Re: [PATCH 2/2] nvme: drop WARN_ON_ONCE on write_stream bounds check

[Christoph Hellwig <hch@lst.de>]

On Tue, Jul 28, 2026 at 07:15:28AM +0200, Greg Kroah-Hartman wrote:
> On Mon, Jul 27, 2026 at 04:51:40PM -0600, Keith Busch wrote:
> > On Mon, Jul 27, 2026 at 09:19:27PM +0200, Greg Kroah-Hartman wrote:
> > > On Mon, Jul 27, 2026 at 08:24:40AM -0600, Keith Busch wrote:
> > > > On Sat, Jul 25, 2026 at 03:51:11PM +0200, Hari Mishal wrote:
> > > > > write_stream is validated against bdev_max_write_streams() in both
> > > > > generic block direct I/O (block/fops.c) and F2FS before a bio
> > > > > carrying it is ever built, so write_stream > nr_plids shouldn't be
> > > > > reachable through any current legitimate path. The remaining users
> > > > > of bio->bi_write_stream elsewhere in the block layer only copy an
> > > > > already-validated value between bios (bio.c, blk-crypto-fallback.c)
> > > > > or compare it for merge eligibility (blk-merge.c); none of them
> > > > > introduce a new, unvalidated value.
> > > > > 
> > > > > Using WARN_ON_ONCE as the backstop for that assumption isn't worth
> > > > > it given how many deployed systems run with panic-on-warn enabled;
> > > > > the existing graceful return BLK_STS_INVAL already handles it on
> > > > > its own.
> > > > 
> > > > That's not a very good reason to remove a WARN_ON. You've left the check
> > > > in for a condition that should never happen, so when it does happen,
> > > > it'll be impossible to debug without the WARN.
> > > > 
> > > > And the WARN also annotates the branch as unlikely, which is desirable
> > > > for this case.
> > > 
> > > But, if it ever does happen, a WARN_ON will reboot the box, given that
> > > billions of Linux systems have panic-on-warn enabled.  
> > 
> > So WARN_ON is the new BUG_ON now?
> 
> It has been that way since syzbot started sending us reports (i.e. for
> many many years...)
> 
> > If the condition happens we need to
> > know how we got here and make it obvious something is wrong. So I guess
> > we'd have to replace every one of these:
> > 
> > 	if (WARN_ON_ONCE(condition)) ...
> > 
> > With an open-coded version like:
> > 
> > 	if (unlikely(condition)) {
> > 		do_once(dump_stack());
> > 		...
> > 	}
> > 
> > ?
> 
> Yes, if userspace can trigger this.  Because again, this will cause a
> box to reboot.
> 
> If we didn't have panic-on-warn, a ton of CVEs would just disappear
> tomorrow.  But that's not the world we live in :(
> 
> > That doesn't seem right, so if that is the suggestion, then I think we
> > need a new macro to provide the result that the WARN_ON usage expected.
> 
> You can provide a tracedump if you really need/want it, no need to call
> WARN_ON(), the macro is there for you to use.
> 
> > > So if this can ever happen, 
> > 
> > But it can't ever happen. This patch's commit message reasoned that as
> > justification to remove the warn, but we need to know how we got here
> > when it does happen because it means somebody broke contract.
> 
> Fair enough, but note that if userspace can trigger this, it should be
> fixed up.  See the other WARN_ON patch fix for nvme that I sent yesterday
> for an example of userspace being able to trigger this type of issue:
> 	https://lore.kernel.org/r/20260727-nvme-tcp-v1-1-61c0e36763eb@linuxfoundation.org
> 
> > > just properly handle it and recover and don't loose user
> > > data.
> > 
> > We can't save the data from this specific condition: the data from the
> > request is unwritable and lost. We've also learned that EINVAL errors
> > are not handled for many DM stacking drivers in very bad ways, so again,
> > we need to know how we got here when something breaks the API contract
> > otherwise it'll be a difficult problem to debug without that visibility.
> 
> Ok, if you want to keep this here, that's fine, because you know you
> can't recover properly and crashing the system is the only acceptable
> thing to do.  But in that case, why not make it a BUG_ON()?

Because we don't want a BUG_ON.  If you set panic on warn in anything
but a debug setup you get what you pay for, and I'm really tired of
all these totally stupid attempts to make WARN_ON the new BUG_ON.
It is not, and that's for a reason.