TT#48647 Improve fake-uname preloaded shared library

- Use hidden visibility by default, and export the symbol explicitly.
- Resolve real_uname() only once.
- Return early if the real_uname() fails, to avoid acting on bogus data.
- Call dlerror() before dlsym() to clear any previous errors.
- Compute the release member size from the utsname struct instead of
  hard-coding it.
- Always NUL-terminate the relese buffer, so protect against very long
  environment strings.
- Make various variables into const.
- Add functional tests.
- Style changes.

Change-Id: Iaf040edcc704fbc50a15d895276c820e71fe19a5

Makefile

@@ -8,6 +8,9 @@ all: build
 build:
 	 $(MAKE) -C src
 
+check:
+	$(MAKE) -C src check
+
 syntaxcheck: shellcheck
 
 shellcheck:

src/Makefile

@@ -4,12 +4,18 @@ CPPFLAGS ?=
 CPPFLAGS += -DPACKAGE_VERSION="\"$(PACKAGE_VERSION)\""
 
 CFLAGS ?= -ggdb -O2 -Wall -Wextra -Wno-unused-parameter
-CFLAGS += -fPIC -ldl
+CFLAGS += -fPIC -ldl -fvisibility=hidden
 
 PLUGIN = fake-uname.so
 
 $(PLUGIN): fake-uname.c
 	$(CC) $(CPPFLAGS) $(CFLAGS) $(LDFLAGS) -o $@ -shared $<
 
+UNAME_CHECK = LD_PRELOAD="$(CURDIR)/$(PLUGIN)" uname
+
+check: $(PLUGIN)
+	test "0.0.0" = "$(shell $(UNAME_CHECK) -r)"
+	test "1.2.3" = "$(shell UTS_RELEASE="1.2.3" $(UNAME_CHECK) -r)"
+
 clean:
 	$(RM) *.o *.so

src/fake-uname.c

@@ -17,28 +17,45 @@
 #define RTLD_NEXT      ((void *) -1l)
 #endif
 
-typedef int (*uname_t) (struct utsname * buf);
+#define SYMBOL_EXPORT __attribute__((visibility("default")))
+
+typedef int uname_func(struct utsname *buf);
 
 static void *get_libc_func(const char *funcname)
 {
   void *func;
   char *error;
 
+  /* Clear any previous errors. */
+  dlerror();
   func = dlsym(RTLD_NEXT, funcname);
-  if ((error = dlerror()) != NULL) {
+  error = dlerror();
-    fprintf(stderr, "Can't locate libc function '%s' error: %s", funcname, error);
+  if (error != NULL) {
+    fprintf(stderr, "Cannot locate libc function '%s' error: %s",
+            funcname, error);
     _exit(EXIT_FAILURE);
   }
   return func;
 }
 
-int uname(struct utsname *buf)
+int SYMBOL_EXPORT uname(struct utsname *buf)
 {
+  static uname_func *real_uname;
+  const char *release;
   int ret;
-  char *env = NULL;
-  uname_t real_uname = (uname_t) get_libc_func("uname");
 
-  ret = real_uname((struct utsname *) buf);
+  if (real_uname == NULL)
-  strncpy(buf->release, ((env = getenv("UTS_RELEASE")) == NULL) ? UTS_RELEASE : env, 65);
+    real_uname = (uname_func *)get_libc_func("uname");
+
+  ret = real_uname(buf);
+  if (ret < 0)
+    return ret;
+
+  release = getenv("UTS_RELEASE");
+  if (release == NULL)
+    release = UTS_RELEASE;
+  strncpy(buf->release, release, sizeof(buf->release) - 1);
+  buf->release[sizeof(buf->release) - 1] = '\0';
+
   return ret;
 }

t/testrunner

@@ -17,5 +17,6 @@ else
 fi
 
 make
+make check
 make syntaxcheck
 make script_version