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.

Cc: stable@vger.kernel.org
Fixes: 42c269c88dc1 ("ftrace: Allow for function tracing to record init functions on boot up")
Link: https://patch.msgid.link/1b5ccfa8095bdb1277f84af1c2c2e2205aca03ae.1785992188.git.jpoimboe@kernel.org
Signed-off-by: Josh Poimboeuf <jpoimboe@kernel.org>
Acked-by: Masami Hiramatsu (Google) <mhiramat@kernel.org>
Signed-off-by: Steven Rostedt <rostedt@goodmis.org>

diff --git a/kernel/trace/ftrace.c b/kernel/trace/ftrace.c
index 414e425c2d8042d8576608eca8d341bc110d54ba..7c50f8ae5a0c4224243ddc6b06a3fc1a4db04695 100644 (file)
--- a/kernel/trace/ftrace.c
+++ b/kernel/trace/ftrace.c
@@ -8305,7 +8305,8 @@ 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;
@@ -8315,6 +8316,9 @@ 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_ptr >= end_ptr)
+               return;
+
        key.ip = start;
        key.flags = end;        /* overload flags, as it is unsigned long */