Am 18.08.21 um 15:13 schrieb Sa, Nuno:
From: Christian König <christian.koenig@xxxxxxx>
Sent: Wednesday, August 18, 2021 2:58 PM
To: Daniel Vetter <daniel@xxxxxxxx>
Cc: Sa, Nuno <Nuno.Sa@xxxxxxxxxx>; linaro-mm-sig@xxxxxxxxxxxxxxxx;
dri-devel@xxxxxxxxxxxxxxxxxxxxx; linux-media@xxxxxxxxxxxxxxx; Rob
Clark <rob@xxxxxx>
Subject: Re: [Linaro-mm-sig] [PATCH] dma-buf: return -EINVAL if
dmabuf object is NULL
[External]
Am 18.08.21 um 14:46 schrieb Daniel Vetter:
On Wed, Aug 18, 2021 at 02:31:34PM +0200, Christian König wrote:
Am 18.08.21 um 14:17 schrieb Sa, Nuno:
From: Christian König <christian.koenig@xxxxxxx>
Sent: Wednesday, August 18, 2021 2:10 PM
To: Sa, Nuno <Nuno.Sa@xxxxxxxxxx>; linaro-mm-
sig@xxxxxxxxxxxxxxxx;
dri-devel@xxxxxxxxxxxxxxxxxxxxx; linux-media@xxxxxxxxxxxxxxx
Cc: Rob Clark <rob@xxxxxx>; Sumit Semwal
<sumit.semwal@xxxxxxxxxx>
Subject: Re: [PATCH] dma-buf: return -EINVAL if dmabuf object
is
NULL
[External]
To be honest I think the if(WARN_ON(!dmabuf)) return -EINVAL
handling
here is misleading in the first place.
Returning -EINVAL on a hard coding error is not good practice and
should
probably be removed from the DMA-buf subsystem in general.
Would you say to just return 0 then? I don't think that having the
dereference is also good..
No, just run into the dereference.
Passing NULL as the core object you are working on is a hard coding
error
and not something we should bubble up as recoverable error.
I used -EINVAL to be coherent with the rest of the code.
I rather suggest to remove the check elsewhere as well.
It's a lot more complicated, and WARN_ON + bail out is rather
well-established code-pattern. There's been plenty of discussions in
the
past that a BUG_ON is harmful since it makes debugging a major
pain, e.g.
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There's also a checkpatch check for this.
commit 9d3e3c705eb395528fd8f17208c87581b134da48
Author: Joe Perches <joe@xxxxxxxxxxx>
Date: Wed Sep 9 15:37:27 2015 -0700
checkpatch: add warning on BUG/BUG_ON use
Anyone who is paranoid about security crashes their machine on any
WARNING
anyway (like syzkaller does).
My rule of thumb is that if the WARN_ON + bail-out code is just an if
(WARN_ON()) return; then it's fine, if it's more then BUG_ON is the
better
choice perhaps.
I think the worst choice is just removing all these checks, because a
few
code reorgs later you might not Oops immediately afterwards
anymore, and
then we'll merge potentially very busted new code. Which is no
good.
Well BUG_ON(some_codition) is a different problem which I agree on
with
Linus that this is problematic.
But "if (WARN_ON(!dmabuf)) return -EINVAL;" is really bad coding
style
as well since it hides real problems which are hard errors behind
warnings.
I agree that doing these kind of checks in the core object of an API is
abusing parameter "validation". I guess a good pattern is having the
warning and let the code flow. But since these checks are already all
over the place I'm not sure the way to go. I'm very new to dma-buf
and I was just checking the code and saw this was not be coherent with
the rest of the API so I thought it would be a straight easy patch... Well,
I could not be more wrong :)
Well that existing stuff might actually depend on this is a really good
argument to keep it for now or at least until we have a consent on what
to do.
Anyways, depending on what's decided, I can send another patch trying
to make these stuff more coherent. At this point, my feeling is that this
patch is to be dropped...
At least for now I would hold it back.
Thanks,
Christian.
- Nuno Sá
Returning -EINVAL indicates a recoverable error which is usually caused
by userspace giving invalid parameters and should never be abused to
indicate a driver coding error.
Functions are either intended to take NULL as valid parameter, e.g. like
kfree(NULL). Or they are intended to work on an object which is
mandatory to provide.
Christian.
-Daniel
Christian.
- Nuno Sá
Christian.
Am 18.08.21 um 13:58 schrieb Nuno Sá:
On top of warning about a NULL object, we also want to return
with a
proper error code (as done in 'dma_buf_begin_cpu_access()').
Otherwise,
we will get a NULL pointer dereference.
Fixes: fc13020e086b ("dma-buf: add support for kernel cpu
access")
Signed-off-by: Nuno Sá <nuno.sa@xxxxxxxxxx>
---
drivers/dma-buf/dma-buf.c | 3 ++-
1 file changed, 2 insertions(+), 1 deletion(-)
diff --git a/drivers/dma-buf/dma-buf.c b/drivers/dma-
buf/dma-
buf.c
index 63d32261b63f..8ec7876dd523 100644
--- a/drivers/dma-buf/dma-buf.c
+++ b/drivers/dma-buf/dma-buf.c
@@ -1231,7 +1231,8 @@ int dma_buf_end_cpu_access(struct
dma_buf *dmabuf,
{
int ret = 0;
- WARN_ON(!dmabuf);
+ if (WARN_ON(!dmabuf))
+ return -EINVAL;
might_lock(&dmabuf->resv->lock.base);
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