Messages in this thread | | | Date | Wed, 13 Apr 2022 13:20:39 +0200 | Subject | Re: [PATCH] binder: Address corner cases in deferred copy and fixup | From | Alessandro Astone <> |
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On 13/04/2022 12:00, Greg KH wrote:
> On Wed, Apr 13, 2022 at 10:54:27AM +0200, Alessandro Astone wrote: >> When handling BINDER_TYPE_FDA object we are pushing a parent fixup >> with a certain skip_size but no scatter-gather copy object, since >> the copy is handled standalone. >> If BINDER_TYPE_FDA is the last children the scatter-gather copy >> loop will never stop to skip it, thus we are left with an item in >> the parent fixup list. This will trigger the BUG_ON(). >> >> Furthermore, it is possible to receive BINDER_TYPE_FDA object >> with num_fds=0 which will confuse the scatter-gather code. >> >> In the android userspace I could only find these usecases in the >> libstagefright OMX implementation, so it might be that they're >> doing something very weird, but nonetheless the kernel should not >> panic about it. >> >> Fixes: 09184ae9b575 ("binder: defer copies of pre-patched txn data") >> Signed-off-by: Alessandro Astone <ales.astone@gmail.com> >> --- >> drivers/android/binder.c | 11 +++++++++-- >> 1 file changed, 9 insertions(+), 2 deletions(-) >> >> diff --git a/drivers/android/binder.c b/drivers/android/binder.c >> index 8351c5638880..18ad6825ba30 100644 >> --- a/drivers/android/binder.c >> +++ b/drivers/android/binder.c >> @@ -2295,7 +2295,7 @@ static int binder_do_deferred_txn_copies(struct binder_alloc *alloc, >> { >> int ret = 0; >> struct binder_sg_copy *sgc, *tmpsgc; >> - struct binder_ptr_fixup *pf = >> + struct binder_ptr_fixup *tmppf, *pf = > Just make this a new line: > struct binder_ptr_fixup *tmppf; > above the existing line. > Ack.
>> list_first_entry_or_null(pf_head, struct binder_ptr_fixup, >> node); >> >> @@ -2349,7 +2349,11 @@ static int binder_do_deferred_txn_copies(struct binder_alloc *alloc, >> list_del(&sgc->node); >> kfree(sgc); >> } >> - BUG_ON(!list_empty(pf_head)); > So you are hitting this BUG_ON() today? > Correct, both on 5.17, stable 5.17.2 and current master
>> + list_for_each_entry_safe(pf, tmppf, pf_head, node) { >> + BUG_ON(pf->skip_size == 0); >> + list_del(&pf->node); >> + kfree(pf); >> + } >> BUG_ON(!list_empty(sgc_head)); >> >> return ret > 0 ? -EINVAL : ret; >> @@ -2486,6 +2490,9 @@ static int binder_translate_fd_array(struct list_head *pf_head, >> struct binder_proc *proc = thread->proc; >> int ret; >> >> + if (fda->num_fds == 0) >> + return 0; > Why return 0? > > This feels like a separate issue from above, should this be 2 different > commits? > return 0 because I want it to be handled as it was handled before 09184ae9b575 ("binder: defer copies of pre-patched txn data")
Function `binder_do_deferred_txn_copies` distinguishes between a copy-fixup and a skip with `if (pf->skip_size)` so if the skip_size is 0, which happens if fda->num_fds is 0, it would accidentally enter the wrong branch. By returning 0 early i make sure a skip of size 0 is not added. It's not an error because it was never an error before commit 09184ae9b575 and some userspace in android is hitting this path.
I would agree it's a separate issue. I originally merged it in this same patch because 1) Both are fixups to 09184ae9b575 2) Both are triggered by the same real-world android transaction that looks something like this: obj[0] BINDER_TYPE_PTR, parent obj[1] BINDER_TYPE_PTR, child obj[2] BINDER_TYPE_PTR, child obj[3] BINDER_TYPE_FDA with num_fds=0 3) In the other hunk of this patch i replace the BUG_ON with: BUG_ON(pf->skip_size == 0) to only BUG if any item remaining in the pf_head list is not a skip, but as observed we may receive skips of size 0. 4) With this hunk only, you would no longer reproduce the BUG_ON because the only transaction we receive in android with BINDER_TYPE_FDA as last child coincidentally always has num_fds=0. Certainly some weird behaviour...
So if I split them, patch A would depend on patch B (see point 3), but the BUG of patch B would only be reproducible without patch A (see point 4).
But let me know if you still prefer them split.
> thanks, > > greg k-h
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