On 2019/6/10 2:28 上午, Rolf Fokkens wrote:
> 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.
>
Hi Rolf,
I see the point, if the indirected pointers became performance bottle
neck in future, let's fix it then.
Thank you for the help!
Coly Li
> 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 <[email protected]>
>> Reviewed-by: Rolf Fokkens <[email protected]>
>> Reviewed-by: Pierre JUHEN <[email protected]>
>> Tested-by: Shenghui Wang <[email protected]>
>> Cc: Kent Overstreet <[email protected]>
>> Cc: Nix <[email protected]>
>> ---
>> 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)
>> {
