On Sun, 2 Aug 2026 20:08:35 -0700
Josh Poimboeuf <[email protected]> wrote:
> 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.
Nice catch.
>
> 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 */
I'd like to keep this consistent with lookup_rec().
>
> mutex_lock(&ftrace_lock);
>
Can you do this instead?
-- Steve
diff --git a/kernel/trace/ftrace.c b/kernel/trace/ftrace.c
index 6c47a94f5924..dbb0fc2928d8 100644
--- a/kernel/trace/ftrace.c
+++ b/kernel/trace/ftrace.c
@@ -8296,7 +8296,8 @@ static void add_to_clear_hash_list(struct list_head
*clear_list,
void ftrace_free_mem(struct module *mod, void *start_ptr, void *end_ptr)
{
unsigned long start = (unsigned long)(start_ptr);
- unsigned long end = (unsigned long)(end_ptr);
+ /* end is inclusive and end_ptr is exclusive */
+ unsigned long end = (unsigned long)(end_ptr) - 1;
struct ftrace_page **last_pg = &ftrace_pages_start;
struct ftrace_page *tmp_page = NULL;
struct ftrace_page *pg;
