[PATCH] ftrace: Fix off-by-one fentry site disable in ftrace_free_mem()

When a module's init text is freed, do_init_module() calls
ftrace_free_mem() with a half-open [start, end) range.  However the
ftrace_cmp_recs() comparator treats the upper bound as inclusive, as all
its other users do, passing 'ip + size - 1'.  So ftrace_free_mem() can
delete a record sitting exactly at 'end', which is outside the freed
range.

For a kernel without CFI or IBT, the first record of a function is at
the function start, which for the first function in a module is also the
base of its text allocation.  As the module allocator packs its regions,
that address is often the 'end' passed by a neighboring module's
do_init_module(), causing the first function's ftrace location to get
disabled, preventing an attempt to livepatch it:

  livepatch: failed to find location for function 'pcspkr_probe'

Convert the exclusive end to the inclusive 'end - 1' the comparator
expects, and return early for an empty range to avoid the subtraction
from underflowing when the init text size is zero.

Fixes: 42c269c88dc1 ("ftrace: Allow for function tracing to record init functions on boot up")
Signed-off-by: Josh Poimboeuf <[email protected]>
---
 kernel/trace/ftrace.c | 5 ++++-
 1 file changed, 4 insertions(+), 1 deletion(-)

diff --git a/kernel/trace/ftrace.c b/kernel/trace/ftrace.c
index f93e34dd2328..7d8b736f0d86 100644
--- a/kernel/trace/ftrace.c
+++ b/kernel/trace/ftrace.c
@@ -8293,8 +8293,11 @@ void ftrace_free_mem(struct module *mod, void *start_ptr, void *end_ptr)
 	struct ftrace_init_func *func, *func_next;
 	LIST_HEAD(clear_hash);
 
+	if (start >= end)
+		return;
+
 	key.ip = start;
-	key.flags = end;	/* overload flags, as it is unsigned long */
+	key.flags = end - 1;	/* overload flags, as it is unsigned long */
 
 	mutex_lock(&ftrace_lock);
 
-- 
2.54.0