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[157.157.127.103]) by smtp.gmail.com with ESMTPSA id qa30sm2832559ejc.54.2021.12.24.08.49.30 (version=TLS1_3 cipher=TLS_AES_256_GCM_SHA384 bits=256/256); Fri, 24 Dec 2021 08:49:30 -0800 (PST) Received: from avar by gmgdl with local (Exim 4.95) (envelope-from ) id 1n0nl3-000260-Cq; Fri, 24 Dec 2021 17:49:29 +0100 From: =?utf-8?B?w4Z2YXIgQXJuZmrDtnLDsA==?= Bjarmason To: Philip Oakley Cc: Johannes Schindelin , git@vger.kernel.org, Junio C Hamano , Jeff King , Erik Faye-Lund , Jonathan Nieder Subject: Re: [RFC PATCH 00/10] range-diff: fix segfault due to integer overflow Date: Fri, 24 Dec 2021 17:46:26 +0100 References: <59ec39af-fdb1-a86a-d2be-37e5954e245f@iee.email> <211222.86r1a5plsm.gmgdl@evledraar.gmail.com> User-agent: Debian GNU/Linux bookworm/sid; Emacs 27.1; mu4e 1.6.10 In-reply-to: Message-ID: <211224.86zgoqgd7q.gmgdl@evledraar.gmail.com> MIME-Version: 1.0 Content-Type: text/plain; charset=utf-8 Content-Transfer-Encoding: quoted-printable Precedence: bulk List-ID: X-Mailing-List: git@vger.kernel.org On Fri, Dec 24 2021, Philip Oakley wrote: > On 21/12/2021 23:36, =C3=86var Arnfj=C3=B6r=C3=B0 Bjarmason wrote: >> On Tue, Dec 21 2021, Philip Oakley wrote: >> >>> Sorry for the late comment.. >>> >>> On 10/12/2021 14:31, Johannes Schindelin wrote: >>>> Hi =C3=86var, >>>> >>>> On Thu, 9 Dec 2021, =C3=86var Arnfj=C3=B6r=C3=B0 Bjarmason wrote: >>>> >>>>> The difference between "master" and "git-for-windows/main" is large >>>>> enough that comparing the two will segfault on my system. This is >>>>> because the range-diff code does some expensive calculations and will >>>>> overflow the "int" type. >>>> You are holding this thing wrong. >>>> >>>> The `main` branch of Git for Windows uses merging rebases, therefore y= ou >>>> need to use a commit range like >>>> `git-for-windows/main^{/^Start.the.merging}..git-for-windows/main` and >>>> compare it to `git-for-windows/main..master`. >>> I'm not sure that a Git repo has an established way of indicating to how >>> it's branching/merging/releasing workflow is set up, especially for >>> projects with non-normative use cases, such as Git for Windows. We don't >>> have a git document for covering=C2=A0 the different workflows in commo= n use >>> for easy reference and consistent terminology. >>> >>> The merging rebase flow, with 'fake' merge does solve a problem that >>> git.git doesn't have but could easily be a common process for 'friendly >>> forks' that follow an upstream with local patches. The choice of >>> '{/^Start.the.merging}' is currently specific to the Git-for-Windows >>> case making it harder to discover this useful maintainer method. >> Yes, but let's not get lost in the weeds here. As I noted I just picked >> GFW as a handy example of a large history & that command as a handy >> example of something that segfaults on "master". > > Had you already experienced the segfault locally, without using the GFW > example? How many commits were present in that case? Yes, I ran into it "orginally" just range-diffing as part of a local build process. I could dig up what revision range it was exactly, but does it matter? > The GFW example seems like it's taken the discussion in the wrong directi= on. > > For me: > $ git log git/master..origin/main --pretty=3Doneline | wc -l > 62105 > > That's a lot of commits to have in a range diff. It's almost as big as > the whole of git/master > > $ git log git/master --pretty=3Doneline | wc -l > 65400 > > Personally I'd like a way of trimming 'deadheads' that's a bit easier > that needing to remember Dscho's magic string [1], but time will tell. There are some repos that move forward by 500-1k commits/day, and people do cherry-pick patches etc. So wanting to range-diff after a couple of months is something you might do... >> So the point really isn't to say that we should fix range-diff becase >> it'll allow us to run this practically useful command on a git.git fork. >> >>> I fully agree that the range-diff should probably have a patch limit at >>> some sensible value. >> Why would it? If I'm willing to spend the CPU to produce a range-diff of >> an absurdly large range and I've got the memory why shouldn't we support >> it? > > There will always be a limit somewhere, and if it's not commit count or > other easily explained & checked limit it will be hard to rationalise > about why Git suddenly fails with an error (or segfault) in those > humungous case. I think it's fairly easy to explain the "your system wouldn't let us malloc more, we're dying" that we get from xmalloc(), st_*() and the like. >> >> We don't in cases like xdiff where it's not trivial to just raise the >> limits, but here it seems relatively easy. >> >> I think limits to save users from spending CPU time they didn't expect >> are reasonable, but then we can handle them like the diff/merge rename >> detection limits, i.e. print a warning/advice, and allow the user to >> opt-out. >> >> That also doesn't really apply here since "diff/merge" will/might still >> do something useful in those scenarios, whereas range-diff would just >> have truncated output. >> >>> The 'confusion' between the types size_t, long and int, does ripple >>> through a lot of portable code, as shown in the series. Not an easy pro= blem. >> Yes, although here we're not just casting and overflowing types, but >> overflowing on multiplication and addition, whereas usually we'd just >> overflow on "nr" being too big for "int" or similar. > I've been very slowly looking at the `long` limits on GFW which have > very similar arithmetic issues for pointers, often with no clear answers. Right, that's to do with the whole "long" or whatever use in the object.c and related code, but I don't think that's applicable here, is it?