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Subject[PATCH 3.16 56/87] bcache: fix stack corruption by PRECEDING_KEY()
3.16.75-rc1 review patch.  If anyone has any objections, please let me know.

------------------

From: Coly Li <colyli@suse.de>

commit 31b90956b124240aa8c63250243ae1a53585c5e2 upstream.

Recently people report bcache code compiled with gcc9 is broken, one of
the buggy behavior I observe is that two adjacent 4KB I/Os should merge
into one but they don't. Finally it turns out to be a stack corruption
caused by macro PRECEDING_KEY().

See how PRECEDING_KEY() is defined in bset.h,
437 #define PRECEDING_KEY(_k) \
438 ({ \
439 struct bkey *_ret = NULL; \
440 \
441 if (KEY_INODE(_k) || KEY_OFFSET(_k)) { \
442 _ret = &KEY(KEY_INODE(_k), KEY_OFFSET(_k), 0); \
443 \
444 if (!_ret->low) \
445 _ret->high--; \
446 _ret->low--; \
447 } \
448 \
449 _ret; \
450 })

At line 442, _ret points to address of a on-stack variable combined by
KEY(), the life range of this on-stack variable is in line 442-446,
once _ret is returned to bch_btree_insert_key(), the returned address
points to an invalid stack address and this address is overwritten in
the following called bch_btree_iter_init(). Then argument 'search' of
bch_btree_iter_init() points to some address inside stackframe of
bch_btree_iter_init(), exact address depends on how the compiler
allocates stack space. Now the stack is corrupted.

Fixes: 0eacac22034c ("bcache: PRECEDING_KEY()")
Signed-off-by: Coly Li <colyli@suse.de>
Reviewed-by: Rolf Fokkens <rolf@rolffokkens.nl>
Reviewed-by: Pierre JUHEN <pierre.juhen@orange.fr>
Tested-by: Shenghui Wang <shhuiw@foxmail.com>
Tested-by: Pierre JUHEN <pierre.juhen@orange.fr>
Cc: Kent Overstreet <kent.overstreet@gmail.com>
Cc: Nix <nix@esperi.org.uk>
Signed-off-by: Jens Axboe <axboe@kernel.dk>
Signed-off-by: Ben Hutchings <ben@decadent.org.uk>
---
drivers/md/bcache/bset.c | 16 +++++++++++++---
drivers/md/bcache/bset.h | 34 ++++++++++++++++++++--------------
2 files changed, 33 insertions(+), 17 deletions(-)

--- a/drivers/md/bcache/bset.c
+++ b/drivers/md/bcache/bset.c
@@ -823,12 +823,22 @@ unsigned bch_btree_insert_key(struct btr
struct bset *i = bset_tree_last(b)->data;
struct bkey *m, *prev = NULL;
struct btree_iter iter;
+ struct bkey preceding_key_on_stack = ZERO_KEY;
+ struct bkey *preceding_key_p = &preceding_key_on_stack;

BUG_ON(b->ops->is_extents && !KEY_SIZE(k));

- m = bch_btree_iter_init(b, &iter, b->ops->is_extents
- ? PRECEDING_KEY(&START_KEY(k))
- : PRECEDING_KEY(k));
+ /*
+ * If k has preceding key, preceding_key_p will be set to address
+ * of k's preceding key; otherwise preceding_key_p will be set
+ * to NULL inside preceding_key().
+ */
+ if (b->ops->is_extents)
+ preceding_key(&START_KEY(k), &preceding_key_p);
+ else
+ preceding_key(k, &preceding_key_p);
+
+ m = bch_btree_iter_init(b, &iter, preceding_key_p);

if (b->ops->insert_fixup(b, k, &iter, replace_key))
return status;
--- a/drivers/md/bcache/bset.h
+++ b/drivers/md/bcache/bset.h
@@ -417,20 +417,26 @@ static inline bool bch_cut_back(const st
return __bch_cut_back(where, k);
}

-#define PRECEDING_KEY(_k) \
-({ \
- struct bkey *_ret = NULL; \
- \
- if (KEY_INODE(_k) || KEY_OFFSET(_k)) { \
- _ret = &KEY(KEY_INODE(_k), KEY_OFFSET(_k), 0); \
- \
- if (!_ret->low) \
- _ret->high--; \
- _ret->low--; \
- } \
- \
- _ret; \
-})
+/*
+ * Pointer '*preceding_key_p' points to a memory object to store preceding
+ * key of k. If the preceding key does not exist, set '*preceding_key_p' to
+ * NULL. So the caller of preceding_key() needs to take care of memory
+ * which '*preceding_key_p' pointed to before calling preceding_key().
+ * Currently the only caller of preceding_key() is bch_btree_insert_key(),
+ * and it points to an on-stack variable, so the memory release is handled
+ * by stackframe itself.
+ */
+static inline void preceding_key(struct bkey *k, struct bkey **preceding_key_p)
+{
+ if (KEY_INODE(k) || KEY_OFFSET(k)) {
+ (**preceding_key_p) = KEY(KEY_INODE(k), KEY_OFFSET(k), 0);
+ if (!(*preceding_key_p)->low)
+ (*preceding_key_p)->high--;
+ (*preceding_key_p)->low--;
+ } else {
+ (*preceding_key_p) = NULL;
+ }
+}

static inline bool bch_ptr_invalid(struct btree_keys *b, const struct bkey *k)
{
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