selftests/x86/lam: Zero out buffer for readlink()

Zero out the buffer for readlink() since readlink() does not append a
terminating null byte to the buffer.  Also change the buffer length
passed to readlink() to 'PATH_MAX - 1' to ensure the resulting string
is always null terminated.

Fixes: 833c12c ("selftests/x86/lam: Add inherit test cases for linear-address masking")
Signed-off-by: Binbin Wu <binbin.wu@linux.intel.com>
Signed-off-by: Ingo Molnar <mingo@kernel.org>
Reviewed-by: Kirill A. Shutemov <kirill.shutemov@linux.intel.com>
Link: https://lore.kernel.org/r/20231016062446.695-1-binbin.wu@linux.intel.com

Author: Binbin Wu

tools/testing/selftests/x86/lam.c

@@ -573,7 +573,7 @@ int do_uring(unsigned long lam)
 	char path[PATH_MAX] = {0};
 
 	/* get current process path */
-	if (readlink("/proc/self/exe", path, PATH_MAX) <= 0)
+	if (readlink("/proc/self/exe", path, PATH_MAX - 1) <= 0)
 		return 1;
 
 	int file_fd = open(path, O_RDONLY);
@@ -680,14 +680,14 @@ static int handle_execve(struct testcases *test)
 		perror("Fork failed.");
 		ret = 1;
 	} else if (pid == 0) {
-		char path[PATH_MAX];
+		char path[PATH_MAX] = {0};
 
 		/* Set LAM mode in parent process */
 		if (set_lam(lam) != 0)
 			return 1;
 
 		/* Get current binary's path and the binary was run by execve */
-		if (readlink("/proc/self/exe", path, PATH_MAX) <= 0)
+		if (readlink("/proc/self/exe", path, PATH_MAX - 1) <= 0)
 			exit(-1);
 
 		/* run binary to get LAM mode and return to parent process */