[tip: locking/urgent] static_call: Fix the module key fixup

[tip-bot2 for Peter Zijlstra]

The following commit has been merged into the locking/urgent branch of tip:

Commit-ID:     8b97c027dfe4ba195be08fd0e18f716005763b8a
Gitweb:        https://git.kernel.org/tip/8b97c027dfe4ba195be08fd0e18f716005763b8a
Author:        Peter Zijlstra <peterz@xxxxxxxxxxxxx>
AuthorDate:    Thu, 25 Feb 2021 23:03:51 +01:00
Committer:     Peter Zijlstra <peterz@xxxxxxxxxxxxx>
CommitterDate: Mon, 01 Mar 2021 11:02:10 +01:00

static_call: Fix the module key fixup

Provided the target address of a R_X86_64_PC32 relocation is aligned,
the low two bits should be invariant between the relative and absolute
value.

Turns out the address is not aligned and things go sideways, ensure we
transfer the bits in the absolute form when fixing up the key address.

Fixes: 73f44fe19d35 ("static_call: Allow module use without exposing static_call_key")
Reported-by: Steven Rostedt <rostedt@xxxxxxxxxxx>
Signed-off-by: Peter Zijlstra (Intel) <peterz@xxxxxxxxxxxxx>
Tested-by: Steven Rostedt (VMware) <rostedt@xxxxxxxxxxx>
Link: https://lkml.kernel.org/r/20210225220351.GE4746@xxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxx
---
 kernel/static_call.c | 7 ++++---
 1 file changed, 4 insertions(+), 3 deletions(-)

diff --git a/kernel/static_call.c b/kernel/static_call.c
index 6906c6e..ae82529 100644
--- a/kernel/static_call.c
+++ b/kernel/static_call.c
@@ -349,7 +349,8 @@ static int static_call_add_module(struct module *mod)
 	struct static_call_site *site;
 
 	for (site = start; site != stop; site++) {
-		unsigned long addr = (unsigned long)static_call_key(site);
+		unsigned long s_key = (long)site->key + (long)&site->key;
+		unsigned long addr = s_key & ~STATIC_CALL_SITE_FLAGS;
 		unsigned long key;
 
 		/*
@@ -373,8 +374,8 @@ static int static_call_add_module(struct module *mod)
 			return -EINVAL;
 		}
 
-		site->key = (key - (long)&site->key) |
-			    (site->key & STATIC_CALL_SITE_FLAGS);
+		key |= s_key & STATIC_CALL_SITE_FLAGS;
+		site->key = key - (long)&site->key;
 	}
 
 	return __static_call_init(mod, start, stop);