[PATCH] ftrace: Initialize modname and offset in print_rec()

[Aaron Ahmed <aarnahmd@amazon.com>]

print_rec() leaves 'modname' and 'offset' uninitialized and
kallsyms_lookup() only sets them on success. When the lookup fails like
when a module is being loaded or unloaded, 'modname' holds a garbage
value, passes the 'if (modname)' check, and is dereferenced by
seq_printf().

  Oops: general protection fault
  RIP: 0010:string+0x48/0xe0
  Call Trace:
   vsnprintf+0x1f5/0x650
   seq_printf+0x8e/0xb0
   print_rec+0x6c/0xc0
   t_show+0x4c/0x440
   seq_read_iter+0x119/0x480

This can be triggered by reading available_filter_functions while a
module is being loaded or unloaded.

Initialize both variables so a failed lookup skips the module name.

Fixes: b39181f7c690 ("ftrace: Add FTRACE_MCOUNT_MAX_OFFSET to avoid adding weak function")
Cc: stable@vger.kernel.org
Tested-by: Gavin Inglis <giinglis@amazon.com>
Signed-off-by: Aaron Ahmed <aarnahmd@amazon.com>
---
Reproducer: https://gist.github.com/ginglis13/dd3f64426767cd48df55dfe5d26c2299

 kernel/trace/ftrace.c | 4 ++--
 1 file changed, 2 insertions(+), 2 deletions(-)

diff --git a/kernel/trace/ftrace.c b/kernel/trace/ftrace.c
index 673a54fdf3929..164fbc07bce05 100644
--- a/kernel/trace/ftrace.c
+++ b/kernel/trace/ftrace.c
@@ -4426,9 +4426,9 @@ subsys_initcall(ftrace_check_for_weak_functions);
 
 static int print_rec(struct seq_file *m, unsigned long ip)
 {
-	unsigned long offset;
+	unsigned long offset = 0;
 	char str[KSYM_SYMBOL_LEN];
-	char *modname;
+	char *modname = NULL;
 	const char *ret;
 
 	ret = kallsyms_lookup(ip, NULL, &offset, &modname, str);

base-commit: 70dd4ee72eb2d5793c62e71c14e3aba78e5a4e56
-- 
2.47.3