I haven't tested the fix (yet), but just looking at the code I'm
perfectly fine with the proposed replacement of the macro PRECEDING_KEY
by the preceding_key function.
I have some minor concerns about the efficiency of the amount of
indirections, but the gcc optimizer may take care of this. This is for
later concern anyway.
On 6/9/19 5:24 PM, Coly Li wrote:
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.
Signed-off-by: Coly Li <colyli@xxxxxxx>
Reviewed-by: Rolf Fokkens <rolf@xxxxxxxxxxxxxx>
Reviewed-by: Pierre JUHEN <pierre.juhen@xxxxxxxxx>
Tested-by: Shenghui Wang <shhuiw@xxxxxxxxxxx>
Cc: Kent Overstreet <kent.overstreet@xxxxxxxxx>
Cc: Nix <nix@xxxxxxxxxxxxx>
---
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)
{
