mcount: Protect FD for message channel

uftrace and libmcount communicate using a pipe in the data directory.
We need to keep the file descriptor open to ensure that uftrace and libmcount
can communicate correctly.

Some daemon programs will close all file descriptors during they startup, we
need to protect file descriptor which will be used to communicate uftrace and
libmcount to avoid broken communication issue.

Signed-off-by: Rongsong Shen <rshen@shenrs.eu>

libmcount/wrap.c

@@ -267,6 +267,7 @@ static int (*real_posix_spawnp)(pid_t *pid, const char *file,
 static int (*real_execve)(const char *path, char *const argv[], char *const envp[]);
 static int (*real_execvpe)(const char *file, char *const argv[], char *const envp[]);
 static int (*real_fexecve)(int fd, char *const argv[], char *const envp[]);
+static int (*real_close)(int fd);
 
 void mcount_hook_functions(void)
 {
@@ -284,6 +285,7 @@ void mcount_hook_functions(void)
 	real_execve = dlsym(RTLD_NEXT, "execve");
 	real_execvpe = dlsym(RTLD_NEXT, "execvpe");
 	real_fexecve = dlsym(RTLD_NEXT, "fexecve");
+	real_close = dlsym(RTLD_NEXT, "close");
 }
 
 __visible_default int backtrace(void **buffer, int sz)
@@ -608,6 +610,18 @@ __visible_default int fexecve(int fd, char *const argv[], char *const envp[])
 	return real_fexecve(fd, argv, new_envp);
 }
 
+__visible_default int close(int fd)
+{
+	if (unlikely(real_close == NULL))
+		mcount_hook_functions();
+
+	if (unlikely(fd == pfd)) {
+		return 0;
+	}
+
+	return real_close(fd);
+}
+
 #ifdef UNIT_TEST
 
 TEST_CASE(mcount_wrap_dlopen)