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 Cc: stable@vger.kernel.org --- Changlog: V2: Fix a pointer assignment problem in preceding_key(), which is pointed by Rolf Fokkens and Pierre JUHEN. V1: Initial RFC patch for review and comment.

drivers/md/bcache/bset.c | 16 +++++++++++++--- drivers/md/bcache/bset.h | 34 ++++++++++++++++++++-------------- 2 files changed, 33 insertions(+), 17 deletions(-)

diff --git a/drivers/md/bcache/bset.c b/drivers/md/bcache/bset.c index 8f07fa6e1739..268f1b685084 100644 --- a/drivers/md/bcache/bset.c +++ b/drivers/md/bcache/bset.c @@ -887,12 +887,22 @@ unsigned int bch_btree_insert_key(struct btree_keys *b, struct bkey *k, 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; diff --git a/drivers/md/bcache/bset.h b/drivers/md/bcache/bset.h index bac76aabca6d..c71365e7c1fa 100644 --- a/drivers/md/bcache/bset.h +++ b/drivers/md/bcache/bset.h @@ -434,20 +434,26 @@ static inline bool bch_cut_back(const struct bkey *where, struct bkey *k) 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) {