Date: Wed, 3 Feb 2016 12:43:55 -0700 From: Warner Losh <imp@bsdimp.com> To: John Baldwin <jhb@freebsd.org> Cc: "freebsd-arch@freebsd.org" <freebsd-arch@freebsd.org>, Brooks Davis <brooks@freebsd.org>, Mike Belopuhov <mike@belopuhov.com>, Ryan Stone <rysto32@gmail.com>, "freebsd-arch@freebsd.org" <arch@freebsd.org> Subject: Re: OpenBSD mallocarray Message-ID: <CANCZdfqeR4nHEWPSv-EdW3OA1UcJTXtLhXijHX7p01-FTpo6Sg@mail.gmail.com> In-Reply-To: <1955470.jNCaThvui8@ralph.baldwin.cx> References: <CAB815ZafpqJoqr1oH8mDJM=0RxLptQJpoJLexw6P6zOi7oSXTQ@mail.gmail.com> <20160201224854.GB1747@spindle.one-eyed-alien.net> <CANCZdfqDDE5jigsDUSpLPjoqGNPCqn4kipX-eqYBBYakuTiBWA@mail.gmail.com> <1955470.jNCaThvui8@ralph.baldwin.cx>
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On Wed, Feb 3, 2016 at 12:39 PM, John Baldwin <jhb@freebsd.org> wrote: > On Monday, February 01, 2016 04:01:14 PM Warner Losh wrote: > > On Mon, Feb 1, 2016 at 3:48 PM, Brooks Davis <brooks@freebsd.org> wrote: > > > > > On Mon, Feb 01, 2016 at 02:12:20PM -0700, Warner Losh wrote: > > > > > > > > > On Feb 1, 2016, at 2:02 PM, Mike Belopuhov <mike@belopuhov.com> > wrote: > > > > > > > > > > On Mon, Feb 01, 2016 at 15:56 -0500, Ryan Stone wrote: > > > > >> On Mon, Feb 1, 2016 at 3:16 PM, Conrad Meyer <cem@freebsd.org> > wrote: > > > > >> > > > > >>> > > > > >>> Sure. +1 from me. I don't think we want the M_CANFAIL hack, > though. > > > > >>> > > > > >>> Best, > > > > >>> Conrad > > > > >>> > > > > >>> > > > > >> That may be the OpenBSD equivalent of M_NOWAIT. > > > > > > > > > > Not quite. From the man page: > > > > > > > > > > M_CANFAIL > > > > > > > > > > In the M_WAITOK case, if not enough memory is available, > > > > > return NULL instead of calling panic(9). If mallocarray() > > > > > detects an overflow or malloc() detects an excessive > > > > > allocation, return NULL instead of calling panic(9). > > > > > > > > Yea, we don???t want it calling panic. Ever. That turns an overflow > > > > into a DoS. Arguments should be properly checked so we can > > > > properly return EINVAL for bat-**** crazy ones. FreeBSD???s malloc > > > > doesn???t cave an excessive detector in it. > > > > > > > > My concern with this is that we have a number of different allocation > > > > routines in FreeBSD. This only goes after the malloc() vector, and > > > > even then it requires code changes. > > > > > > > > At best, CANFAIL is a kludge to fail with a panic instead of an > > > > overflow. That???s got to be at most a transient thing until all the > > > > code that it is kludged into with out proper thought is fixed. I???m > not > > > > sure that???s something that we want to encourage. I???m all for > > > > safety, but this flag seems both unsafe and unwise. > > > > > > Given that current code could be doing literally anything in the > > > overflow case (and thanks to modern undefined behavior optimizations is > > > likely doing something extraordinarily bizarre), I think turning > overflows > > > into panics is a good thing. Yes, you can argue that means you've > added > > > a DoS vector, but best case you had an under allocation and bizarre > > > memory corruption before. If the default or even only behavior is > going > > > to be that overflow fails then we need a static checker that ensure we > > > check the return value even in the M_WAITOK. Otherwise there will be > > > blind conversions of malloc to mallocarray that go unchecked. > > > > > > > Returning NULL should be sufficient. Blind conversion of malloc to > > mallocarray in the kernel is also stupid. Intelligent conversion is > > needed to ensure that the error conditions are handled correctly. > > There's no need for a flag to say 'I am going to do the right thing > > if you give me NULL back'. The conversion should do the right > > thing when you get NULL back. A quick survey of the current kernel > > shows that there's not very many that could be using user defined > > values, but does show a large number of places where we could > > use this API. > > > > I guess this comes down to 'why is it an unreasonable burden to > > test the return value in code that's converted?' We're already changing > > the code. > > > > If you absolutely must have a flag, I'd prefer M_CANPANIC or something > > that is also easy to add for the 'mindless' case that we can easily > > grep for so we know when we're removed all the stupid 'mindless' > > cases from the tree. > > Having M_WAITOK-anything return NULL will be a POLA violation. It doesn't > return NULL for anything else. I think having a separate M_CANFAIL flag > is also rather pointless. If we want to have this, I think it should > work similar to malloc(). M_WAITOK panics if you do stupid things > (malloc(9) does this for sufficiently large overflow when it exhausts kmem > contrary to Warner's earlier claim), M_NOWAIT returns NULL. > Exausting kmem isn't influenced by simple args. But I do stand corrected. > In general I think I most prefer Bruce's approach of having a separate > macro > to check for overflow explicitly so it can be handled as a separate error. > In particular, if mallocarry(..., M_NOWAIT) fails, is it because of memory > shortage (in which case retrying in the future might be sensible) or is it > due to overflow (in which case it will fail no matter how many times you > retry)? You'd then have to have the macro anyway to differentiate and > handle > this case. > > Warner also seems to be assuming that we should do check for overflow > explicitly for any user-supplied values before calling malloc() or > malloc()-like things. This means N hand-rolled (and possibly buggy) > checks, > or a shared macro to do the check. I think this is another argument in > favor > of Bruce's approach. :) > I like Bruce's approach. And it works for more than just malloc. > If you go that route, then mallocarray() is really just an assertion > checker. It should only fail because a programmer ommitted an explicit > overflow check for a user-supplied value or screwed up an in-kernel > value. In that case I think panic'ing sooner when the overflow is obvious > is more useful for debugging the error than a NULL pointer deference > some time later (or requests that get retried forever and go possibly > unnoticed). > That would be fine. On its own, mallocarray() has all the issues we've talked about. Warner
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