() == (void *) LTTNG_UST_DLSYM_FAILED_PTR) {
+		return;
+	}
+
+	(void) lttng_ust_safe_closefrom_fd(lowfd, __lttng_ust_fd_plibc_close);
 }
 #elif defined(__NetBSD__) || defined(__OpenBSD__)
 /* NetBSD and OpenBSD. */
 int closefrom(int lowfd)
 {
-	return lttng_ust_safe_closefrom(lowfd, __lttng_ust_fd_plibc_close);
+	if (_lttng_ust_fd_init_plibc_close() == (void *) LTTNG_UST_DLSYM_FAILED_PTR) {
+		errno = ENOSYS;
+		return -1;
+	}
+
+	return lttng_ust_safe_closefrom_fd(lowfd, __lttng_ust_fd_plibc_close);
 }
 #else
 /* As far as we know, this OS does not implement closefrom. */